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Adaptive evolution in Paleozoic coiled Cephalopods
Abstract
Coiled cephalopods constitute a major part of the Paleozoic nekton. They emerged in the Early Ordovician but nearly vanished in the Silurian. The Emsian appearance of ammonoids started a story of evolutionary success of coiled cephalopods, which lasted until the end-Permian extinction event. This story is investigated by using a taxonomic database of 1346 species of 253 genera of coiled nautiloids and 1114 genera of ammonoids. The per capita sampling diversities, the Van Valen metrics of origination and extinction, and the probabilities of origination and ex- tinction were calculated at stage intervals. The outcome of these estimations largely reflects the known biotic events of the Paleozoic. The polyphyletic, iterative appearance of coiled cephalopods within this time frame is interpreted to be a process of adaptation to shell-crushing predatory pres- sure. The evolution of the diversity of coiled nautiloids and ammonoids is strongly correlated with- in the time intervals. Once established, assemblages of coiled cephalopods are related to changes in sea level. The general trends of decreasing mean (or background) origination and extinction rates during the Paleozoic are interpreted to reflect a successive stabilization of the coiled cephalopod assemblages. Different reproduction strategies in ammonoids and nautiloids apparently resulted in different modes of competition and morphological trends. Significant morphological trends to- ward a stronger ornamentation and a centrally positioned siphuncle characterize the evolution of Paleozoic nautiloids.
... These trends, as well as the trend towards a higher degree of whorl overlap in the adult stage, may have affected the swimming capabilities and fecundity in ammonoids (Klug and Korn 2004;De Baets et al. 2012). The repeated coiling trends might be a response to increased predatory pressure as proposed for various molluscs groups (e.g., Nützel and Frýda 2003;Kröger 2005;De Baets et al. 2012Klug et al. 2017;Dzik 2020). During the Devonian, various predatory groups capable of preying on ammonoids were already well established (Whalen and Briggs 2018;Ferrón and Donoghue 2022). ...
... The loosely coiled conch is considered as an optimised morphology for an economy of shell material (Tendler et al. 2015). However, they are mechanically weaker and hydrodynamically less favourable; they can be crushed more easily by predators than tightly coiled conchs (Brett and Walker 2002;Nützel and Frýda 2003;Kröger 2005;Wagner and Erwin 2006;Klug 2007;De Baets et al. 2012). Following this idea, this trend towards increased coiling of the conch was usually interpreted as the result of adaptative pressures coupled with improved hydrodynamics. ...
... The diversification of vertebrate nektonic predators, including fishes, and the escalation of their swimming capabilities has been demonstrated to be more complex and gradual (Whalen and Briggs 2018;Andreev et al. 2022;Ferrón and Donoghue 2022;Friedman 2022;Zhu et al. 2022) than previously understood . Nevertheless, various or repeated pulses of increase in coiling may have occurred in conjunction with radiation pulses of active predators (Kröger 2005). But other factors might be involved; increased coiling also correlates with increasing fecundity as well as decreasing embryo size, leading to an increase in reproductive rates (Klug 2001a(Klug , 2007De Baets et al. 2012compare Ritterbush et al. 2014). ...
Fossils of Devonian ammonoids are abundant and well-preserved in the Anti-Atlas of Morocco; as such they provide
an invaluable record of regional morphological disparity changes (diversity of shapes) that characterise the first steps of
ammonoid evolution. However, they were rarely analysed quantitatively with respect to their morphological spectrum.
Here, we investigated the morphological disparity of the Early–Middle Devonian ammonoids of the Moroccan Anti-
Atlas by analysing the shape of their whorl profile. A geometric morphometric approach based on the acquisition of
outline semilandmark coordinates was used to analyse the whorl profiles. For comparison, morphometric ratios based on
classical conch measurements were also analysed to investigate the overall conch geometry. Several standard disparity
estimators were computed to measure different aspects of morphological disparity fluctuations through time. It appears
that a major increase in disparity occurred throughout the Early Devonian, followed by fluctuating disparity during
the Middle Devonian constituting a general decreasing trend. Only the end-Eifelian Kačák Event shows a significant
decrease in disparity. Thus, the ammonoids explored the range of possible shapes fairly quickly during their initial radiation;
however, we found no evidence for an early burst of shape diversity (i.e., the rise does not exceed the expectations
given diversity). Nevertheless, correlation tests between diversity and disparity time series support that they are partially
decoupled. The highly resolved biozone record highlights that the increase in disparity began earlier than the increase in
diversity that characterises the late Emsian.
... As far as thelodont genera in the Devonian are concerned, they Table 2. Diversity of mid-Palaeozoic Discosorida, Nautilida, Oncocerida and Tarphycerida. The data compilation was performed by B.K. (Kröger 2003(Kröger , 2005(Kröger , 2008, largely based on Sepkoski's raw data (2002). (Märss et al. 2007). ...
... Many members of these groups occur frequently in black shales and thus certainly lived in the water column. Most orthocerids were probably capable of minor horizontal movements but they were ineffective swimmers and migrated predominantly vertically and ⁄ or drifted passively (Hewitt & Watkins 1980;Westermann 1999;Mutvei 2002;Kröger 2003Kröger , 2005Kröger & Mutvei 2005;Mutvei et al. 2007). This is suggested by their poorly differentiated muscle-attachment structures, the absence of significant endosiphonal or endocameral deposits and, in some cases, also shell morphology. ...
... Diversity of Devonian bony fish. The data compilation was performed by B.K. (Kröger 2005) based on Sepkoski's raw data (2002). In Fig. 2 of the main text, two curves of Carr (1995) Additional groups Diversity data of the radiodonts, the eurypterids and graptoloids were extracted from Sepkoski's compendium (2002) and the Paleobiology Database. ...
Traditional analyses of Early Phanerozoic marine diversity at the genus level show an explosive radiation of marine life until the Late Ordovician, followed by a phase of erratic decline continuing until the end of the Palaeozoic, whereas a more recent analysis extends the duration of this early radiation into the Devonian. This catch-all approach hides an evolutionary and ecological key event long after the Ordovician radiation: the rapid occupation of the free water column by animals during the Devonian. Here, we explore the timing of the occupation of the water column in the Palaeozoic (including certain fish groups) and test the hypothesis that ecological escalation led to fundamental evolutionary changes in the mid-Palaeozoic marine water column. According to our analyses, demersal and nektonic modes of life were probably initially driven by competition in the diversity-saturatedbenthic habitats together with the availability of abundant planktonic food. Escalatory feedback then promoted the rapid rise of nekton in the Devonian as suggested by the
sequence and tempo of water-column occupation.
... Many members of these groups occur frequently in black shales and thus certainly lived in the water column. Most orthocerids were probably capable of minor horizontal movements but they were ineffective swimmers and migrated predominantly vertically and ⁄ or drifted passively (Hewitt & Watkins 1980; Westermann 1999; Mutvei 2002; Kröger 2003 Kröger , 2005 Kröger & Mutvei 2005; Mutvei et al. 2007). This is suggested by their poorly differentiated muscle-attachment structures, the absence of significant endosiphonal or endocameral deposits and, in some cases, also shell morphology. ...
... Diversity of Devonian bony fish. The data compilation was performed by B.K. (Kröger 2005) based on Sepkoski's raw data (2002). InFig. 2 of the main text, two curves of Carr (1995) are reproduced showing the diversity of sarcopterygian and actinopterygian genera per stage. ...
... 2. Diversification of various higher rank taxa of phytoplankton, zooplankton, demersal and pelagic nekton in the Devonian and Early Carboniferous (for data see Tables 2–15). the radiation of gnathostomes then increased predatory pressure on cephalopods, selecting for higher mobility (Klug & Korn 2004; Kröger 2005). Assuming this hypothesis is correct, an initial high diversity in the demersal zone, followed by a radiation of some transitional demersal to pelagic-nektonic predators, again followed by a nekton diversification and a plankton decrease, would be expected. ...
... Many members of these groups occur frequently in black shales and thus certainly lived in the water column. Most orthocerids were probably capable of minor horizontal movements but they were ineffective swimmers and migrated predominantly vertically and ⁄ or drifted passively (Hewitt & Watkins 1980; Westermann 1999; Mutvei 2002; Kröger 2003 Kröger , 2005 Kröger & Mutvei 2005; Mutvei et al. 2007). This is suggested by their poorly differentiated muscle-attachment structures, the absence of significant endosiphonal or endocameral deposits and, in some cases, also shell morphology. ...
... Diversity of Devonian bony fish. The data compilation was performed by B.K. (Kröger 2005) based on Sepkoski's raw data (2002). InFig. 2 of the main text, two curves of Carr (1995) are reproduced showing the diversity of sarcopterygian and actinopterygian genera per stage. ...
... 2. Diversification of various higher rank taxa of phytoplankton, zooplankton, demersal and pelagic nekton in the Devonian and Early Carboniferous (for data see Tables 2–15). the radiation of gnathostomes then increased predatory pressure on cephalopods, selecting for higher mobility (Klug & Korn 2004; Kröger 2005). Assuming this hypothesis is correct, an initial high diversity in the demersal zone, followed by a radiation of some transitional demersal to pelagic-nektonic predators, again followed by a nekton diversification and a plankton decrease, would be expected. ...
... Relation between conch form, mode of life, and evolution− ary success.-According to Kröger (2005), tight coiling of cephalopod shells must be explained by adaptive evolution. One important question about coiling is as follows: What made the difference between ammonoids and other cephalo− pods with respect to the predator−prey relationship among cephalopods as well as between cephalopods and gnatho− stome fish. ...
... One important question about coiling is as follows: What made the difference between ammonoids and other cephalo− pods with respect to the predator−prey relationship among cephalopods as well as between cephalopods and gnatho− stome fish. As discussed by various authors (e.g., Klug and Korn 2004;Kröger 2005), shell curvature that ultimately evolved into tightly coiled shells probably played an impor− tant role with respect to the origin of sublethal injuries in sev− eral respects discussed below: (i) Coiling probably enabled even early ammonoids to increase their maximum swimming velocity compared to cephalopods with orthoconic shells (for explanations see e.g., Jacobs 1992;Jacobs and Cham− berlain 1996;Westermann 1996;Korn and Klug 2002;Klug and Korn 2004); (ii) Increased coiling enhanced manoeuvra− bility because the distance between the centre of gravity and the aperture rises with increased coiling, and thus, the lever− age effect from the hyponome action increases (see also Stridsberg 1985;Saunders and Shapiro 1986;Jacobs 1992;Jacobs and Chamberlain 1996;Westermann 1996Westermann , 1999Seki et al. 2000;Klug and Korn 2004); (iii) Coiling produces an additional advantage in that the animal spans less space with the same volume, and thus, the animal is more difficult to be detected and captured by a predator. In this respect, the optimised morphology would be spherical with a more or less closed umbilicus. ...
... With regard to cephalopod coiling and given the above in− formation, the question arises why were early coiled nautiloids less "diverse" than the ammonoids after the Devonian? Possi− bly, the origin for this phenomenon can-at least partiallybe sought in differing reproductive strategies and early onto− genies (compare Kröger 2005). Many fossil nautiloids proba− bly produced a smaller number of much larger eggs than ammonoids (see dimensions listed in Landman et al. 1996;Chirat 2001). ...
Internal moulds of the relatively small- to moderate-size shells of Early Devonian ectocochleate cephalopods (typically <150 mm diameter) occasionally display traces of repaired shell damage. Presumably, these animals with their highly specialized buoyancy device, the phragmocone, lived in the water column. It is uncertain as to how the shells of these animals were damaged; one likely cause would be predatory attacks but the identity of the perpetrator remains uncertain. So far, no remains of arthropods capable of breaking or cutting shells have been found in the fossiliferous outcrops of this age in the Anti-Atlas (Morocco). The only macrovertebrate remains of this age are of acanthodian and placodermfish which probably lived a more or less benthonic life style. Additionally, a fish attack on these cephalopods would probably have destroyed most of the thin-shelled conch and killed the animal. Most of the repaired shell breaks are triangular in shape which is characteristic for cephalopod bite marks. Additionally, the paired arrangement of the fractures in over 70 bactritoids supports the hypothesis that it was a cephalopod attacking another cephalopod. It cannot be excluded with certainty that occasional vertebrate attacks left traces on their shells. Fossil evidence indicates that the development of tightly coiled conchs was a rapid evolutionary event in the Ammonoidea in the Early Devonian; however, the evolution of coiling is probably not directly related to predation pressures because the ratio of injured to healthy specimens is roughly the same in Zlíchovian bactritoids with orthoconic and ammonoids with coiled shells.
... Although the authors interpreted this pattern as a response to the evolution of increasing numbers of shell crushing predators, the qualitative data of Ward (1981) suggest a passive trend constrained by a left-wall effect. For instance, convergent evolution of spines in marine mollusk shells have classically been interpreted as having repeatedly evolved as a defense in response to shell-crushing predators (e.g., Vermeij 1987;Kröger 2005;Ifrim 2013), but other interpretations are also available (see Ifrim 2013). Recent studies (Moulton et al. 2012(Moulton et al. , 2015Chirat et al. 2013) have demonstrated that a large diversity of ornamentation and spine structures can be accounted for through small variations in control parameters of the mechanical interaction between the secreting mantle edge and the calcified shell edge, which suggests that convergent evolution of spines can also be understood through a generic morphogenetic process without selective pressures. ...
... It has been widely demonstrated that, for shells with oxyconic shell shapes (involute and compressed), the energy consumption for swimming is the lowest and potential maximal swimming speed is the highest (decreasing drag, increasing streamlining, etc.; Schmidt 1930; Raup 1967;Chamberlain , 1980Chamberlain and Westermann 1976;Jacobs 1992;Jacobs and Chamberlain 1996;Hassan et al. 2002;Klug and Korn 2004;Klug et al. 2008;Naglik et al. 2015). Increased involution of the shell therefore appears to represent an adaptation toward improved hydrodynamic properties of the shell ( Fig. 5.11; but for alternative interpretations including sea-level changes and predatory pressure, see, e.g., McGhee 1984, 1985;McGhee et al. 1991;Neige et al. 1997;Olóriz et al. 1997Olóriz et al. , 1999Kröger 2005; Wilmsen and Mosavinia 2011). For example, the gradual shift during the Devonian from marine communities dominated by organisms with openly-coiled shells to communities dominated by tightly coiled shells in distantly related organisms (e.g., nautilids, ammonoids, gastropods) was probably caused by predatory selective forces (Nützel and Frýda 2003;Kröger 2005;Klug et al. 2010), assuming that open coiling makes shells weak and vulnerable to attack and force but also, they are simply larger (this selection pressure is also linked with swimming abilities). ...
... Increased involution of the shell therefore appears to represent an adaptation toward improved hydrodynamic properties of the shell ( Fig. 5.11; but for alternative interpretations including sea-level changes and predatory pressure, see, e.g., McGhee 1984, 1985;McGhee et al. 1991;Neige et al. 1997;Olóriz et al. 1997Olóriz et al. , 1999Kröger 2005; Wilmsen and Mosavinia 2011). For example, the gradual shift during the Devonian from marine communities dominated by organisms with openly-coiled shells to communities dominated by tightly coiled shells in distantly related organisms (e.g., nautilids, ammonoids, gastropods) was probably caused by predatory selective forces (Nützel and Frýda 2003;Kröger 2005;Klug et al. 2010), assuming that open coiling makes shells weak and vulnerable to attack and force but also, they are simply larger (this selection pressure is also linked with swimming abilities). ...
A major goal in evolutionary biology is to characterize and understand the patterns and processes that shape the evolutionary trajectory of clades through time and space. One common pattern in extinct and extant lineages is the pervasive occurrence of persistent phenotypic evolutionary trends. These trends of evolutionary lineages suggest that adaptation by means of natural selection is a major cause of homoplastic patterns such as convergence and parallel evolution. Such patterns can result from other processes, which are usually ignored or difficult to identify, such as chance, environmental changes, constructional or developmental constraints. How the evolution of organisms follows preferentially specific phenotypic trajectories remains poorly known. The ammonoids are well-known to display persistent phenotypic shifts through time. Commonly described trends in ammonoid lineages are increase in adult size, changes in coiling and increase in suture frilling. Some new methods for the study of ammonoid evolution are here exemplified by recent case studies.
... The phylogeny of most of the major cephalopod clades has been reconstructed with some confidence using morphological, developmental and molecular data in the last decades and some general macroevolutionary patterns are beginning to crystalize (e.g., Dzik 1981Dzik , 1984Woodruff et al. 1987;Engeser 1996;Young et al. 1998;Peterson et al. 2004;Kröger 2005;Bergmann et al. 2006;Strugnell et al. 2006;Strugnell and Nishiguchi 2007;Bizikov 2008;Shigeno et al. 2008Shigeno et al. , 2010Warnke et al. 2011). Undoubtedly, the sister group of cephalopods lies within the Mollusca, although the sister group of cephalopods is under debate. ...
... Classically, ammonoids have been separated from their bactritid ancestors by the presence of at least one full whorl (e.g., House 1988). This character, however, appears somewhat arbitrary, although coiling undoubtedly represents an important character in this context (e.g., Kröger 2005). ...
... In combination with their morphology, this might indicate that Kokenia and potentially even Cyrtobactrites and Pseudobactrites represent independent lineages of coiled bactritoids, only resembling the transitional morphology (Erben 1966;Klug 2001b;De Baets et al. 2013b). This would indicate iterative coiling trends in bactritoids around the origin of ammonoids (see also Kröger 2005). Therefore, only the earliest coiled Anetoceratinae and closely related more derived ammonoids (excluding bactritoids and Kokenia) would be included in the Ammonoidea until better preserved material becomes known and the bactritoid/ammonoid transition can be further refined. ...
In order to put the origin of the Ammonoidea into the broader evolutionary context, we review the hypothesis on the origin of cephalopods in general, the origin of bactritids as well as the origin of bactritids with their respective Bauplan characters. We also list major morphological changes that occurred between the origin of cephalopods until the early evolution of ammonoids.
... The phylogeny of most of the major cephalopod clades has been reconstructed with some confidence using morphological, developmental and molecular data in the last decades and some general macroevolutionary patterns are beginning to crystalize (e.g., Dzik 1981Dzik , 1984 Woodruff et al. 1987; Engeser 1996; Young et al. 1998; Peterson et al. 2004; Kröger 2005; Bergmann et al. 2006; Strugnell et al. 2006; Strugnell and Nishiguchi 2007; Bizikov 2008; Shigeno et al. 2008 Shigeno et al. , 2010 Warnke et al. 2011). Undoubtedly, the sister group of cephalopods lies within the Mollusca, although the sister group of cephalopods is under debate. ...
... Less distinct characters of early ammonoids are the more strongly sinuous sutures with external and lateral lobes, the laterally compressed whorls and the distinct hyponomic sinus. However, all of the latter characters are also known to some degree from a few bactritids such as Lobobactrites, Cyrtobactrites (Erben 1964a, b, 1966 Dzik 1984; Klug 2001b; Kröger 2005 Kröger , 2008b Klug et al. 2008a). Hardly anything is known with respect to the jaws or soft parts of early ammonoids (Korn and Klug 2003). ...
... 1.5, 1.6.). This holds true for the initial chamber and the shaft included in the ammonitella (Erben 1960Erben , 1964aErben , 1966 Bogoslovsky 1969; Klug and Korn 2004; Kröger 2005; Klug et al. 2008; De Baets et al. 2012, 2013a), for the juvenile shell and the neanoconch until the adult shell (Klug and Korn 2004; Kröger 2005; De Baets et al. 2012, 2013a ). This evolutionary trend in the increase in coiling is only rarely reversed; extreme examples for evolutionary trends towards looser coiling are the Mesozoic groups of heteromorph ammonites (Cecca 1997; Guex 2006; Monnet et al. 2015). ...
This two-volume work is a testament to the abiding interest and human fascination with ammonites. We offer a new model to explain the morphogenesis of septa and the shell, we explore their habitats by the content of stable isotopes in their shells, we discuss the origin and later evolution of this important clade, and we deliver hypotheses on its demise. The Ammonoidea produced a great number of species that can be used in biostratigraphy and possibly, this is the macrofossil group, which has been used the most for that purpose. Nevertheless, many aspects of their anatomy, mode of life, development or paleobiogeographic distribution are still poorly known.
Themes treated are biostratigraphy, paleoecology, paleoenvironment, paleobiogeography, evolution, phylogeny, and ontogeny. Advances such as an explosion of new information about ammonites, new technologies such as isotopic analysis, tomography and virtual paleontology in general, as well as continuous discovery of new fossil finds have given us the opportunity to present a comprehensive and timely "state of the art" compilation. Moreover, it also points the way for future studies to further enhance our understanding of this endlessly fascinating group of organisms.
Table of contents:
PART I – Macroevolution
1 Ancestry, origin and early evolution of ammonoids
Christian Klug, Björn Kröger, Jakob Vinther, Dirk Fuchs and Kenneth De Baets
2 Evolutionary trends of Triassic ammonoids
Claude Monnet, Arnaud Brayard and Morgane Brosse
3 Evolutionary trends within Jurassic ammonoids
Pascal Neige and Isabelle Rouget
4 Buckman’s rules of covariation
Claude Monnet, Kenneth De Baets and Margaret M. Yacobucci
5 Evolutionary patterns of ammonoids: phenotypic trends, convergence, and parallel evolution
Claude Monnet, Christian Klug and Kenneth De Baets
PART II – Paleobiogeography of ammonoids
6 Biogeography of Paleozoic ammonoids
Dieter Korn and Kenneth De Baets
7 Biogeography of Triassic ammonoids
Arnaud Brayard, Gilles Escarguel, Claude Monnet, James F. Jenks and Hugo Bucher
8 Macroevolution and paleobiogeography of Jurassic-Cretaceous ammonoids
Margaret M. Yacobucci
9 Paleobiogeography of Early Cretaceous ammonites
Jens Lehmann, Christina Ifrim, Luc Bulot and Camille Frau
10 Paleobiogeography of Late Cretaceous Ammonoidea
Christina Ifrim, Jens Lehmann and Peter D. Ward
PART III – Ammonoids through time
11 Ammonoids and quantitative biochronology – a unitary association perspective
Claude Monnet, Arnaud Brayard and Hugo Bucher
12 Paleozoic ammonoid biostratigraphy
Dieter Korn and Christian Klug
13 Biostratigraphy of Triassic ammonoids
James F. Jenks, Claude Monnet, Marco Balini, Arnaud Brayard and Maximiliano Meier
14 Ammonoid biostratigraphy in the Jurassic
Günter Schweigert
15 Ammonite biostratigraphy of the Cretaceous
Jens Lehmann
16 Taxonomic diversity and morphological disparity of Paleozoic ammonoids
Dieter Korn, Christian Klug and Sonny A. Walton
17 Permian-Triassic extinctions and rediversifications
Arnaud Brayard and Hugo Bucher
18 Ammonoids at the Triassic-Jurassic transition: pulling back from the edge of extinction
Louise M. Longridge and Paul L. Smith
19 Ammonites on the brink of extinction: diversity, abundance, and ecology of the Order Ammonoidea at the
Cretaceous/Paleogene (K/Pg) boundary
Neil H. Landman, Stijn Goolaerts, John W.M. Jagt, Elena A. Jagt-Yazykova and Marcin Machalski
20 Ammonoid Taphonomy
Ryoji Wani and Neal S. Gupta
... Although a full treatment of this topic would fill at least an article of its own, it is necessary to explain some general patterns for a better understanding of our systematic assignments. As mentioned before, the middle Palaeozoic is marked by increasing coiling in several mollusc groups (Alberti 1993, House 1996, Nützel & Frýda 2003, Klug & Korn 2004, Kröger 2005, Korenet al. 2007, Klug et al. 2008b, De Baets et al. 2012a. As shown in Te xt- fig.8, this increase in several cases concerns the embryonic shell (some gastropods and ammonoids), the entire shell (some gastropods and ammonoids), and the postembryonic shell (some dacryoconarids, bactritoids, some ammonoids). ...
... In the lineage from Orthocerida via Bactritoidea and plesiomorphic Ammonoidea to derived Ammonoidea, a general increase in coiling can be seen (Erben 1964a, b, 1965, Klug & Korn 2004, Kröger 2005, Kröger & Mapes 2007. This trend begins with a gradual ventral displacement of the siphuncle in orthocerids. ...
... Ultimately, the question for the driving factor remains. It has been suggested that it was the increasing predatory pressure by the rapidly diversifying jawed fish (e.g., Nützel & Frýda 2003, Kröger 2005 or this trend was "driven by competition in the diversity-saturated benthic habitats together with the availability of abundant planktonic food" (Klug et al. 2010, p. 1). In any case, all these early evolutionary morphological transformations of the ammonoids can be seen in the light that most likely, these changes enhanced swimming and reproductive capabilities of these cephalopods (Klug & Korn 2004, Klug et al. 2008b, De Baets et al. 2012a. ...
The ammonoids from the well-studied German Fossillagerstätte of the Hunsrück Slate have the reputation of being the oldest known representatives of this cephalopod group. This material is of great interest not only because of the global scarcity of the earliest ammonoids, but also because it includes the first record of stratigraphically-controlled specimens, which could be assigned to the middle Kaub Formation in the Bundenbach/Gemünden area. Accordingly, some of the Hunsrück Slate ammonoids are indeed the stratigraphically oldest ammonoids because they are associated with the index dacryoconarid Nowakia praecursor and thus derive from the Nowakia zlichovensis Biozone of the early Emsian (Zlíchovian), while younger dacryoconarids and other ammonoid samples (Mimagoniatites fecundus, Mimosphinctes tripartitus) indicate the Nowakia barrandei to N. elegans biozones. In spite of this special importance, these Early Devonian cephalopods have never been revised comprehensively.Our study includesmore than 300 Hunsrück Slate specimens from both public and private collections. For the first time, ammonoids from the Altlay Hunsrück Slate in the Northern Hunsrück/Mosel region are reported, while all the materials from older collections derive from the middle Kaub Formation of the Central Hunsrück Basin (central Hunsrück, Taunus). These early ammonoids thus prove to be a valuable source of information for biostratigraphic correlation within the Hunsrück Slate and with early Emsian occurrences in other regions. Based on conch characters (geometry, ornament, suture lines) and ontogenetic traits, we describe the species Metabactrites fuchsi n. sp., Ivoites hunsrueckianus (Erben 1960), Ivoites schindewolfi n. sp., ?Ivoites sp., ?Ivoites opitzi n. sp., Anetoceras mittmeyeri n. sp., Erbenoceras advolvens (Erben 1960), Erbenoceras solitarium (Barrande 1865), Chebbites sp., Mimosphinctes primigenitus (Erben 1965), Mimosphinctes tripartitus Eichenberg, 1931, Gyroceratites heinricherbeni n. sp., ?Teicherticeras sp., and Mimagoniatites fecundus (Barrande 1865). Supposedly endemic species of the Hunsrück Slate such as “Anetoceras recticostatum Erben, 1962” and “Mimagoniatites falcistria (Fuchs 1915)” are here synonymized with the widely distributed species Erbenoceras solitarium and Mimagoniatites fecundus, both known to occur also outside Europe. Furthermore, we studied their taphonomy and assigned them to sevengroups of preservation.Theresults of this taphonomic study corroborate previous interpretations of the depositional environment and diagenesis. We also discuss the evolution of the shell in the earliest ammonoids and their closest relatives as well as structures (“Opitzian pits”) possibly caused by parasitic infestations of these early Emsian ammonoids.
... As far as thelodont genera in the Devonian are concerned, they Table 2. Diversity of mid-Palaeozoic Discosorida, Nautilida, Oncocerida and Tarphycerida. The data compilation was performed by B.K. (Kröger 2003(Kröger , 2005(Kröger , 2008, largely based on Sepkoski's raw data (2002). (Märss et al. 2007). ...
... Many members of these groups occur frequently in black shales and thus certainly lived in the water column. Most orthocerids were probably capable of minor horizontal movements but they were ineffective swimmers and migrated predominantly vertically and ⁄ or drifted passively (Hewitt & Watkins 1980;Westermann 1999;Mutvei 2002;Kröger 2003Kröger , 2005Kröger & Mutvei 2005;Mutvei et al. 2007). This is suggested by their poorly differentiated muscle-attachment structures, the absence of significant endosiphonal or endocameral deposits and, in some cases, also shell morphology. ...
... Diversity of Devonian bony fish. The data compilation was performed by B.K. (Kröger 2005) based on Sepkoski's raw data (2002). In Fig. 2 of the main text, two curves of Carr (1995) Additional groups Diversity data of the radiodonts, the eurypterids and graptoloids were extracted from Sepkoski's compendium (2002) and the Paleobiology Database. ...
Klug, C., Kröger, B., Kiessling, W., Mullins, G.L., Servais, T., Frýda, J., Korn, D. & Turner, S. 2009: The Devonian nekton revolution. Lethaia, 10.1111/j.1502-3931.2009.00206.x
Traditional analyses of Early Phanerozoic marine diversity at the genus level show an explosive radiation of marine life until the Late Ordovician, followed by a phase of erratic decline continuing until the end of the Palaeozoic, whereas a more recent analysis extends the duration of this early radiation into the Devonian. This catch-all approach hides an evolutionary and ecological key event long after the Ordovician radiation: the rapid occupation of the free water column by animals during the Devonian. Here, we explore the timing of the occupation of the water column in the Palaeozoic and test the hypothesis that ecological escalation led to fundamental evolutionary changes in the mid-Palaeozoic marine water column. According to our analyses, demersal and nektonic modes of life were probably initially driven by competition in the diversity-saturated benthic habitats together with the availability of abundant planktonic food. Escalatory feedback then promoted the rapid rise of nekton in the Devonian as suggested by the sequence and tempo of water-column occupation. □Devonian, diversity, ecology, food webs, nekton, plankton, radiation.
... From an evolutionary point of view, the siphuncle is the fundamentally new and outstanding skeletal element of cephalopod mollusks (e.g. Ward, 1982;Bandel, 1988;Crick, 1988;Kröger, 2003Kröger, , 2005Barskov et al., 2008;Mutvei, 2016). The siphuncle gave rise to the class of cephalopods (Kröger, 2003(Kröger, , 2005, as it made possible for shelled cephalopods to develop new life-styles, e.g. to shift from a benthic to a nektic life. ...
... Ward, 1982;Bandel, 1988;Crick, 1988;Kröger, 2003Kröger, , 2005Barskov et al., 2008;Mutvei, 2016). The siphuncle gave rise to the class of cephalopods (Kröger, 2003(Kröger, , 2005, as it made possible for shelled cephalopods to develop new life-styles, e.g. to shift from a benthic to a nektic life. For Spirula spirula the siphuncle is a tube, in contrast to the siphuncle of Sepiida, where it is a zone. ...
Structural biological hard tissues fulfill diverse tasks: protection, defence, locomotion, structural support, reinforcement, buoyancy. The cephalopod mollusk Spirula spirula has a planspiral, endogastrically coiled, chambered, endoskeleton consisting of the main elements: shell-wall, septum, adapical-ridge, siphuncular-tube. The cephalopod mollusk Sepia officinalis has an oval, flattened, layered-cellular endoskeleton, formed of the main elements: dorsal-shield, wall/pillar, septum, siphuncular-zone. Both endoskeletons are light-weight buoyancy devices that enable transit through marine environments: vertical (S. spirula), horizontal (S. officinalis). Each skeletal element of the phragmocones has a specific morphology, component structure and organization. The conjunction of the different structural and compositional characteristics renders the evolved nature of the endoskeletons and facilitates for Spirula frequent migration from deep to shallow water and for Sepia coverage over large horizontal distances, without damage of the buoyancy device. Based on Electron-Backscatter-Diffraction (EBSD) measurements and TEM, FE-SEM, laser-confocal-microscopy imaging we highlight for each skeletal element of the endoskeleton its specific mineral/biopolymer hybrid nature and constituent arrangement. We demonstrate that a variety of crystal morphologies and biopolymer assemblies are needed for enabling the endoskeleton to act as a buoyancy device. We show that all organic components of the endoskeletons have the structure of cholesteric-liquid-crystals and indicate which feature of the skeletal element yields the necessary mechanical property to enable the endoskeleton to fulfill its function. We juxtapose structural, microstructural, texture characteristics and benefits of coiled and planar endoskeletons and discuss how morphometry tunes structural biomaterial function. Both mollusks use their endoskeleton for buoyancy regulation, live and move, however, in distinct marine environments.
... Subsequently, in the early Ordovician, cephalopod shell shape evolved substantially, and long straight-shelled orthocerids and coiled-shelled nautiloids appeared (Kröger et al., 2011). Later, during the Devonian, ammonites independently evolved coiled shells (Kröger, 2005). It is unclear whether coiling enabled the shell to withstand higher pressures, such as those applied by predators, or those exerted hydrostatically under greater depths, or if it provided greater buoyancy control and mobility (Lewy, 2002;Kröger, 2005;Peŕez-Claros et al., 2007). ...
... Later, during the Devonian, ammonites independently evolved coiled shells (Kröger, 2005). It is unclear whether coiling enabled the shell to withstand higher pressures, such as those applied by predators, or those exerted hydrostatically under greater depths, or if it provided greater buoyancy control and mobility (Lewy, 2002;Kröger, 2005;Peŕez-Claros et al., 2007). In contrast, belemnites possessed elongated linear shells that had an inorganic lining which increased their strength, enabling them to exploit deep ocean habitats (Doguzhaeva et al., 2014), which probably contained lower predator densities. ...
Predation is a major evolutionary driver of animal adaptation. However, understanding of anti-predator evolution is biased toward vertebrate taxa. Cephalopoda, a class in the invertebrate phylum Mollusca, are known for their diverse anti-predator strategies, characterised by their behavioural flexibility. While ancestral cephalopods were protected by a hard outer shell, extant cephalopods have greatly reduced their reliance on physical defences. Instead, cephalopods have evolved highly developed senses to identify potential threats, cryptic skin patterns to avoid detection, startle responses to deter attack, and elaborate means of escape. While cephalopod anti-predator repertoires are relatively well described, their evolution, and the selective pressures that shaped them, have received much less attention. This is despite their potential relevance, in turn, to elucidate evolution of the remarkable cognitive abilities of cephalopods. Here, we review cephalopod anti-predator evolution, considering four key aspects: (i) shell reduction and loss; (ii) the skin patterning system; (iii) the ecological context accompanying the evolution of advanced cognit.ive abilities; (iv) why the evolutionary trajectory taken by cephalopods is so unique among invertebrates. In doing so, we consider the unique physiology of cephalopods and discuss how this may have constrained or aided the development of their anti-predator repertoire. In particular, cephalopods are poorly equipped to defend themselves physically and escape predation by fish, due to a lack of comparable weaponry or musculature. We argue that this may have selected for alternative forms of defence, driving an evolutionary trajectory favouring crypsis and complex behaviours, and the promotion of sensory and cognitive adaptations. Unravelling the complexities of cephalopod anti-predator evolution remains challenging. However, recent technological developments available for cephalopod field and laboratory studies, coupled with new genomic data and analysis approaches, offer great scope to generate novel insights.
... These cephalopods have a wide distribution in all oceansfrom the poles to the tropicson continental margins and in oceanic areas (Hanlon and Messenger, 1996;Jereb and Roper, 2005). Coleoids represent the majority of the diversity of modern cephalopods and constituted an important part of the Paleozoic nekton (Kröger 2005). ...
... According to the fossil record, cephalopods originated in the Early Cambrian period (~530 Mya) from a monoplacophoran-like mollusk (Ward and Bandel, 1987;Mutvei et al., 2007;Kröger et al., 2011) that evolved quickly, possibly due to its ability to regulate buoyancy, and that diversified into several lineages during the Ordovician (Kröger, 2005). Paleontological records dating from the Early Carboniferous (~330 Mya) suggest an origin of the subclass Coleoidea from Devonian Bactritoidea (Nishiguchi and Mapes, 2008;Kröger et al., 2011;Klug et al., 2019). ...
Coleoids are the most diverse group of cephalopod mollusks. While their origin is date during the Mesozoic, the diversification pattern is unknown. However, two hypotheses have been proposed. The first suggests an increasing diversification rate after the Cretaceous-Paleogene extinction event (K-Pg) as consequence of empty habitats left by the ammonites and belemnites. The second hypothesis proposes a mid-Cenozoic increase in diversification rate related to distributional changes during ice ages and biotic interactions. To test these hypotheses, we estimated a lineage through time (LTT) and the gamma-statistic along with model-based diversification rates. These analyses were conducted on a dated molecular phylogeny for coleoids that we reconstructed using five molecular markers (cytochrome b, 16S rRNA, cytochrome oxidase I, rhodopsin, and PAX-6). Our divergence time estimation suggests that coleoids originated in the Mesozoic Era (Middle Triassic) and that both main clades (Decapodiformes and Octopodiformes) diverged in the Cretaceous/Jurassic Period. The LTT, gamma statistic, and diversification rates inferred with the Bayesian Analysis of Macro-evolutionary Mixtures (BAMM), indicate an acceleration in diversification rate over time since the origin of coleoids. Additionally, BAMM allowed us to detect abrupt increases in diversification rate before and after the K-Pg boundary. Our results partially support both hypotheses as all analyses indicate that the coleoid diversification rate was increasing during the Cenozoic. However, our results also indicate increasing diversification rates before the K-Pg boundary. We propose that the radiation of coleoids has been shaped by an acceleration in diversification rate over time, including exceptional episodes of abrupt increases before and after the K-Pg boundary.
... Alternatively , the female may have carried the eggs, as in Recent pelagic octopods (e.g., Argonauta). Recent Nautilus produces less than 10 eggs of a diameter of c. 20 mm (Tanabe et al. 1993b; Kröger 2005), laying the egg capsule directly on the sea floor ( Landman 1987, 2010). Differing reproductive strategies in nautiloids and ammonoids in early ontogenies are reported by Stephen and Stanton (2002), Kröger (2005) and Klug (2007). ...
... Tight coiling implies an increased resistance against breakage of entire whorls when compared to loosely coiled shells (Nützel and Frýda 2003 and references therein). Consequently, a simple change in morphology had a major impact on ecological fitness, partially explaining the evolutionary success of ammonoids (Kröger 2005). Coiling probably enabled even early forms to increase their maximum swimming velocity compared to cephalopods with orthoconic shells (e.g., Jacobs 1992a; Jacobs and Chamberlain 1996; Westermann 1996 ...
The current knowledge about the ammonoid/habitat relation is reviewed and in part newly interpreted. The autecology of ammonoids, such as ontogeny and habitat, based on morphological and geochemical analyses in association with results from modern relatives, forms the foundation for subsequent interpretations. Synecological interactions (predator-prey, infestation) are discussed with reference to sedimentary facies and the corresponding biofacies. Arguments for a possible mode of life and habitat are given based on the modern data on the food/prey habits and predation habitats of ammonoids. The state of the art in scientific investigations on ammonoid life and habitat is summarized, reviewed and in part reinterpreted. Traditional assumptions based on facies analyses are strengthened or contradicted by more recent methods such as morphospace and stable isotope analyses. Not yet tested hypotheses, speculations and mathematical models are tested by comparing the results with new geophysical data.
... Several groups of externally shelled molluscs (ammonoids, bactritoids, dacryoconarids, gastropods, nautiloids) show trends towards increased shell coiling during the Devonian (Alberti 1993;Koren et al. 2007;House 1996;Nützel and Frýda 2003;Kröger 2005;Klug et al. 2010a), which are related to the "Devonian nekton revolution". These changes have usually been interpreted to reflect increased predatory pressure (Nützel and Frýda 2003;Kröger 2005). ...
... Several groups of externally shelled molluscs (ammonoids, bactritoids, dacryoconarids, gastropods, nautiloids) show trends towards increased shell coiling during the Devonian (Alberti 1993;Koren et al. 2007;House 1996;Nützel and Frýda 2003;Kröger 2005;Klug et al. 2010a), which are related to the "Devonian nekton revolution". These changes have usually been interpreted to reflect increased predatory pressure (Nützel and Frýda 2003;Kröger 2005). Constructional and developmental constraints might create similar trends (Morita 2003;Wagner & Erwin 2006;Futuyma 2009) and other factors such as competition, changes in reproductive strategy, and sorting trends related with extinction events might also have played a role (House 1996;Frýda et al. 2008;Klug et al. 2010a). ...
The Devonian (418.1 – 360.7 Ma) was a time of a major macroecological turnover in marine ecosystems. It coincides with the rapid rise of the nekton (cephalopods, gnathostomes, i.e. active swimmers). The ammonoids, an extinct group of nektonic, externally shelled cephalopods, originated at this time from the bactritoids and played a key role in this event. Ammonoid shells, just like many other mollusk groups (gastropods, other cephalopods), show a progressive coiling of their conch in the Devonian. Their high fossilization potential, wide dispersal, record of growth in their accretionary shell and high evolutionary rates have made them widely used tools for biostratigraphy, variability and evolutionary studies, especially Mesozoic forms.
The earliest ammonoids have been little investigated in this respect due to the scarcity of complete or well-preserved material, low stratigraphic resolution, and difficulties in stratigraphic correlation. New Early Devonian ammonoid material from Belgium, Germany, Morocco and Uzbekistan gave the possibility to improve the knowledge on their stratigraphy, quantify intraspecific variability for the first time and test several macroevolutionary hypotheses.
To reconstruct the early evolution of ammonoids and constrain important events in the Devonian nekton revolution, a good biostratigraphic and phylogenetic framework is a necessary prerequisite. The existing biozonation schemes for Germany, Morocco, and Uzbekistan could be improved by combining data on ammonoids, brachiopods, corals, and dacryoconarids. The age of several sedimentary successions in classical ammonoid localities could be better constrained. One of these is the Hunsrück, which was long thought to have yielded the oldest ammonoids and probably the most famous Devonian Fossillagerstätte. Combined stratigraphic ranges of ammonoids and dacryoconarids indicate an early Emsian age for the Hunsrück Slate and radiometrically dated tuffs within it. Correlation between the more neritic and more pelagic facies in Germany and Morocco could be improved. Moroccan sections up to 350 km apart could be correlated based on faunal associations.
Variability in early Emsian ammonoids was quantified for the first time and revealed large intraspecific variation in coiling and rib spacing. Several previously defined taxa fall in the range of variability of a single taxon, which has important implications for diversity studies and comparison of taxa between different regions. This and other systematic studies reveal the importance of using the entire ontogeny to distinguish taxa, especially in case of large intraspecific variability.
Several macroevolutionary patterns and events in the early evolution of ammonoids were discovered and described. Proof, that certain deformities (pits in internal moulds, which formed as casts of blister pearls), in shell of Devonian ammonoids is caused by parasites is presented. The changes in shape, size, arrangement, and distribution of these pits over time in some lineages taxa represents some of the oldest evidence of co-evolution between host and parasite of at least 20 Ma. Another study portrays a case of parallel evolution in several characters of the shell between two ammonoid lineages with the same ancestor. It revealed that not only adaptation, but also covariation plays a role in the parallel evolution between these two lineages. A final study quantitatively corroborates the progressive coiling of the embryonal shell and umbilical window through time and phylogeny in the Devonian. The umbilical window disappeared convergently in at least 3 lineages at more or less the same time, which is usually thought to indicate that selective factors such as predatory pressure were involved. The increased coiling probably also reflects changes in reproductive strategy.
Further improvements to the stratigraphic resolution and correlation can be achieved by combining data of multiple groups and quantitative biochronology on suitable data sets. The phylogeny of early ammonoids could be refined by quantitatively integrating ontogeny and intraspecific variability on a larger scale. This would be of interest to test concurrent changes in evolutionary pathways of early ammonoids. Several of the studies presented herein support the hypothesis that many more evolutionary trends might be present at this phase of major changes of marine food webs.
... Many of the giant orthoconic cephalopods have rather large embryonic shells, which measured several centimetres in endocerids (Flower 1940, 1955; Teichert 1964a,b; Kr€ oger 2013) and actinocerids (Flower 1940; Teichert 1964b). This is in contrast to many orthocerids, where the embryonic shells often measured some millimetres only (Kr€ oger 2005; Klug et al. 2010). However, it cannot be generalized that the dimensions of the embryonic shells in cephalopods are positively correlated with adult shell size (De Baets et al. 2012). ...
... 475 Ma; Teichert & Kummel 1960), the largest trilobites (468–460 Ma; Rudkin et al. 2003; Guti errez-Marco et al. 2009) and the largest known anomalocaridids (488–472 Ma; Van Roy & Briggs 2011) are all confined to a 28 Ma interval of Ordovician time. This could be termed a coincidence taking the long time interval into account, but it appears likely to reflect shared biological and sampling controls that all these groups then found beneficial ecological conditions, which are reflected in the Ordovician diversity peaks in all three taxa (Kr€ oger 2005; Servais et al. 2008; Klug et al. 2010). A link between maximum body size and diversity has been shown by Trammer (2005). ...
During and after the Cambrian explosion, very large marine invertebrate species have evolved in several groups. Gigantism in Carboniferous land invertebrates has been explained by a peak in atmospheric oxygen concentrations, but Palaeozoic marine invertebrate gigantism has not been studied empirically and explained comprehen-sively. By quantifying the spatiotemporal distribution of the largest representatives of some of the major marine invertebrate clades (orthoconic cephalopods, ammonoids, trilobites, marine eurypterids), we assessed possible links between environmental parameters (atmospheric or oceanic oxygen concentrations, ocean water temperature or sea level) and maximum body size, but we could not find a straightforward rela-tionship between both. Nevertheless, marine invertebrate gigantism within these groups was temporally concentrated within intervals of high taxonomic diversity (Ordovician, Devonian) and spatially correlated with latitudes of high occurrence fre-quency. Regardless of whether temporal and spatial variation in sampled diversity and occurrence frequency reflect true biological patterns or sampling controls, we find no evidence that the occurrences of giants in these groups were controlled by optimal conditions other than those that controlled the group as a whole; if these conditions shift latitudinally, occurrences of giants will shift as well. It is tempting to attribute these shifts to contemporary changes in temperature, oxygen concentrations in the atmosphere and the oceans as well as global palaeogeography over time, but further collection-based studies are necessary on finer stratigraphic and phylogenetic resolution to corroborate such hypotheses and rule out sampling or collection biases
... In the pelagic-carnivore megaguild (Table 5) the nautiloid orthocerid, lithulitid, and tarphyncerid cephalopods were dominant carnivores in the Late Ordovician (Servais et al., 2010), whereas in the Late Devonian the ammonoid cephalopods and placoderm fishes were the dominant carnivores (McGhee, 1996). The Hirnantian biodiversity crisis reduced the diversity of nautiloid cephalopods to levels not seen since their origination in the Early Ordovician (Crick, 1990) and eliminated the coiled tarphyncerids (Kröger et al., 2009), but by the middle Silurian the nautiloids had rediversified and regained their ecologically prominent position in the pelagic-carnivore megaguild (Crick, 1990; Kröger, 2005; Kröger et al., 2009). In comparison, the Famennian biodiversity crisis also sharply reduced the diversity of cephalopod predators, now ammonoids rather than nautiloids (only two genera survived the crisis; House, 2002 ), but the ammonoids rediversified in the early Carboniferous , rapidly evolving no less than nine new families (House, 2002). ...
... In the pelagic-carnivore megaguild, the conodonts lost 85% of their species assemblages in the early Ludlordian Event and only a few subspecies variants of Ozarkodina eosteinhornensis survived until the end of the Silurian (Kaljo et al., 1996). A major extinction of nautiloids occurred in the middle Ludfordian Event, of equal of greater magnitude to that that occurred in the Hirnantian (Kröger, 2005). In the attached-epifaunal-filter-feeding megaguild (Table 7 ), the rugose , tabulate, and heliolitid corals lost around 45 genera in the middle Ludfordian Event (Kaljo et al., 1996). ...
A new ecological-severity ranking of the major Phanerozoic biodiversity crises is proposed in which the Capitanian crisis is ranked lesser than the Frasnian (Late Devonian) but greater than the Serpukhovian (end-Mississippian), and the Famennian (end-Devonian) crisis is ranked as equal in ecological impact to the Hirnantian (end-Ordovician). Two new decouplings between taxonomic severity and ecological severity are revealed in these analyses, the Capitanian and Famennian crises, in which the ecological impact of the biodiversity loss was markedly different from the magnitude of the biodiversity loss. These analyses also reveal that the "Great Devonian Interchange" (GDI) invasive-species event in the Givetian biodiversity crisis may pro-vide an important palaeoecological analog for the study of present-day extinction and homogenization in ecosys-tems produced by modern invasive species. © 2012 Published by Elsevier B.V.
... Both mechanisms have also been suggested to explain trends toward smaller, more tightly coiled gastropod juvenile shells (Nützel). This fits quite well with the presence and diversification of various predatory groups in the Devonian, which were capable of preying on ammonoid hatchlings , which include phyllocarids, conodonts, gnathostomes, ammonoids , and other cephalopods (Brett and Walker 2002; Kröger 2005; Berkyová et al. 2007; Klug et al. 2008). This would be in line with the evidence of predation on Recent pelagic cephalopod and gastropod juveniles by gnathostomes, cephalopods, arthropods, and various other pelagic invertebrates and their young (Nixon 1987; Vecchione 1987; Hickman 2001; Ibáñez and Keyl 2010). ...
... ings already had a hyponome and were capable of active locomotion by jetting water from their hyponome as indicated by a bend in the growth lines (hyponomic sinus) in Devonian ammonoids directly after hatching (Erben 1964; House 1965). The smaller, more coiled hatchlings without an umbilical window (∼1 mm) might have a less unstable locomotion (cf. Kröger 2005) and their more spherical shape might have reduced the surface area and friction at their small size (Jacobs and Chamberlain 1996; Klug 2007). However, shape might play a subordinate role in embryonic shells of ammonoids as smaller forms have less power relative to drag than in larger forms, which would have made the effect on the absol ...
During the Devonian Nekton Revolution, ammonoids show a progressive coiling of their shell just like many other pelagic mollusk groups. These now extinct, externally shelled cephalopods derived from bactritoid cephalopods with a straight shell in the Early Devonian. During the Devonian, evolutionary trends toward tighter coiling and a size reduction occurred in ammonoid embryonic shells. In at least three lineages, descendants with a closed umbilicus evolved convergently from forms with an opening in the first whorl (umbilical window). Other lineages having representatives with open umbilici became extinct around important Devonian events whereas only those with more tightly coiled embryonic shells survived. This change was accompanied by an evolutionary trend in shape of the initial chamber, but no clear trend in its size. The fact that several ammonoid lineages independently reduced and closed the umbilical window more or less synchronously indicates that common driving factors were involved. A trend in size decrease of the embryos as well as the concurrent increase in adult size in some lineages likely reflects a fundamental change in reproductive strategies toward a higher fecundity early in the evolutionary history of ammonoids. This might have played an important role in their subsequent success as well as in their demise.
... The use of CMR thinking and modeling was first introduced in the paleontological literature in the 1980s by statistical ecologists who realized that the methods that they were developing in ecology could benefit paleontologists (Nichols and Pollock, 1983;Conroy and Nichols, 1984;Nichols et al., 1986). However, the CMR approach was only used very sporadically in the paleontological literature following those papers (Connolly and Miller, 2001a, 2001bChen et al., 2005;Kröger, 2005;Liow et al., 2008). We think there are several reasons for this. ...
We rely on observations of occurrences of fossils to infer the rates and timings of origination and extinction of taxa. These estimates can then be used to shed light on questions such as whether extinction and origination rates have been higher or lower at different times in earth history or in different geographical regions, etc. and to investigate the possible underlying causes of varying rates. An inherent problem in inference using occurrence data is one of incompleteness of sampling. Even if a taxon is present at a given time and place, we are guaranteed to detect or sample it less than 100% of the time we search in a random outcrop or sediment sample that should contain it, either because it was not preserved, it was preserved but then eroded, or because we simply did not find it. Capture-mark-recapture (CMR) methods rely on replicate sampling to allow for the simultaneous estimation of sampling probability and the parameters of interest (e.g. extinction, origination, occupancy, diversity). Here, we introduce the philosophy of CMR approaches especially as applicable to paleontological data and questions. The use of CMR is in its infancy in paleobiological applications, but the handful of studies that have used it demonstrate its utility and generality. We discuss why the use of CMR has not matched its development in other fields, such as in population ecology, as well as the importance of modelling the sampling process and estimating sampling probabilities. In addition, we suggest some potential avenues for the development of CMR applications in paleobiology.
... That is, these cephalopods would have reduced energy expenditure and self-generated wake during rotation while having 360° access to closely surrounding prey ). In addition to these physical properties, other factors are likely involved in selection (other functional/morphogenetic factors, Kröger 2005;Tendler et al. 2015;Parent et al. 2020;Peterman et al. 2021;Hebdon, Polly, et al. 2022;Weber et al. 2022;competition, Ritterbush 2016). While the endmembers of the planispiral morphospace (Ritterbush and Bottjer 2012) represent extreme cases, the majority of cephalopods occupied spaces in between. ...
Synopsis
Stability–maneuverability tradeoffs impose various constraints on aquatic locomotion. The fossil record houses a massive morphological dataset that documents how organisms have encountered these tradeoffs in an evolutionary framework. Externally shelled cephalopods (e.g., ammonoids and nautiloids) are excellent targets to study physical tradeoffs because they experimented with numerous conch morphologies during their long-lived evolutionary history (around 0.5 billion years). The tradeoff between hydrostatic stability and maneuverability was investigated with neutrally buoyant biomimetic models, engineered to have the same mass distributions computed for their once-living counterparts. Monitoring rocking behavior with 3D motion tracking reveals how stability influenced the life habits of these animals. Cephalopods with short body chambers and rapid whorl expansion (oxycones) more quickly attenuate rocking, while cephalopods with long body chambers (serpenticones and sphaerocones) had improved pitch maneuverability. Disparate conch morphologies presented broad functional opportunities to these animals, imposing several advantages and consequences across the morphospace. These animals navigated inescapable physical constraints enforced by conch geometry, illuminating key relationships between functional diversity and morphological disparity in aquatic ecosystems. Our modeling techniques correct for differences in material properties between physical models and those inferred for their living counterparts. This approach provides engineering solutions to the obstacles created by buoyancy, mass distributions, and moments of inertia, permitting more lifelike, free-swimming biomechanical models and aquatic robots.
... Cephalopods are a prime example of predator-driven evolution. The coiling of their shells was considered an adaptation by Paleozoic nautiloids and ammonoids to avoid predation by other large cephalopods and early gnathostome fishes (Kröger, 2005;Mapes & Chaffin, 2003, and references therein), and strong sculpture with a deep living chamber in Mesozoic ammonites were seen as adaptations against shell-breaking predators (Keupp, 2006;Kröger, 2002). ...
Aim
Nautilus and Allonautilus , last members of the once widespread nautiloid cephalopods, are today restricted to the deep central Indo‐West Pacific Ocean, for reasons that remain unclear. Cephalopod evolution is generally considered as being driven by vertebrate predation; therefore, we investigated the role of whales and seals in the decline of nautiloids through the Cenozoic.
Location
Global.
Taxon
Nautiloids, pinnipeds, cetaceans.
Methods
Distribution data for nautiloids, pinnipeds and cetaceans through the Cenozoic were compiled and plotted on a series of paleogeographic maps. Nautiloid shell sizes were compiled and plotted against the first appearance of pinnipeds and cetaceans in key regions.
Results
From the Oligocene onward, nautiloids became extinct in areas where pinnipeds appeared. The exception is the agile nautiloid Aturia , extinct globally at the end of the Miocene. A major role of odontocetes in the demise of nautiloids is not apparent, except for a few brevirostrine Oligocene taxa from the North American Atlantic and Pacific coasts, which appeared in these areas at the same time as nautilids disappeared. The Oligocene disappearance of nautiloids (except Aturia ) from the American Pacific coasts coincides with the development of oxygen minimum zones (OMZs) in this region.
Main conclusions
We hypothesize that the Cenozoic spread of pinnipeds drove nautiloids into their present‐day central Indo‐West Pacific refuge. Additional factors for the local extinction of nautiloids in the Oligocene include predation by short‐snouted whales and the development of OMZs, preventing nautiloids from retreating into deeper water.
... A similarly abrupt transition from straight bactritoids to coiled ammonoids is observed in the Early Devonian Pragian-Zlichovian boundary beds (Kröger & Mapes, 2007). The reason(s) for the seemingly rapid evolution of coiled forms remains enigmatic, although the simultaneous appearance of durophagous predators may have played an important role (Kröger, 2005). Indeed, strong evidence of durophagous predation on cephalopods is reported from the lower Middle Ordovician (Darriwilian) strata of Baltoscandia (Kröger, 2004). ...
The Rochdale Formation of eastern New York (= Fort Ann and lower Bascom formations, designations abandoned) is now recognized to record the earliest stages of the Great Ordovician Radiation of cephalopods. The earliest Bassleroceratidae, Tarphyceratidae and endoceridans on the east Laurentian shallow carbonate platform occur in the upper, thrombolite-bearing member of the Rochdale. This fauna demonstrates that the earliest radiation of Ordovician nautiloids took place in the late Tremadocian and is best recorded in tropical platform facies. Revision of this cephalopod fauna based on approximately 190 specimens collected along a 200 km, N–S belt in easternmost New York has provided new information on inter- and intraspecific variation of earlier described species. The ellesmerocerid Vassaroceras and the endocerids Mcqueenoceras and Paraendoceras are emended. New taxa include Bassleroceras champlainense sp. nov. and B. triangulum sp. nov., Mccluskiceras comstockense gen. et sp. nov., Exoclitendoceras rochdalense gen. et sp. nov. and Paraendoceras depressum sp. nov. A rank abundance plot of 146 specimens from a locality in the Lake Champlain lowlands provides information on the community structure of a nautiloid fauna in which the longiconic cyrtoconic Bassleroceras is shown to dominate strongly. The nautiloid community structure of the Rochdale Formation is similar to that of the underlying Tribes Hill Formation (late early Tremadocian) with respect to the depositional setting, diversity and evenness but displays a remarkably increased taxonomic distinctness.
... CMR was originally developed for demographic studies of extant animal populations [20] but subsequently adapted for palaeontological studies [21][22][23][24]. A few methods have increasingly been used to generate origination and extinction rates for fossil taxa [25], with CMR being one of these [26][27][28][29][30][31][32][33][34]. Somewhat less frequently, CMR is also used to generate diversity curves [9,30]. ...
The history of insects’ taxonomic diversity is poorly understood. The two most common methods for estimating taxonomic diversity in deep time yield conflicting results: the ‘range through’ method suggests a steady, nearly monotonic increase in family-level diversity, whereas ‘shareholder quorum subsampling’ suggests a highly volatile taxonomic history with family-level mass extinctions occurring repeatedly, even at the midpoints of geological periods. The only feature shared by these two diversity curves is a steep increase in standing diversity during the Early Cretaceous. This apparent diversification event occurs primarily during the Aptian, the pre-Cenozoic interval with the most described insect occurrences, raising the possibility that this feature of the diversity curves reflects preservation and sampling biases rather than insect evolution and extinction. Here, the capture–mark–recapture (CMR) approach is used to estimate insects’ family-level diversity. This method accounts for the incompleteness of the insect fossil record as well as uneven sampling among time intervals. The CMR diversity curve shows extinctions at the Permian/Triassic and Cretaceous/Palaeogene boundaries but does not contain any mass extinctions within geological periods. This curve also includes a steep increase in diversity during the Aptian, which appears not to be an artefact of sampling or preservation bias because this increase still appears when time bins are standardized by the number of occurrences they contain rather than by the amount of time that they span. The Early Cretaceous increase in family-level diversity predates the rise of angiosperms by many millions of years and can be better attributed to the diversification of parasitic and especially parasitoid insect lineages.
... [39] Since then, there have been subsequent radiations of squids and cuttlefishes in the Jurassic and incirrate octopuses in the Cretaceous that have been linked to the radiation of bony fishes. [39] Morphological novelties in cephalopods include camera-type eyes that evolved convergently with those found in vertebrates [40] and flexible arms and tentacles that are thought to be derived from the molluscan foot. [41,42] Together with the partial or complete loss of the molluscan shell, the arm crown has been proposed as a major morphological innovation for their diversification as it likely enabled these animals to become agile predators. ...
How genomic innovation translates into organismal organization remains largely unanswered. Possessing the largest invertebrate nervous system, in conjunction with many species-specific organs, coleoid cephalopods (octopuses, squids, cuttlefishes) provide exciting model systems to investigate how organismal novelties evolve. However, dissecting these processes requires novel approaches that enable deeper interrogation of genome evolution. Here, the existence of specific sets of genomic co-evolutionary signatures between expanded gene families, genome reorganization, and novel genes is posited. It is reasoned that their co-evolution has contributed to the complex organization of cephalopod nervous systems and the emergence of ecologically unique organs. In the course of reviewing this field, how the first cephalopod genomic studies have begun to shed light on the molecular underpinnings of morphological novelty is illustrated and their impact on directing future research is described. It is argued that the application and evolutionary profiling of evolutionary signatures from these studies will help identify and dissect the organismal principles of cephalopod innovations. By providing specific examples, the implications of this approach both within and beyond cephalopod biology are discussed.
... This would not be so sur prising because coiling evolved independently many times in the course of cephalopod phylogeny (e.g. Kröger 2005, Kröger et al. 2011). This phylogenetic hypothesis is supported by the presence of "Ritzstreifen" and similar wrinkle layer (see review and references in Korn et al. 2014) in, e.g. ...
Thick Early Devonian carbonatic sedimentary successions, exposed in the Zeravshan Mountains of Uzbekistan, display a transition from a reefal to a pelagic facies. This allows us to document and analyze the history of sedimentation and changes in marine faunas of this region. The late Pragian succession of Bursykhirman Mountain is documented with the transition from platform carbonates to pelagic sediments. Lithology and microfacies through the early Emsian sedimentary sequence of two ammonoid-bearing sections were investigated with a focus on the Dzhaus Beds. In
addition to this sedimentological analysis, we discuss the palaeobiogeographically peculiar situation of Uzbekistan (palaeocontinent Kazakhstania). Many species found in the Kitab State Geological Reserve are endemic and at least restricted to the South Tien Shan. We suggest a moderately close relationship to southern Chinese and Vietnamese faunas, even though more palaeontological data from the latter two regions is needed for a test. We also revise the cephalopod fauna from the Kitab Reserve and introduce the following new taxa: Beckeroceras gen. nov., Uzbekisphinctes gen. nov., Ivoites meshchankinae sp. nov., Kitabobactrites salimovae gen. et sp. nov., and Metabactrites rakhmonovi sp. nov.
... This study focuses on cephalopods from the Pennsylvanian-early Permian (Morrowan, Atokan, Desmoinesian, Missourian, Virgilian, and Wolfcampian) in the Midcontinent Sea of the United States as knowledge of their systematic affinities, geographic distribution and overall diversity is relatively well understood (Miller, Dunbar & Condra, 1933;Newell, 1936;Plummer & Scott, 1937;Miller & Youngquist, 1949;Nassichuk, 1975;Boardman II et al., 1994;Landman, Tanabe & Davis, 1996;Kröger, 2005;Klug et al., 2015;Korn, Klug & Walton, 2015), the stratigraphy of the region is well constrained (Heckel, 2008;Heckel, 2013), and there are extensive exposures of fossiliferous units in the region. Moreover, at this time the Midcontinent Sea was bordered by the Antler Orogeny to the north, the Ancestral Rocky Mountain Orogeny to the west/northwest and the Ouachita Mountain belt to the south/southeast (as well as various structural arches), such that it constituted a distinct biogeographic region for marine invertebrates (Wells et al., 2007;Nelson & Lucas, 2011;Joachimski & Lambert, 2015). ...
Geographic range is an important macroevolutionary parameter frequently considered in paleontological studies as species' distributions and range sizes are determined by a variety of biotic and abiotic factors well known to affect the differential birth and death of species. Thus, considering how distributions and range sizes fluctuate over time can provide important insight into evolutionary dynamics. This study uses Geographic Information Systems (GIS) and analyses of evolutionary rates to examine how in some species within the Cephalopoda, an important pelagic clade, geographic range size and rates of speciation and extinction changed throughout the Pennsylvanian and early Permian in the North American Midcontinent Sea. This period is particularly interesting for biogeographic and evolutionary studies because it is characterized by repetitive interglacial-glacial cycles, a global transition from an icehouse to a greenhouse climate during the Late Paleozoic Ice Age, and decelerated macroevolutionary dynamics, i.e. low speciation and extinction rates. The analyses presented herein indicate that cephalopod species diversity was not completely static and actually fluctuated throughout the Pennsylvanian and early Permian, matching findings from other studies. However, contrary to some other studies, the mean geographic ranges of cephalopod species did not change significantly through time, despite numerous climate oscillations; further, geographic range size did not correlate with rates of speciation and extinction. These results suggest that pelagic organisms may have responded differently to late Paleozoic climate changes than benthic organisms, although additional consideration of this issue is needed. Finally, these results indicate that, at least in the case of cephalopods, macroevolution during the late Paleozoic was more dynamic than previously characterized, and patterns may have varied across different clades during this interval.
... Originally shelled, sea-floor-dwelling molluscs, cephalopods are descended from superficially limpet-like ancestors in the Cambrian [4,5]. The protective shell later became adapted as a chambered buoyancy organ [6], giving rise to free-swimming forms by the latest Cambrian that radiated into several Ordovician lineages [7]. Subsequently, internalization and reduction of the mineralized shell facilitated adaptation for alternative ecologies in the coleoids [8]. ...
Coleoid cephalopod molluscs comprise squid, cuttlefish and octopuses, and represent nearly the entire diversity of modern cephalopods. Sophisticated adaptations such as the use of colour for camouflage and communication, jet propulsion and the ink sac highlight the unique nature of the group. Despite these striking adaptations, there are clear parallels in ecology between coleoids and bony fishes. The coleoid fossil record is limited, however, hindering confident analysis of the tempo and pattern of their evolution. Here we use a molecular dataset (180 genes, approx. 36 000 amino acids) of 26 cephalopod species to explore the phylogeny and timing of cephalopod evolution. We show that crown cephalopods diverged in the Silurian?Devonian, while crown coleoids had origins in the latest Palaeozoic. While the deep-sea vampire squid and dumbo octopuses have ancient origins extending to the Early Mesozoic Era, 242 ? 38 Ma, incirrate octopuses and the decabrachian coleoids (10-armed squid) diversified in the Jurassic Period. These divergence estimates highlight the modern diversity of coleoid cephalopods emerging in the Mesozoic Marine Revolution, a period that also witnessed the radiation of most ray-finned fish groups in addition to several other marine vertebrates. This suggests that that the origin of modern cephalopod biodiversity was contingent on ecological competition with marine vertebrates.
... Tarphycerids (Early Ordovician-late Silurian) with tightly coiled conchs are the oldest cephalopods with a supposedly nektonic mode of life resembling extant Nautilus (Kröger 2005, Kröger & Landing 2008. The time of hatching can be determined in fossil nautiloids by applying an actualistic approach using information from the early embryonic development of extant Nautilus (Stenzel 1964, Mutvei et al. 1993, Mutvei & Doguzhaeva 1997). ...
Our study of the early ontogeny of the Silurian Ophioceras has led to the revision of the current concept that many juvenile tarphycerids possessed a coiled conch upon hatching and thus resembled adults in habit as is in extant Nautilus. In fact, there is no evidence that any Early Palaeozoic coiled nautiloid possessed an embryonic conch exceeding half a whorl. A change in conch coiling, occasionally accentuated by a dorsolateral groove analogous to the nepionic constriction and the appearance of conspicuous growth anomalies indicate that, after hatching, Ophioceras possessed a cap-shaped, slightly curved conch, usually approximately a quarter whorl long. A hatchling thus differed substantially from the likely nektonic late juveniles with coiled conchs and their obliquely oriented aperture as in Nautilus. A relatively large first phragmocone chamber and very short body chamber possibly resulted in positive buoyancy and a planktonic habit of hatchlings. The embryonic conch size is highly variable and the height of the first chamber varies between 1.2-2.6 mm. Changes in sculpture across the embryonic/juvenile conch boundary are sometimes gradual, but frequently, hatching is manifested by an abrupt increase in growth line spacing and the appearance of longitudinal ridges. The cicatrix is here documented in the Tarphycerida for the first time. A distinct chamber length decrease, commonly present close to the end of the first whorl, is not linked with hatching. Anomalous conch structures in Ophioceras including healed injuries, atypical shapes of ribs, atypical courses of septa and pits occurring in late juvenile growth stages are described in the light of the autecology of Ophioceras and the determination of early post-embryonic growth anomalies.
... lopod clades documents an ecological transition from an essentially 909 planktonic to a nektonic habit(Kröger, 2005), anticipating similar910 coiling events in the 'Devonian Nekton Revolution' (Klug and Korn, 911 2004; Klug et al., 2010). 912 The probable change in the mode of life from benthic to planktonic 913 in the late Cambrian at the monoplacophoran-cephalopod-transition 914 and from planktonic to nektonic lifestyles early in the Ordovician within 915 several cephalopod clades, may reflect the rich supply of planktonic 916 food sources developing in the later part of the Cambrian. ...
... The decline of the Eurypterina, the predatory suborder of eurypterids, occurs at the same time as the diversification of gnathostomes, during the Early Devonian (Lamsdell and Braddy 2009). This shift in dominant predator is visible in cephalopods: coiled nautiloids diversify once in coordination with the emergence of eurypterids, and then reradiate and produce the ammonoids at the same time as the appearance of gnathostomes (Kröger 2005). Are the crinoid patterns related to the shift in dominant predators? ...
As animal life diversified over the course of the Phanerozoic, the intensity of predator-prey interactions increased in several phases. Crinoids (Phylum Echinodermata: Class Crinoidea) were a dominant constituent of Paleozoic shallow marine faunas and constitute a lesser component of post-Paleozoic faunas; as most of them are sessile suspension feeders, they provide a good case study for the effect of increasing predation pressure on the Paleozoic evolutionary fauna. Herein are presented injury frequencies and examples of anti-predatory adaptations from a variety of modern and fossil crinoids, discussed with reference to those from previous studies.
Rates of regenerating injuries in the modern sessile bathyal crinoid Holopus mikihe are shown to be comparable to those of shallow-water Mesozoic relatives and many Paleozoic taxa, and lower than any other measured injury rates in living crinoids. Regenerating injuries on disarticulated spines of Paleozoic crinoids similarly show an increase in regeneration frequency between the Paleozoic and Recent. Changes in arm branching morphology that increase resilience to predation are shown to have begun in the Early Paleozoic and reached their maximum by the Early Devonian; on this basis we infer that predators influenced crinoid evolution from the Ordovician, before the appearance of predatory vertebrates and echinoids known to prey on crinoids during later times. Beginning in the Devonian, snails parasitizing crinoids are associated with more frequent crinoid arm regeneration and with the presence of spines on the oral surface near their usual position, consistent with the hypothesis that the snails were targeted by predators with crinoids incurring collateral damage. Calyx spines were common in the Devonian and less common in the Mississippian, but tegmen spines associated with predation on parasitic snails persist up to the Late Mississippian. We also find indications that predation on crinoids decreased into the Permian. Our results support the hypothesis that escalation in the crinoid-predator relationship occurred during the Paleozoic during several different episodes of escalation related to new ecological developments. However, rather than a consistent upward trend in all types of defensive adaptations, we find that some defenses may be associated with types of hostile interaction that later lost ecological importance.
... Wells and O'Dor (1991) thought that other ectocochleates such as ammonoids may have pursued a similar low energy mode of life. They supported this hypothesis by pointing out that increasing numbers of fish occupied high energy nektonic habitats (for these macroecological changes, see Signor and Brett 1984;Bambach 1999;Kröger 2005;Klug et al. 2010) and would have competitively excluded most ammonoids from these habitats. The problem with this hypothesis is twofold: (1) As Jacobs and Chamberlain (1996) pointed out, ammonoids are more closely related to coleoids (some of which use considerable energy in relation to body size and also swim at high velocities) than they are to low energy nautilids (Jacobs and Landman 1993;Kröger et al. 2011). ...
Because ammonoids have never been observed swimming, there is no alternative to seeking indirect indications of the locomotory abilities of ammonoids. This approach is based on actualistic comparisons with the closest relatives of ammonoids, the Coleoidea and the Nautilida, and on the geometrical and physical properties of the shell. Anatomical comparison yields information on the locomotor muscular systems and organs as well as possible modes of propulsion while the shape and physics of ammonoid shells provide information on buoyancy, shell orientation, drag, added mass, cost of transportation and thus on limits of acceleration and swimming speed. On these grounds, we conclude that ammonoid swimming is comparable to that of Recent nautilids and sepiids in terms of speed and energy consumption, although some ammonoids might have been slower swimmers than nautilids.
... The interpretation of potential trends in ornamentation is currently complicated by a poor understanding of shell morphogenesis. For instance, convergent evolution of spines in marine mollusk shells have been interpreted as having repeatedly evolved as a defense in response to shell-crushing predators (e.g., Ward 1981;Vermeij 1987;Kröger 2005;Ifrim 2013). However, recent studies ( Moulton et al. 2012;Chirat et al. 2013) have demonstrated that a large diversity of ornamentation and spine structures can be accounted for by small variations in control parameters of the mechanical interaction between the secreting mantle edge and the calcified shell edge, suggesting that convergent evolution of spines can also be understood through a generic morphogenetic process without such selective pressures. ...
The Triassic represents a key interval in the evolutionary history of ammonoids. Characterized by the dominance of the Ceratitida with their typical suture line indented on the lobes only, the Triassic quasi-monophyletic clade shows a remarkable biostratigraphic and geographic record. However, very few studies have thoroughly investigated their evolutionary trends, except for taxonomic richness. Although Triassic ammonoids show a very large range of morphologies, suture complexity and adult size, little is currently known about their trends, except for peculiar time intervals or taxonomic groups. Nevertheless, it seems that taxonomic diversity and morphological disparity of Triassic ammonoids are uncoupled, at least during part of the Early Triassic recovery. Finally, Triassic ammonoids still have many properties to contribute to evolutionary biology, but going further now requires the construction of quantitative databases of the various morphological characters and reconstruction of Triassic ammonoid phylogeny.
... Klug et al. (2010) proposed that the Devonian nekton revolution (affecting, e.g., cephalopods and fishes) might have resulted from sea bottom predator pressure. An important event in the history of the Palaeozoic nekton is the initial radiation of gnathostomes in the Early Devonian (Emsian; Kröger, 2005), with the simultaneous appearance of the Chondrichthyes, Placodermi, and Paleoniscides, the main late Palaeozoic competitors and predators of cephalopods (Mapes et al., 1995;Mapes and Chaffin, 2003). The radiation of gnathostomes may have increased predator pressure on cephalopods, selecting for higher mobility forms (Klug and Korn, 2004) and forcing an invasion of benthic or demersal organisms into the water column (Klug et al., 2010). ...
Arthropods are a major component of the marine zooplankton, functioning as intermediates between primary producers and tertiary consumers in pelagic food webs. They have likely occupied the water column ecospace since the early Cambrian, co-evolving with several major plankton groups through the Phanerozoic. Analysis of the fossil record of arthropods indicates that a zooplanktonic lifestyle has arisen independently across several arthropod groups, and that the arthropod body plan has shown adaptability to fundamental environmental change. Key anatomical adaptations and reproductive strategies that are compatible with and/or facilitate a zooplanktonic lifestyle include well-developed swimming appendages, buoyancy, an active metabolism supported by efficient respiration, visual or photoreception organs for use in detecting mates, food and predators, and the nurturing of young within the exoskeleton. Many of these characters are apparent in the earliest record of arthropods. The development and diversification of arthropod zooplankton into the food chain helped enable large secondary and tertiary consumers to enter the water column and also contributed to the export of organic matter to the seabed via the faecal stream. Although the fossil record of arthropod zooplankton is extremely poor for most of the Phanerozoic, their position in marine food webs suggests they have been fundamental to rebuilding marine trophic structure following major extinction events, and to maintaining marine plankton diversity through Phanerozoic time.
... n the Early Emsian (early Zlíchovian). As far as ammonoids are concerned, they underwent profound morphological changes during the Emsian, such as an increasingly tightly coiled shell, a decreasing size of the umbilical window, an increasing whorl expansion rate (Klug, 2001a, b) and also a differentiation of muscle attachment (Kröger et al., 2005). Kröger (2005) suggested that these evolutionary events were adaptations to increasing predatory pressure which is in accordance with the hypothesis of Signor and Brett (1984). Contrariwise, there were several conservative groups such as some orthoceratids which kept an overall morphology strikingly similar to, e.g., Ordovician forms. This might refle ...
A new specimen of a Dalejan (Late Emsian, Early Devonian) orthoconic cephalopod is described from the southwestern Tafi lalt (Anti-Atlas, Morocco). It has a nonagonal cross section (with nine edges) and resembles Kionoceras. Since it differs in several characters from Kionoceras, we introduce the new taxon Amessouiceras enneagon gen. et. sp. nov. and shortly discuss evolutionary events among cephalopods in the Early Devonian.
... The use of CMR thinking and modeling was first introduced in the paleontological literature in the 1980s by statistical ecologists who realized that the methods that they were developing in ecology could benefit paleontologists (Nichols and Pollock, 1983; Conroy and Nichols, 1984; Nichols et al., 1986). However, the CMR approach was only used very sporadically in the paleontological literature following those papers (Connolly and Miller, 2001a, 2001b Chen et al., 2005; Kröger, 2005; Liow et al., 2008). We think there are several reasons for this. ...
We rely on observations of occurrences of fossils to infer the rates and timings of origination and extinction of taxa. These estimates can then be used to shed light on questions such as whether extinction and origination rates have been higher or lower at different times in earth history or in different geographical regions, etc. and to investigate the possible underlying causes of varying rates. An inherent problem in inference using occurrence data is one of incompleteness of sampling. Even if a taxon is present at a given time and place, we are guaranteed to detect or sample it less than 100% of the time we search in a random outcrop or sediment sample that should contain it, either because it was not preserved, it was preserved but then eroded, or because we simply did not find it. Capture-mark-recapture (CMR) methods rely on replicate sampling to allow for the simultaneous estimation of sampling probability and the parameters of interest (e.g. extinction, origination, occupancy, diver-sity). Here, we introduce the philosophy of CMR approaches especially as applicable to paleontological data and questions. The use of CMR is in its infancy in paleobiological applications, but the handful of studies that have used it demonstrate its utility and generality. We discuss why the use of CMR has not matched its development in other fields, such as in population ecology, as well as the importance of modelling the sampling process and estimating sampling probabilities. In addition, we suggest some potential avenues for the development of CMR applications in paleobiology.
... Thus, Nützel and Frýda (2003) interpreted the loss of the openly coiled larval shell morphology during the Paleozoic as a result of increasing predation pressure in the plankton. Similarly, openly or loosely coiled Paleozoic ammonoids were increasingly replaced by tightly coiled forms as a result of increasing predation by fish and others (Kröger 2005;Klug 2007;Klug et al. 2010). ...
Early and middle Paleozoic gastropod protoconchs generally differ strongly from their corresponding adult morphologies, that is, most known protoconchs are smooth and openly coiled, whereas the majority of adult shells are ornamented and tightly coiled. In contrast, larval and adult shells of late Paleozoic gastropods with planktotrophic larval development (Caenogastropoda, Neritimorpha) commonly resemble each other in shape and principle ornamentation. This is surprising because habitat and mode of life of planktonic larvae and benthic adults differ strongly from each other.
Generally, late Paleozoic to Recent protoconchs are tightly coiled. This modern type of larval shell resembles the adult shell morphology and was obviously predisplaced onto the larval stage during the middle Paleozoic. The oldest known planktonic-armored (strongly ornamented) larval shells are known from the late Paleozoic. However, smooth larval shells are also common among the studied late Paleozoic gastropods. The appearance of larval armor at the beginning of the late Paleozoic could reflect an increase of predation pressure in the plankton. Although there are counter examples in which larval and adult shell morphology differ strongly from each other, there is statistical evidence for a heterochronic predisplacement of adult characters onto the larval stage. Larval and adult shells are built in the same way, by accretionary secretion at the mantle edge. It is likely that the same underlying gene expression is responsible for that. If so, similarities of larval and adult shell may be explained by gene sharing, whereas differences may be due to different (planktic vs. benthic life) epigenetic patterns.
... Present cephalopods are far from their Paleozoic splendor (Krö ger 2005), but they continue to be a very diverse and abundant group inhabiting all marine environments of the world, from surface waters to more than 5000 m depth (Roper et al. 1984; Jereb & Roper 2005). Although some cephalopods are stenothermic, most species are considered stenohaline and eurithermic, and thus salinity would be a main determinant of their geographical distribution (Boyle & Rodhouse 2005; Jereb & Roper 2005). ...
Cephalopods are increasingly acknowledged as an ecologically important group in Chilean ecosystems, but are also one of their less-known biogeographic components. Notably, this group is represented virtually exclusively by non-endemic species, although we hypothesized that their distribution over the coast should be constrained by similar physical determinants to those affecting endemic taxa. We thus present a first evaluation of the latitudinal patterns of diversity and distribution of cephalopod species in Chile, based on geographical data obtained from a review of the available literature. We constructed presence-absence binary matrices of coastal and oceanic species in 20 latitudinal units (2 degrees), for then calculating the respective similarity matrices to obtain a distribution dendrogram using hierarchical cluster analysis (UPGM). The original binary matrices were resampled performing 1000 stochastic reassignments to calculating the 95th percentile as the criterion to identify significant clusters. Statistical comparisons between distributional groupings were performed using ANOSIM. We recorded 86 cephalopods in Chile, including oceanic (7 1) and coastal (15) species. Species richness showed two major breaks at 30 degrees S and 42 degrees S, and decreased toward higher latitudes. Cephalopod species showed well-defined endpoints of distribution within the Chilean coast, differentiating three main biogeographical units: northern (18-30 degrees S), central (30-42 degrees S) and southern (42-56 degrees S) areas. Biogeographical patterns of cephalopod species in Chile showed no particular difference with those already described for most Chilean taxa. The marked distribution breaks of cephalopods at 30 degrees and 42 degrees S suggest that external forcing and physical factors other than temperature gradients may strongly constrain their dispersal.
... This period is characterized by the rapid diversification of land plants and by an explosive trend from planktonic and demersal toward true nektonic marine animals as represented by the radiation of several groups of jawed fish. It is also marked by the origination of important cephalopod groups such as Bactritoidea, Ammonoidea, perhaps Coleoidea and Nautilida (see Erben 1964Erben , 1965Erben , 1966Dzik 1981Dzik , 1984House 1981House , 1996Bandel & Boletzky 1988;Klug 2001aKlug , 2002aKorn & Klug 2003;Klug & Korn 2004;Kröger 2005;Fuchs 2006; Kröger & Mapes 2007;Klug et al. 2008a,b;De Baets et al. 2009, 2010. Several significant geo-events with distinctive sedimentary and ⁄ or faunal perturbations also occurred during the Devonian (see House 1985House , 2002Walliser 1986Walliser , 1996, including the Choteč-and Kačák-events briefly discussed here. ...
Monnet, C., Klug, C., Goudemand, N., De Baets, K. & Bucher, H. 2011: Quantitative biochronology of Devonian ammonoids from Morocco and proposals for a refined unitary association method. Lethaia, Vol. 44, pp. 469–489.
Based on a rich dataset, the biostratigraphy of the late Emsian and the Eifelian (Early–Middle Devonian) ammonoids from the Moroccan Tafilalt is re-evaluated. We processed this dataset (comprising 53 species from 15 sections) with the unitary association method (UAM), by means of the UA-graph freeware. This led to the construction of a sequence of 17 UAs (maximal sets of actually or virtually coexisting taxa), which are grouped into 10 laterally reproducible association zones. This biostratigraphical subdivision of this interval is in some parts finer than the classically used empirical stratigraphical scheme and than a previous graphic correlation analysis. It enabled us to measure regional ammonoid diversity of this interval in detail. The UAM is a powerful biochronological method, which benefits from complementary tools to analyse conflicting inter-taxon stratigraphical relationships inherent to complex biostratigraphical datasets. In cases of under-constrained superpositional relationships between the taxa, the UAM can yield results, which are not parsimonious in terms of reconstructed virtual coexistences. We suggest several additions to complement the algorithmic steps of the method. The most important is the exhaustive or heuristic reconstruction of possible solutions resolving the observed biostratigraphical contradictions (conflicting inter-taxon superpositional relationships and cycles between maximal cliques) and the selection among the solutions of the most-parsimonious one(s) in terms of reconstructed virtual coexistences. Multiple equivalent results may then be processed with standard consensus techniques. Finally, the robustness of the results can be tested by bootstrapping methods to provide confidence estimates on the ranges and associations of studied taxa. □Ammonites, Anti-Atlas, biostratigraphy, correlation, zonation, diversity.
... Dollo's conclusions on ecology and lifestyle of fossil cephalopod groups are now only interesting from a historical point of view, but his principle of ecological interpretation, in which genera are assigned to plankton, nekton, or benthos based on morphological analysis retains its full importance. Dollo's ideas correspond more to reality than a widespread misconception that is perpetuated in text-books and even some scientific papers (e.g., Kröger, 2005) that all ammonoids were active nektonic predators. The latter is not only incorrect and cannot be assumed taking into account the design of the outer shell of cephalopods functioning as a gas-liquid float, but simply contradicts common sense. ...
The dynamics of morphological diversity of shells of Carboniferous ammonoids in Central Asia and Kazakhstan was studied based on changes over time of shell morphotypes. It is shown that the overall morphological diversity in the Carboniferous in Central Asia and Kazakhstan corresponded to its general trends world-wide, including in the pre-crisis and post-crisis eras. Major crisis events affected the entire morphological diversity (and hence all adaptive types). The changes in the morphological diversity dynamics in the Carboniferous ammonoids of Central Asia and Kazakhstan are most similar to those of the Urals, and considerably different from those known from Western Europe.
Measuring locomotion tactics available to ancient sea animals can link functional morphology with evolution and ecology over geologic timescales. Externally-shelled cephalopods are particularly important for their central roles in marine trophic exchanges, but most fossil taxa lack sufficient modern analogues for comparison. In particular, phylogenetically diverse cephalopods produced orthoconic conchs (straight shells) repeatedly through time. Persistent re-evolution of this morphotype suggests that it possesses adaptive value. Practical lateral propulsion is ruled out as an adaptive driver among orthoconic cephalopods due to the stable, vertical orientations of taxa lacking sufficient counterweights. However, this constraint grants the possibility of rapid (or at least efficient) vertical propulsion. We experiment with this form of movement using 3D-printed models of Baculites compressus , weighted to mimic hydrostatic properties inferred by virtual models. Furthermore, model buoyancy was manipulated to impart simulated thrust within four independent scenarios ( Nautilus -like cruising thrust; a similar thrust scaled by the mantle cavity of Sepia ; sustained peak Nautilus -like thrust; and passive, slightly negative buoyancy). Each model was monitored underwater with two submerged cameras as they rose/fell over ~2 m, and their kinematics were computed with 3D motion tracking. Our results demonstrate that orthocones require very low input thrust for high output in movement and velocity. With Nautilus -like peak thrust, the model reaches velocities of 1.2 m/s (2.1 body lengths per second) within one second starting from a static initial condition. While cephalopods with orthoconic conchs likely assumed a variety of life habits, these experiments illuminate some first-order constraints. Low hydrodynamic drag inferred by vertical displacement suggests that vertical migration would incur very low metabolic cost. While these cephalopods likely assumed low energy lifestyles day-to-day, they may have had a fighting chance to escape from larger, faster predators by performing quick, upward dodges. The current experiments suggest that orthocones sacrifice horizontal mobility and maneuverability in exchange for highly streamlined, vertically-stable, upwardly-motile conchs.
Larger body size has long been assumed to correlate with greater risk of extinction, helping to shape body-size distributions across the tree of life, but a lack of comprehensive size data for fossil taxa has left this hypothesis untested for most higher taxa across the vast majority of evolutionary time. Here we assess the relationship between body size and extinction using a data set comprising the body sizes, stratigraphic ranges, and occurrence patterns of 9408 genera of fossil marine animals spanning eight Linnaean classes across the past 485 Myr. We find that preferential extinction of smaller-bodied genera within classes is substantially more common than expected due to chance and that there is little evidence for preferential extinction of larger-bodied genera. Using a capture–mark–recapture analysis, we find that this size bias of extinction persists even after accounting for a pervasive bias against the sampling of smaller-bodied genera within classes. The size bias in extinction also persists after including geographic range as an additional predictor of extinction, indicating that correlation between body size and geographic range does not provide a simple explanation for the association between size and extinction. Regardless of the underlying causes, the preferential extinction of smaller-bodied genera across many higher taxa and most of geologic time indicates that the selective loss of large-bodied animals is the exception, rather than the rule, in the evolution of marine animals.
Tragoceras falcatum (Schlotheim, 1820) is a common, loosely coiled estonioceratid (Tarphycerida, Cephalopoda) occurring in the Kunda Regional Stage (early Darriwilian, Middle Ordovician) of Estonia. Although the species is quite well-known, we document some features for the first time. For example, one specimen from the Harku quarry (Estonia) with a phosphatized replacement shell exhibits growth halts (megastriae) on the body chamber. As they are not preserved in smaller specimens, we suggest that these megastriae formed at the approach of maturity, possibly also reflecting sexual dimorphism and cycles of reproduction (iteroparity?). Additionally, the specimen shows minute soft-tissue imprints (drag bands and pseudosutures). These imprints are comparable to patterns in other cephalopods such as ammonoids, bactritids and other nautiloids, but have not yet been reported from Palaeozoic nautiloids. However, they might have been misinterpreted as oncomyarian muscle attachment scars previously. Lastly, we discuss the taphonomy of the specimen, which was encrusted by multiple bryozoan colonies post-mortem. Furthermore, it shows questionable traces of bioerosion.
Tarphycerids were diverse and abundant in Ordovician marine faunas. Beginning at the Late Ordovician extinction, the diversity of tarphycerids declined throughout the Silurian, until their extinction in the latest Silurian. Two genera survived the Late Ordovician extinction: Trocholites Conrad, 1838 (from which Ophioceras Barrande, 1865 probably diverged) and Discoceras Barrande, 1867 (= Graftonoceras Foerste, 1925). Discoceras graftonense (Meek and Worthen, 1870), so far known from the US, China, and Australia, is recorded from the Silurian of Bohemia and Gotland. Discoceras stridsbergi n. sp., D . lindstroemi n. sp., and D . sp. indet. from the Wenlock of Gotland and D . amissus (Barrande, 1865) from the Llandovery of Bohemia are all endemic species probably derived from D . graftonense . The distribution of D . graftonense and the origin of four species of Discoceras in the latest Sheinwoodian and early Homerian represent the last diversification and dispersion of the Tarphycerida. No tarphycerid species originated after the mid-Homerian extinction (Mulde and Lundgreni events). Silurian Discoceras retained the morphology and habitats of their Ordovician ancestors. The hatching time and autecology of juveniles has remained unclear. Evidence from the material studied suggests that juveniles were planktonic in habit, possessing a minute curved shell with few phragmocone chambers. Discoceras lindstroemi n. sp. is exceptional owing to its heteromorphic planispiral shell with coiling that changed during ontogeny, resulting in a changing aperture orientation and decreased maneuverability.
Directed evolution of life through millions of years, such as increasing adult body size, is one of the most intriguing patterns displayed by fossil lineages. Processes and causes of such evolutionary trends are still poorly understood. Ammonoids (externally shelled marine cephalopods) are well known to have experienced repetitive morphological evolutionary trends of their adult size, shell geometry and ornamentation. This study analyses the evolutionary trends of the family Acrochordiceratidae Arthaber, 1911 from the Early to Middle Triassic (251–228 Ma). Exceptionally large and bed‐rock‐controlled collections of this ammonoid family were obtained from strata of Anisian age (Middle Triassic) in north‐west Nevada and north‐east British Columbia. They enable quantitative and statistical analyses of its morphological evolutionary trends. This study demonstrates that the monophyletic clade Acrochordiceratidae underwent the classical evolute to involute evolutionary trend (i.e. increasing coiling of the shell), an increase in its shell adult size (conch diameter) and an increase in the indentation of its shell suture shape. These evolutionary trends are statistically robust and seem more or less gradual. Furthermore, they are nonrandom with the sustained shift in the mean, the minimum and the maximum of studied shell characters. These results can be classically interpreted as being constrained by the persistence and common selection pressure on this mostly anagenetic lineage characterized by relatively moderate evolutionary rates. Increasing involution of ammonites is traditionally interpreted by increasing adaptation mostly in terms of improved hydrodynamics. However, this trend in ammonoid geometry can also be explained as a case of Cope’s rule (increasing adult body size) instead of functional explanation of coiling, because both shell diameter and shell involution are two possible paths for ammonoids to accommodate size increase.
The Rochdale Formation of eastern New York (= Fort Ann and lower Bascom formations, designations abandoned) is now recognized to record the earliest stages of the Great Ordovician Radiation of cephalopods. The earliest Bassleroceratidae, Tarphyceratidae and endoceridans on the east Laurentian shallow carbonate platform occur in the upper, thrombolite-bearing member of the Rochdale. This fauna demonstrates that the earliest radiation of Ordovician nautiloids took place in the late Tremadocian and is best recorded in tropical platform facies. Revision of this cephalopod fauna based on approximately 190 specimens collected along a 200 km, N–S belt in easternmost New York has provided new information on inter- and intraspecific variation of earlier described species. The ellesmerocerid Vassaroceras and the endocerids Mcqueenoceras and Paraendoceras are emended. New taxa include Bassleroceras champlainense sp. nov. and B. triangulum sp. nov., Mccluskiceras comstockense gen. et sp. nov., Exoclitendoceras rochdalense gen. et sp. nov. and Paraendoceras depressum sp. nov. A rank abundance plot of 146 specimens from a locality in the Lake Champlain lowlands provides information on the community structure of a nautiloid fauna in which the longiconic cyrtoconic Bassleroceras is shown to dominate strongly. The nautiloid community structure of the Rochdale Formation is similar to that of the underlying Tribes Hill Formation (late early Tremadocian) with respect to the depositional setting, diversity and evenness but displays a remarkably increased taxonomic distinctness.
Early Emsian claystones and marls of the Tafilalt yielded two diverse and prolific faunas with nearly 5000 specimens belonging to at least 100 species being recovered and identified. The older of the two faunas contains what may be the oldest bactritids, pyrgocystid edrioasteroids, phyllocarid carapaces, complete asteropygid trilobites, acanthodian fin spines rarely preserved as pairs, and articulated machaeridians. Additionally, the lower interval yielded a diverse and largely infaunal bivalve assemblage. The younger fauna is marked by the appearance of the first ammonoids, which are represented by the genera Cbebbites, Erbenoceras, Gracilites, Gyroceratites, Irdanites, Lenzites, and Metabactrites. These are accompanied by other cephalopods such as bactritoids, predominantly epibyssate bivalves, and gastropods. Based on the differences in the identified faunal elements of the two assemblages, the preservation of the fossils and the lithology from which the fossils were recovered, it can be concluded that the palaeoenvironment had more or less normal oxic conditions and a moderate water depth within the deeper part of the photic zone being below storm wave base. The high diversity of the infaunal benthonic community and the sediment itself indicate a soft bottom environment. The change in faunal composition between the two faunas strongly suggests that the palaeoenvironment of the younger fauna had decreased oxygen content in the deepest part of the water column and the sediment. It is possible that the environmental stresses created by the oxygen deprivation in the deeper part of the water column facilitated regionally the early radiation of bactritoids and ammonoids during the Emsian. Globally, the rising sea-level and the radiation of the Gnathostomata certainly played an important role in the early bactritoid and ammonoid evolution. The majority of the macrofossils from the two faunas are systematically described; the following new taxa are introduced: the bivalve Eonuculoma babini n. gen. et sp., the orthoceratids Cbebbioceras erfoudense n. gen. et sp., Infundibuloceras brevimira n. gen. et sp. and Plagiostomoceras bassicbebbiense n. sp., the bactritoid Cyrtobactrites scbeffoldi n. sp., the ammonoid Metabactrites ernsti n. sp., and the machaerid Aulakolepos rugatus n. sp., the phyllocarids Nahecaris jannae n. sp.and N. malvinae n. sp. as well as the edrioasteroid Rbenopyrgus flos n. sp. The latter taxa represent the first records of the families Pyrgocystidae, Lepidocoleidae and of phyllocarid carapaces from Africa.
Nautiloids of the superfamily Rutoceratoidea from the late Emsian (late Early Devonian) of the Prague Basin (Czech Republic) are commented upon. Species recognized include the hercoceratids Hercoceras mirum, H.? transiens, Ptenoceras proximum, P. nudum, P. minusculum and Anomaloceras anomalum, as well as the rutoceratids Adelphoceras bohemicum, Homoadelphoceras devonicans, Pseudorutoceras bolli and Goldringia? devonicans. In addition, four new species are described: Parauloceras regulare sp. nov., Roussanoffoceras chlupaci sp. nov., Otomaroceras sp. nov. and Goldringia sp. nov. Morphology and distribution patterns of Pragian and late Emsian rutoceratoid faunas from the Prague Basin are compared. They show that an increased diversity was accompanied by a higher level of specialization of rutoceratoids, which manifested itself in low abundance, increased facies dependence and greater variation in shell size during the Early Devonian. The evolution of sculpture and a contracted aperture, both regarded as protective adaptive features, was also examined, but no adaptive trend towards more pronounced sculpture and constriction of the aperture was found to have occurred in the Early Devonian. A more distinctive sculpture was, however, observed in shallow-water assemblages of P. proximum in comparison with deeper-water faunules, and two additional cephalopod species were examined in order to obtain comparative data. The presence of distinct sculpture patterns in coeval shallow- and deeper-water assemblages suggests limited migration between them and consequently reflects some degree of territoriality in Devonian nautiloids. New data on early shell development in P. proximum are presented. During the Choteč Event, rutoceratoid generic diversity dropped dramatically, one family became extinct and the Early Devonian diversification of the group came to an end. The recovery of nautiloid faunas was slower than that of other cephalopods and associated, unrelated invertebrates. The absence of change in abundance patterns between Pragian and late Emsian rutoceratoid faunas, i.e. prior to and subsequent to ammonoid radiation, suggests that the appearance and radiation of the latter group in the early Emsian did not affect the structure of nautiloid assemblages, i.e. these two clades did not occupy the same niches.
The hatchling sizes of the Cretaceous nautiloids (Hercoglossa forbesianus and Cimomia angustus; Hercoglossidae) are reported here for the first time as ranging between 20.4–22.2 mm in diameter. These new data for Hercoglossidae and the previously reported data for Nautilidae and Cymatoceratidae suggest that all Cretaceous nautiloids had large hatchlings irrespective of their taxonomy relative to those in ammonoids. Based on the relationship between hatching events and the nepionic constriction in modern nautili, the hatchling size of nautiloids can be recognized by the constriction on the shell surface of the innermost whorl. The range of hatchling diameter in all Cretaceous nautiloids (9–35 mm) is comparable to those of modern nautili (20–32 mm), although the Cretaceous nautiloids are characterized by a wider range with the smaller minimum diameter. The large hatchling size in Cretaceous nautiloids stands in marked contrast to those in ammonoids, which had much smaller hatchling size (mostly <2 mm in diameter) and became extinct at the end of the Cretaceous. Such distinct differences in reproductive strategy (i.e., hatchling and egg sizes) suggest that these differences may have played a significant role in the fate of ammonoids and nautiloids through the K/T mass extinction, as previously hypothesized by several paleontologists.
2008. Assessing confidence intervals for stratigraphic ranges of higher taxa: The case of Lissamphibia. Acta Palaeontologica Polonica 53 (3): 413–432. To evaluate stratigraphic evidence for the time of origin of the clade of extant amphibians (Lissamphibia), we attempt to establish a confidence interval on the lower bound of the stratigraphic range of this clade. This is based on the strati− graphic distribution of 1207 fossiliferous localities that have yielded lissamphibians, the relative area of sedimentary rocks from various periods (upper Paleozoic to present) exposed on the continents, and ten exponential−growth models of lissamphibian diversity that differ by the assumed effects of three major biological crises and the assumed starting times of lissamphibian diversification. The results suggest a more recent origin of Lissamphibia than advocated in most recent molecular studies. They are also more compatible with monophyly than with polyphyly of the extant amphibians, but heavily depend on poorly constrained assumptions about lissamphibian extinction rates during biological crises. Counts of lissamphibian diversity through time that consider ghost lineages and stage durations show moderate declines across the Cretaceous–Paleogene and Oligocene–Miocene boundaries.
The 70 Ma Ordovician Period is characterized by extensive epeiric seas, paleocontinent dispersal, intervals of intense volcanism and black shale deposition, a greenhouse climate state deteriorating to a brief icehouse state, strong faunal provincialism, and profound changes to the biota including the changeover from the Cambrian Fauna to the Paleozoic Fauna. Although many invertebrate phyla diversify during the Ordovician, precise biostratigraphic and global biogeographic data are provided best by conodonts, trilobites and graptolites. These three groups are used in this chapter to recognize five major bio-events four of which correspond closely to Series boundaries: Basal Tremadoc (BTc), Basal Arenig (B’Ag), Basal Llanvirn (B’Ln), Basal Caradoc (B’Cc) and Upper Ashgill (U’Al). Most of these correspond to significant eustatic events and the latter to the terminal Ordovician glaciation. The first four are each characterized by extinctions but these are overshadowed by a rapid innovation event with a radiation of a more diversified fauna; the U’Al is a severe extinction event, second only to the terminal Permian event in the entire Phanerozoic. Compared to many other Phanerozoic systems, the Ordovician is a period of considerable biologic, climatic and oceanographic complexity within which the balance between the forcing processes that produced the major and minor events is still not well understood.
Methods currently used to estimate taxonomic extinction probabilities from fossil data.generally assume that the probability of encountering a specimen in a particular stratum, given that the taxon was extant in the time period and location represented by the stratum, either equals one or else is a constant for all strata. Methods used to estimate taxonomic diversity (number of taxa) and speciation rate generally assume that encounter probabilities equal one. We suspect that these assumptions are often false. Capture-recapture models were historically developed for estimation in the face of variable and unknown sampling probabilities. These models can thus be used to estimate parameters of interest from paleobiological data when encounter probabilities are unknown and variable over time. These models also permit estimation of sampling veriances, and goodness-of-fit tests are available for assessing the fit of data to most models. Here, we describe capture-recapture models that should be useful in paleobiological analyses and discuss underlying assumptions. We illustrate these models with examples and discuss aspects of study design. We conclude that these models should prove useful in paleobiological analyses.
Ammonoids retain a record of growth in their shells, and, therefore, material is readily available for studies of early ontogeny. Such studies were performed first in the mid-19th century and have been pursued with vigor ever since. Using optical and scanning electron microscopy, ammonoid workers have described the morphology of the early whorls and have attempted to reconstruct the sequence of early ontogenetic development and to identify the embryonic shell.
The Guamunian xanthids Carpilius maculatus (L.), C. convexus (Forskal), and Eriphia sebana (Shaw & Nodder), and the parthenopid Daldorfia horrida (L.), possess large master claws with molariform teeth than are used to crush the shells of hermit crabs and snails. These crabs typically sever the spire of their prey, or make a gash in the body whorl. They tend to employ sustained pressure on the prey shell, and, except for Eriphia, rarely attack the outer lip, so that the outer lip of the shell typically remains undamaged, except in shells near the critical size, i.e., the maximum size of vulnerability to predation. Temperate species of Cancer (C. productus Randall and C. oregonensi Rathbun) may also crush shelled prey in the larger of their two claws, but more commonly they use both claws together in breaking open their victims. Sustained pressure is applied for only short periods by these crabs.Gastropod adaptations conferring resistance to crushing by crabs include a thick shell, narrow or otherwise small aperture, thickened outer lip, strong sculpture, and a low spire. Emphasis on these traits lowers the critical size of the prey, i.e., permits escape from cushing at a smaller size. An equatorward increase in the expression of the characteristics of crushing-resistance parallels an increase in crushing power of the crabs.
We present new and published isotopic data on brachiopod shells from the Urals and Moscow Basin to explore paleoclimate, global change, and chemostratigraphy in the Carboniferous. A total of 134 shells were analyzed from the Askyn, Sokol, Zilim, and Kamen Perevolochny sections in the Urals. The most important feature of this record is a sharp increase in δ18O and δ13C at the Serpukhovian-Bashkirian boundary. A Mid-Carboniferous increase had been reported for Moscow Basin and North American sections, but the timing had never been so well constrained. This shift bears witness to the transition from greenhouse to icehouse climate in the Carboniferous. The magnitude of the δ 18 O and δ 13 C shifts in the Urals (Askyn section) appears exaggerated because of anomalously low Serpukhovian values. Using the δ 18 O increase recorded in North American and Moscow Basin sections as conservative estimates, this shift implies an ice sheet at least the size of modern glaciers, and perhaps a lowering of tropical temperature several degrees celsius. The simultaneous increase of δ18O and δ13C provides evidence that cooling was promoted by increased burial of organic carbon. These δ18O and δ13C shifts appear global and should be useful as chronostratigraphic markers for the Serpukhovian-Bashkirian boundary. Mid- Kasimovian δ18O and δ13C minima in the Moscow Basin may represent a warm interval in the otherwise icehouse Late
From the Foreword:
"Predator-prey interactions are among the most significant of all organism-organism interactions....It will only be by compiling and evaluating data on predator-prey relations as they are recorded in the fossil record that we can hope to tease apart their role in the tangled web of evolutionary interaction over time. This volume, compiled by a group of expert specialists on the evidence of predator-prey interactions in the fossil record, is a pioneering effort to collate the information now accumulating in this important field. It will be a standard reference on which future study of one of the central dynamics of ecology as seen in the fossil record will be built."
(Richard K. Bambach, Professor Emeritus, Virginia Tech, Associate of the Botanical Museum, Harvard University)
The kinetic model of taxonomic diversity predicts that the long-term diversification of taxa within any large and essentially closed ecological system should approximate a logistic process controlled by changes in origination and extinction rates with changing numbers of taxa. This model is tested with a new compilation of numbers of metazoan families known from Paleozoic stages (including stage-level subdivisions of the Cambrian). These data indicate the occurrence of two intervals of logistic diversification within the Paleozoic. The first interval, spanning the Vendian and Cambrian, includes an approximately exponential increase in families across the Precambrian-Cambrian Boundary and a “pseudo-equilibrium” through the Middle and Late Cambrian, caused by diversity-dependent decrease in origination rate and increase in extinction rate. The second interval begins with a rapid re-diversification in the Ordovician, which leads to a tripling of familial diversity during a span of 50 Myr; by the end of the Ordovician diversity attains a new dynamic equilibrium that is maintained, except for several extinction events, for nearly 200 Myr until near the end of the Paleozoic. A “two-phase” kinetic model is constructed to describe this heterogeneous pattern of early Phanerozoic diversification. The model adequately describes the “multiple equilibria,” the asymmetrical history of the “Cambrian fauna,” the extremely slow initial diversification of the later “Paleozoic fauna,” and the combined patterns of origination and extinction in both faunas. It is suggested that this entire pattern of diversification reflects the early success of ecologically generalized taxa and their later replacement by more specialized taxa.
Implosion of 47 epoxy-sealed shells of Nautilus pompilius indicates that very young shells (20–30 mm diameter at last septum) can withstand 136 atm pressure (= 1360 m equivalent depth), but intermediate and mature shells (30–145 mm diameter) implode at approximately 30–70 atm (= 300–700 m). Strain gages attached to the shells during implosion tests show that the flank and venter are compressed by external pressure, but the septum undergoes tension and structurally should be the most vulnerable to hydrostatic pressure. Overall shell strength is substantially less than reported values of tensile strength (approximately 364 atm) and compressive strength (approximately 1561 atm) of the constituent shell material, nacre. Shell strength as a depth limiting factor appears to be controlled by shell and septal architecture and by stress concentrators such as flaws and microstructural inhomogeneity. The wide range in implosion values and lack of strong correlation between such parameters as septal thickness and implosion pressure indicate that determination of depth ranges for fossil cephalopods may be difficult.
Central to progress in the design and validation of computer adaptive systems will be the possession of the means to evaluate the performance of this type of system. Apart from the procedures for measuring the total performance of the system—its ability to adapt and to affect behavioral measures—one also requires evaluation procedures that support the design process. These latter procedures will support the diagnosis of the relationship between the design and its performance to improve subsequent design. It is also expected that some of these evaluation techniques will be embodied within the system to allow it to self regulate its performance. This chapter discusses different types of evaluation for human–computer interaction. It discusses some of the special problems of evaluating adaptive systems. The chapter describes a number of different techniques for evaluation that have been developed and used on the Adaptive Intelligent Dialogs (AID) project. It also discusses the application of these techniques during and after the design and construction of a computer adaptive system.
In the history of Silurian biota and ecosystem as a whole no “big” catastrophes occurred like the one at the Ordovician-Silurian boundary. Yet it was not a quiet period either. There were established 15 more or less remarkable bio-events, among others the most severe extinction of conodonts and acritarchs in the very beginning of the Wenlock (Ireviken Event), the Great Crisis or lundgreni Event among graptolites in the Homerian and the middle Ludfordian Event comprising many lineages of vertebrates, graptolites, conodonts and corals. Most remarkable diversity rises of the Silurian biota were in the late Rhuddanian, in the Telychian and in the early Gorstian. Both extinctions and originations were in good correlation with the global sea-level curve, but the effect has to be interpreted as an integrated process partly triggered by development of early Silurian glaciation and climate in general.
Since the last review of Jurassic—Cretaceous ammonoid ecology (Westermann, 1990), much additional work has been done on ammonoid autecology (architecture or macrostructure) as well as on the associations and occurrences of ammonoids in the field (synecology). Important works on Paleozoic through Triassic ammonoids, dispersed in the literature, have not been reviewed previously. Quantitative autecological studies, begun in the mid-1980s, concerned buoyancy and orientation. Electron and light microscopic studies of the shells have also contributed to an understanding of the soft parts. Research on shell fabrication, strength, and hydrodynamics has increased greatly, also contributing to ammonoid autecology. Intraspecific morphological variation has been studied intensively but remains poorly understood ecologically; most authors still fail to consider variation in the functional interpretation of shell shape. Ammonoid synecology was significantly advanced in recent years by the renewed interest in Paleozoic and Mesozoic dysoxic black-shale facies and their relation to eustasy and orbitally enforced cycles. Other recent studies in synecology have emphasized the interrelations among sediment, eustasy, and biofacies. Finally, ammonoid taxonomy has been summarized in The Ammonoidea (Special Volume 18, The Systematics Association, 1981).
Information pertaining to the function of ammonoid shells is generated by analogy to living cephalopods, by measurement or experiment designed to elucidate the properties of the ammonoid shell in life situations, and by examination of the distribution and sedimentary environments in which ammonoid fossils are preserved. Virtually all discussions of ammonoid shell function implicitly or explicitly incorporate more than one of these approaches. The combination of analogy with empirical work in the field and laboratory makes the reconstruction of the function of ammonoid shells and interpretation of ammonoid life habits and mode of life particularly intriguing. These interpretations have led to many lively debates among paleobiologists. In this chapter, we examine ammonoid buoyancy and locomotion. We evaluate arguments that have been used to reconstruct the buoyancy and locomotor properties of these extinct cephalopods, discuss recent advances in the understanding of ammonoid locomotion, and suggest directions in which the study of these aspects of ammonoid paleobiology may proceed in the future. Other chapters of this book explore aspects of the structural issues pertaining to the implosion strength of ammonoid shells (Hewitt, Chapter 10, this volume) as well as the environmental information that can be brought to bear on the subject (Westermann, Chapter 16, this volume).
The present chapter concentrates on those aspects of ammonoid morphology that are directly related to the habitat depth of ammonoids and the strength of the shell. Other chapters in the present volume discuss the role of cameral water and ornamentation in locomotion (Chapters 7 and 16, this volume), the growth of septal sutures (Chapter 9, this volume), and ecology (Chapter 16, this volume). The symbols used in this chapter are defined in Table I.
Quantification of Paleozoic, Triassic, Jurassic, and Cretaceous ammonoid shell ornamentation shows that commonality and roughness of ornamentation increased throughout the geologic range of the ammonoids. The two major hypotheses concerning the function of ammonoid shell ornamentation are that 1) ornament served a protective (defensive) function against shell breakage by predators, and 2) it increased hydrodynamic efficiency of the shell during swimming. The heavily ribbed and spined ammonoid shells of the late Mesozoic have ornamentation too coarse to have served any hydrodynamic purpose. The increasing proportion of such shells during the Jurassic and Cretaceous may have been in response to increased numbers of late Mesozoic shell crushing predators and “better armed” ammonoid prey. This trend parallels adaptive trends of other invertebrate groups during the “Mesozoic marine revolution” as defined by Vermeij (1977).
Measurements of diversity and extinction intensity around bio-events in local stratigraphic sections and global taxonomic data bases include some degree of uncertainty, or potential error. Sources of uncertainty involve failure to sample rarer species in all collections and imprecise age correlations and species counts, among others. This chapter presents several methods for evaluating this paleontologic uncertainty. For fossil ranges in local sections, methods are reviewed that might help to discriminate between true gradual (or stepwise) extinction and apparent gradual decline resulting from sampling effects. For tabulated extinctions in regional or global data bases, methods are evaluated for measuring extinction intensity and procedures are presented for calculating relative uncertainty.
The fossil record provides us with a remarkable chronicle of life on Earth. Fossils show how the history of life is characterized by unending change – species originate and become extinct, and clades wax and wane in diversity through the vastness of geological time. One thing is clear – extinction has been just as important as the origination of new species in shaping life's history. It has been estimated that more than 99 per cent of all species that have ever lived on Earth are now extinct. While species of some prokaryotes may be extremely long-lived (Chapter 2), species of multicellular eukaryotes in the Phanerozoic fossil record commonly become extinct within 10 million years (Ma) of their time of origin, with some surviving for less than a million years. Entire groups of previously dominant animals and plants have succumbed to extinction, epitomized by those stalwarts of popular palaeontology, the dinosaurs. The extinction of dominant clades has had positive as well as negative consequences – extinction removes incumbents and opens the way for other clades to radiate. For example, without the extinction of the incumbent dinosaurs and other ‘ruling reptiles’ 65 Ma ago, birds and mammals, including humans, would surely not be the dominant terrestrial animals they are today. Over the past 30 years palaeontologists have increasingly turned their attention towards the documentation of evolutionary patterns and the interpretation of processes responsible for these patterns. As part of this endeavour, extinction has become a major focus of study.
Thirty localities in the Upper Paleozoic (Mississippian-Permian) of North America, have yielded more than 1000 cephalopod mandibles preserved as carbon films, pyrite/limonite internal molds, and as nuclei in phosphate, carbonate, and ironstone concretions. Cephalopod mandibles are rare despite the fact that co-occurring cephalopods may be extremely abundant. Without exception, the mandibles recovered co-occur with ammonoids and bactritoids. Localities containing only nautiloids have not yet yielded cephalopod mandibles. Nekroplanktic accumulations of cephalopods have not yielded mandibles. Except for Lagerstatten deposits (two occurrences), the cephalopod mandibles were always recovered from molluscan-dominated communities; these communities have been interpreted as being in relatively offshore, 'deeper' water in normal marine salinities with relatively low to anoxic oxygen conditions. -from Author
Damage displayed by 18 of 954 relatively mature (> 35 mm diameter) specimens of Gonioloboceras goniolobum (Meek) from North American Midcontinent Upper Carboniferous sediments is interpreted to be bite marks caused by condrichthyans and other fish, particularly the symmoriid shark Symmorium reniforme. It is likely that these and other predators regularly preyed on Gonioloboceras goniolobum because all of the analyzed specimens of Gonioloboceras exhibit some evidence of unrepaired damage, including broached camerae and missing body chambers. Two new characteristics utilized with caution to detect predatory events on ammonoids are raised oval or circular pedestals on internal molds and the breakage and termination of septa in conjunction with a broken external shell. When sufficient damage to the Gonioloboceras conchs occurred during predatory attacks, the broached camerae flooded, the conch became negatively buoyant, and it sank in the vicinity of the attack. When preserved, these conchs form a variable component of an autochthonous fossil accumulation that can include numerous other cephalopod taxa, which also were probably subjected to predatory attacks. Other, but not all, ammonoid accumulations in the fossil record form similar autochthonous deposits, or even autochthonous portions of mixed deposits.
Buoyancy, stability, and orientation of a shelled cephalopod in water are the products of shell geometry, body chamber length, shell, tissue, and water densities. Given such physical characteristics the hydrostatic characteristics of planispiral shelled cephalopods, can be calculated and simulated using microcomputer-based techniques. Individual variables such as geometry, body chamber length, and shell thickness are linked in a calculable manner to orientation, neutral buoyancy, and stability. Living Nautilus provides a means of testing the model and for making hydrostatic comparisons between ammonoids and nautiloids. The close agreement between calculated versus observed body chamber lengths in five species of Mississippian ammonoids show that neutral buoyancy, and Nautilis-like orientations, were at least feasible for these species. -from Authors
Devonian and Carboniferous global events are described. Their magnitude ranges from short-termed geo-events without major biotic changes up to highest order mass-extinctin events. All bio-events are proximately caused by environmental changes such as anoxic episodes and/or sea-level changes.
Results indicate that the very high burial rates of organic carbon calculated for Permo-Carboniferous time cannot be appreciably changed by rapid recycling but that the very high calculated burial ratio of organic-carbon-to-pyrite-sulfur at that time can be lowered. Using a simple oxygen calculation sub-routine, one can also show that oxygen levels were probably higher than at present during the mid-to-late Paleozoic.-from Author
Mass extinctions and glacioeustatic sea-level changes in the lower part of the Hirnantian (final stage of the Ashgill) are accompanied by shifts in marine stable-isotope compositions. Previously published stable-isotope changes have been used to identify the onset and demise of the Gondwana glaciation and to suggest relationships between biotic changes and carbon cycling within the oceans. However, the existing isotopic data set had limitations because it was derived from Ordovician low-latitude settings and from carbonates or organic carbon in separate areas. We report new data from Ordovician high-latitude carbonates and demonstrate parallel shifts in organic and carbonate δ13C from Baltica.Brachiopod shells from a high-palaeolatitude, periglacial setting in Argentina have elevated δ13C values similar to those described previously from low-latitude sites. The new data demonstrate that the positive Hirnantian δ13C excursion, previously only recognised from low-palaeolatitude areas, was widespread and probably global in extent. The poor preservation state of the brachiopods unfortunately prevented the determination of a reliable oxygen isotopic value from the same material.Preliminary carbon isotopic data from thermally immature organic matter from Estonia provide the first indication of a synchronous shift in organic and inorganic δ13C in sediments from the same basin. This work provides new data of critical importance for constraining models of end-Ordovician palaeoceanography and climate change.
Shell growth and morphogenesis were studied in nine species of Bivalvia from the viewpoint of theoretical morphology. The aperture map, or pattern of relative rate of shell accretion for each point around the aperture, received particular attention. Morphometric analyses indicate that the basic pattern of the aperture map is generally maintained throughout ontogeny, whereas both shell convexity and aperture shape commonly change with growth. Computer simulations show that posterior elongation of the aperture with growth cancels the effect of ontogenetic shell inflation to move the maximal growth point anteriorly. In the species examined, the coiling axis is generally inclined to the hinge axis toward the anterodorsal direction and is plunging to the dextral side of the valve. This condition allows ontogenetic shell inflation without modification of the basic pattern of the aperture map. The result indicates that ontogenetic change of shell form is architecturally constrained by a basic pattern of the aperture map, which is kept throughout ontogeny.
The estimation and interpretation of temporal patterns in origination and extinction rates is a major goal of paleobiology. However, the possibility of coincident variation in the quality and completeness of the fossil record makes the identification of such patterns particularly difficult. Previously, Nichols and Pollock (1983) proposed that capture-mark-recapture (CMR) models be adapted to address this problem. These models can be used to estimate both sampling and turnover rates, reducing the risk of confounding the two quantities. Since that time, theoretical advances have made possible the application of these tools to a much broader range of problems. This paper reviews those advances likely to be of greatest relevance in paleobiological studies. They include (1) joint estimation of per-taxon origination and extinction rates, (2) modeling sampling or turnover rates as explicit functions of causal variables, (3) ranking of alternative models according to their fit to the data, and (4) estimation of parameter values using multiple models. These are illustrated by application to an Ordovician database of benthic marine genera from key higher taxa. Robustness of these methods to violation of assumptions likely to be suspect in paleobiological studies further suggests that these models can make an important contribution to the quantitative study of macroevolutionary dynamics.
We still have much to learn about the evolution of taxonomic diversity gradients through geologic time. For example, have latitudinal gradients always been as steep as they are now, or is this a phenomenon linked to some form of Cenozoic global climatic differentiation? The fossil record offers potential to address this sort of problem, and this study reconstructs latitudinal diversity gradients for the last (Tithonian) stage of the Jurassic period using marine bivalves. At this time of relative global warmth, bivalves were cosmopolitan in their distribution and the commonest element within macrobenthic assemblages.
Analysis of 31 regional bivalve faunas demonstrates that Tithonian latitudinal gradients were present in both hemispheres, though on a much smaller magnitude than today. The record of the Northern Hemisphere gradient is more complete and shows a steep fall in values at the tropical/temperate boundary; the Southern Hemisphere gradient exhibits a more regular decline in diversity with increasing latitude.
Tithonian latitudinal gradients were underpinned by a tropical bivalve fauna that comprises almost equal numbers of epifaunal and infaunal taxa. The epifaunal component was dominated by three pteriomorph families, the Pectinidae, Limidae and Ostreidae, that may be regarded as a long-term component of tropical bivalve diversity. Of the mixture of older and newer “heteroconch” families that formed the bulk of the infaunal component, the latter radiated spectacularly through the Late Cretaceous and Cenozoic to dominate tropical bivalve faunas at the present day. This pulse of heteroconch diversification, which was a major cause of the steepening of the bivalve latitudinal gradient, provides important evidence that rates of speciation may be negatively correlated with latitude.
Nevertheless, we cannot exclude the possibility that elevated extinction rates in the highest latitudes also contributed to the marked steepening of bivalve latitudinal gradients over the last 150 Myr. Rates of extinction within epifaunal bivalve taxa appear to have been higher in these regions through the Cretaceous period, but this was largely before any significant global climatic deterioration. Infaunal bivalve clades have had differential success over this time period in the polar regions. Whereas, in comparison with the Tropics, heteroconchs are very much reduced in numbers today, the anomalodesmatans are much better represented, and the protobranchs have positively thrived. We are beginning to appreciate that low temperature per se may not be a primary cause of elevated rates of extinction. Food supply may be an equally important control on both rates of speciation and extinction; those bivalves that have been able to adapt to the extreme seasonality of food supply have flourished in the polar regions.
The characters and body parts of organisms are shaped by mechanical forces at two temporal scales. At the ontogenetic scale, the relevant forces are those of every day, exerted by muscles, other metabolism-powered processes, and normal interactions between the body and the external environment. At the phylogenetic scale, forces are strong enough to kill some individuals or to cause reproductive failure. These forces act more intermittently.
I explore these ideas by examining the characters of molluscan shells, which grow by the addition of skeletal material along the rim of the open end of a hollow, conical tube that is closed at its narrow (apical) end. In the idealized case of a null shell, the skeleton is a right circular cone, in which the magnitude and direction of growth are the same at each point along the rim. The rate of expansion of the cone is determined by the shell-builder's metabolism. Real shell-builders are exposed to, and themselves exert, forces that affect shell shape. These forces are generated by contact between the shell-secreting mantle margin and the substratum, by local or temporary deformations of the mantle margin imposed by other parts of the body and previously formed parts of the shell, and by contraction of muscles that connect the soft tissues to the inner shell surface. Early mollusks whose shells more or less resemble the null shell were slow-moving, epifaunal animals that clamped the shell against the substratum. Evolutionary increases in metabolic rate, associated with greater mobility and faster growth, made some ontogenetically important forces stronger and introduced new forces. As a result, the range of available phenotypes expanded. Refinements in genetic regulation of form, perhaps including an increase in the number of semiautonomous regulatory regions, further added to the specification and range of variation of characters that were subject either to evolutionary conservation or to natural selection. For example, the mantle margin in plesiomorphic gastropods appears to comprise one such region, which produces a growing shell margin in the form of a logarithmic spiral; in more-derived gastropods, the mantle margin may comprise two or more regions, which together produce a growing shell margin that departs strikingly from the logarithmic form of the outer shell lip.
The morphospace occupied by accretionary shells can be described by (1) the number of semiautonomous developmental modules, (2) selective regimes observable as phenotypic adaptive evolution, and (3) metabolic rate. The perspective outlined here implies that shells initially occupied a limited morphospace encompassing one or two modules, adaptation as an epifaunal clamping animal, and slow growth (low expansion rates) and metabolism. Further compartmentalization, together with increased metabolic rates in ecologically dominant taxa, caused the morphospace to expand both in the number of independent descriptors and in the range of values that each parameter spans. These trends in morphospace may characterize all major multicellular clades.