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Phyletic Evolution in the Globorotalia Crassaformis (Galloway and Wissler) Lineage: A Preliminary Report
Abstract
A preliminary report is presented documenting the evolution of the Globorotalia crassaformis (Galloway and Wissler) lineage from the Early Pliocene G. cibaoensis (Bermudez) in Hydraulic Piston Core 72/516 on the Rio Grande Rise (western South Atlantic). A new technique, eigenshape analysis, is used to describe the gross changes in morphology, and the use of “Raupian” analysis of expansion rates, angular displacement, position relative to coiling axis, and translation rates of chambers is suggested as a useful vehicle for inferring the geometric causes of observed evolutionary changes. These changes include size increase, decrease in the number of chambers per whorl, inflation, reduction of relative height of aperture, and increase in the peripheral angle. The sequence is judged to be an example of phyletic gradualism.
... Neogene planktonic foraminiferal fossil lineages have been used to interpret gradualism (Arnold, 1983;Belyea and Thunell, 1984;Wei and Kennett, 1988), PE (Wei and Kennett, 1988), and punctuated anagenesis (Malmgren et al., 1983(Malmgren et al., , 1996. However, the last decade has seen the emergence of sophisticated model-fitting techniques for time series that are ideal tools for testing the evolutionary tempo and mode (Hunt, 2006(Hunt, , 2008Hunt and Carrano, 2010;Hunt et al., 2015). ...
... Recent truncorotalid diversity is related to the evolution of Truncorotalia crassaformis, an extant species that arose after the Miocene/Pliocene boundary from a contentious ancestral species (Hornibrook, 1981;Kennett and Srinivasan, 1983;Cifelli and Scott, 1986;Bylinskaya, 2004;Boudagher-Fadel, 2012;Scott et al., 2015). Notably, Arnold (1983) hypothesized a gradual transition from Truncorotalia juanai (=Hirsutella cibaoensis in Arnold (1983)) toward T. crassaformis across the boundary. However, by using semilandmark geometric morphometrics and maximum likelihood-based time-series analyses (Hunt et al., 2015) we reveal an abrupt evolutionary transition along the Truncorotalia lineage after the Miocene/Pliocene boundary . ...
... Recent truncorotalid diversity is related to the evolution of Truncorotalia crassaformis, an extant species that arose after the Miocene/Pliocene boundary from a contentious ancestral species (Hornibrook, 1981;Kennett and Srinivasan, 1983;Cifelli and Scott, 1986;Bylinskaya, 2004;Boudagher-Fadel, 2012;Scott et al., 2015). Notably, Arnold (1983) hypothesized a gradual transition from Truncorotalia juanai (=Hirsutella cibaoensis in Arnold (1983)) toward T. crassaformis across the boundary. However, by using semilandmark geometric morphometrics and maximum likelihood-based time-series analyses (Hunt et al., 2015) we reveal an abrupt evolutionary transition along the Truncorotalia lineage after the Miocene/Pliocene boundary . ...
The fossil record provides empirical patterns of morphological change through time and is central to the study of the tempo and mode of evolution. Here we apply likelihood-based time-series analyses
to the near-continuous fossil record of Neogene planktonic foraminifera and reveal a morphological shift along the Truncorotalia lineage. Based on a geometric morphometric dataset of 1,459 specimens, spanning 5.9–4.5 Ma, we recover a shift in the mode of evolution from a disparate latest Miocene morphospace to a highly constrained early Pliocene morphospace. Our recovered dynamics
are consistent with those stipulated by Simpson’s quantum evolution and Eldredge-Gould’s punctuated equilibria and supports previous suppositions that even within a single lineage, evolutionary
dynamics require a multi-parameter model framework to describe. We show that foraminiferal lineages are not necessarily gradual and can experience significant and rapid transitions along
their evolutionary trajectories and reaffirm the utility of multivariate datasets for their future research.
... In planktonic foraminifera the "generating curve" (sensu Raup, 1966) of the spiral form is the shape of the chambers (Arnold, 1983). In our initial analysis, we computed medial axes ofthe final chamber in the apertural view (Fig. 5A), as chamber shape is most variable in this view. ...
... We have used centroid size as our size variable. Arnold (1983) used three ofRaup's (1966) spiral parameters to describe the spiral view ofthe foraminiferal test: 0, the angle at which chambers are added; w, the rate of change in chamber radius; and d, the distance of the chambers from the spiral axis. We reduce these to two measures, 0 and r (the expansion rate, incorporating both change in chamber radius and the distance of the chamber from the spiral axis). ...
Analysis of probability distributions of individual organisms provides a common language to describe synchronic and diachronic diversity. When based on an appropriate quantitative description of morphology, this language can be used to explore the temporal component of diversity embedded in the fossil record. Miocene Globorotalia (planktonic foraminifera) from Deep Sea Drilling Project site 593 are described using two-point registration of landmarks in two views (spiral and apertural) and medial-axis analysis of the shape of the final chamber. The equiangular spiral parameters Θ (the angle of increment), r (the expansion rate), and t (the rate of translation down the spiral axis) appear as principal components of the landmark data. Chamber shape variation is described by three principal components of medial-axis curvature. Partial-least-squares analysis demonstrates that the first components of within-morphospace variation also explain the patterns of correlation between the landmark and chamber-shape morphospaces. In the landmark morphospaces, the distribution of sampled individuals is continuous and roughly elliptical with few stratigraphic changes. In the chamber-shape morphospace, the distribution is continuous but shows complex features beyond the elliptical; the occupied morphospace changes stratigraphically, but neither strict cladogenesis nor strict anagenesis explains the derivation of new morphologies. Exemplars of named morphospecies are scattered across these spaces with continuous variation among all forms. These names cannot be assumed to represent discrete entities.
... Although great promise exists, the evolution of only the planktonic foraminifera, and to a lesser degree the radiolarians, has been investigated in any detail using morphometric techniques (e.g., Healy-Williams 1983; Lohmann 1983; Baker 1983). The phylogeny and evolutionary dynamics (i.e., mode and tempo) for several lineages of these groups have been deciphered (e.g., Malmgren and Kennett 1981; Arnold 1983; Lazarus 1986; Wei and Kennett 1988). This approach lags behind considerably in the study of calcareous nannoplankton because of their size (typically less than 10 pm) as it is difficult to capture a clean image in the light microscope at the magnifications required to observe most nannofossils. ...
... We have investigated the evolution of this important group of coccoliths over a period of ten million years. Because of the short-term oscillations in the records of many morphocharacters , the differences between sites, and the lack of high-resolution sampling, it is not possible for the most part to attribute shape change to an evolutionary mode, such as punc-tuated equilibrium (e.g., Eldredge and Gould 1972), gradualism (e.g., Malmgren and Kennett 1981; Arnold 1983; Baker 1983; Wei and Kennett 1986), or punctuated gradualism (e.g., Malmgren et al. 1983; Lazarus 1986 ). Reversals in trends of morphologic evolution have been observed in other microfossil groups such as radiolarians (Kellogg 1983 ) and foraminifera (e.g., Malmgren et al. 1983) and can be included in the "random walk" component of evolution discussed by Bookstein (1987). ...
Investigation of the evolution of calcareous nannoplankton is hindered by the extremely small size (5-10 μm) of their fossils. We introduce new technology, a scanning electron microscope connected to an image analysis system (SEM-IAS), which allows this field to be explored as never before. This system enables a host of morphocharacters to be measured and included in studies of phylogeny and evolutionary dynamics. We have applied the SEM-IAS to study the evolution of Paleocene coccolith genera Cruciplacolithus, Chiasmolithus , and Sullivania. A variety of detailed measurements have been made on over 4000 coccoliths from Deep Sea Drilling Project Site 384 in the temperate North Atlantic and Ocean Drilling Program Site 690 in the Weddell Sea off Antarctica. Our results indicate no simple relationships between morphocharacters, the shapes of all three genera are both complex and highly variable. Although most morphocharacters possess little phylogenetic significance, the areas of different shield cycles show gradual divergence between Chiasmolithus and Sullivania through the Paleocene. Change of most other morphocharacters occurs at variable rates and reversals in trends are common. Minimal correlation exists between the trends and oscillatory shape changes observed at the two sites. We conclude that these trends and oscillations represent local, transitory ecophenotypic variation of the complex form. There is little stasis in the ten-million-year record studied.
... Detailed stratigraphic and morphometric sampling of microfossil lineages was pioneered by Hays (1970) and Kellogg (1975Kellogg ( , 1976 on Pacific and Southern Ocean radiolarians, revealing examples of gradual size change and apparent lineage branching (speciation). Malmgren and Kennett (1981), Arnold (1983), and Malmgren et al. (1983) used a similar approach to investigate a variety of species of planktonic foraminifera, tracing gradual evolution over multi-million-year time scales. These and other subsequent studies are summarized in the Appendix, which compares the type of organism, number of specimens and stratigraphic horizons examined, and the type and number of traits measured on each microfossil. ...
... 37 821 1 Kellogg (1976) Eucyrtidium (R) V20-109 24 av. 26 621 1 Malmgren and Kennett (1981) Globorotalia (¼Globoconella) (F) 284 72 40-60~3600 8 Arnold (1983) Globorotalia crassaformis (F) 72/516 35~35~1225 Eigenshape Malmgren et al. (1983) Globorotalia tumida (F) 214 105 30-75~5460 Eigenshape Lohmann and Malmgren (1983) Globorotalia truncatulinoides 115-88PC 70 40-50~3150 Eigenshape Backman and Hermelin (1986) Reticulofenestra ( Young (1990) Reticulofenestra (N) 219 14~100~1400 1 Young (1990) Reticulofenestra (N) 223 16~100~1600 1 Young (1990) Reticulofenestra (N) 231 33~100~3300 1 Young (1990) Reticulofenestra (N) 242 18~100~1800 1 Young (1990) Reticulofenestra (N) 225 9~100~900 1 Young (1990) Reticulofenestra (N) 227 5~100~500 1 Young (1990) Reticulofenestra ( ...
Marine planktonic microfossils have provided some of the best examples of evolutionary rates and patterns on multi-million-year time scales, including many instances of gradual evolution. Lineage splitting as a result of speciation has also been claimed, but all such studies have used subjective visual species discrimination, and interpretation has often been complicated by relatively small sample sizes and oceanographic complexity at the study sites. Here we analyze measurements on a collection of 10,200 individual tests of the Eocene planktonic foraminifer Turborotalia in 51 stratigraphically ordered samples from a site within the oceanographically stable tropical North Pacific gyre. We use novel multivariate statistical clustering methods to test the hypothesis that a single evolutionary species was present from 45 Ma to its extinction ca. 34 Ma. After identification of a set of biologically relevant traits, the protocol we apply does not require a prior assignment of individuals to species. We find that for most of the record, contemporaneous specimens form one morphological cluster, which we interpret as an evolving species that shows quasi-continuous but non-directional gradual evolutionary change (anagenesis). However, in the upper Eocene from ca. 36 to ca. 34 Ma there are two clusters that persistently occupy distinct areas of morphospace, from which we infer that speciation (cladogenesis) must have occurred.
... Evidence from the equatorial Pacific and the marginal Caribbean Sea in the Atlantic Ocean [26] has shown a striking size increase from small, normally perforated G. menardii during the Late Miocene to large (sometimes giant) menardiiforms that prevailed during the Pliocene in response to an increase in the latitudinal thermal gradients in the upper water column caused by the emergence of the Isthmus of Panama and the intensification of the Northern Hemisphere Glaciation [56]. Such a size evolutionary trend was also documented in other Late Neogene planktonic foraminiferal populations in the level of single species (e.g., G. crassaformis; [90]), and/or lineages (e.g., G. pleisiotumida/G. tumida [91], G. conoidea/G. ...
The Tortonian-Messinian transition is associated with important climatic and oceanographic changes in the Mediterranean Basin, which have shaped both the biotic and abiotic nature of this setting. The morphological variability of the planktonic foraminifera Globorotalia menardii, a species that is highly sensitive to water column structure, has been investigated from the sedimen-tary archive of three Cretan sections across a west-east transect covering the Tortonian-Messinian Boundary. The present work explicitly focuses on test-size and coiling direction changes occurring during the 7.36-7.24 Ma time slice. On such a short timescale, the most important morphological differentiation accounts for the average size of G. menardii, which is mostly associated with evolutionary adaptation to new ecological niches during the latest Tortonian as a response to the environmental perturbations and ecological stress conditions preceding the Tortonian-Messinian Boundary. A combined thermal and/or salinity-driven stratification and thermocline development hypothesis has been suggested to explain the observed size variability. To ameliorate the accuracy of the proposed model and further determine which environmental parameter reflects the optimum conditions of the analysed species, additional sea surface temperature and salinity data derived from the same sampling intervals of the studied or additional Mediterranean sites are needed. The coiling direction of this species within the study time interval remained constant and not environmentally controlled.
... On a temporal scale, morphospecies populations may be dynamic in their morphological change. Gradual morphological change (anagenesis) is characteristic of many Cenozoic planktonic foraminiferal lineages (Malmgren & Kennett 1981;Arnold 1983;Malmgren et al. 1983Malmgren et al. , 1984Hunter et al. 1988;Biolzi 1991;Pearson & Ezard 2014). Many lineages form morphological clines (i.e. ...
The extant morphospecies of the Trilobatus sacculifer plexus (T. sacculifer, T. quadrilobatus, T. immaturus and T. trilobus) have widespread biogeographical distributions and long stratigraphical ranges, and are thus routinely utilized in palaeoceanographical studies. The descendant morphospecies Globigerinoidesella fistulosa is comparatively short-ranging (Pliocene–Pleistocene) and an important biostratigraphical marker. However, taxonomic concepts of these morphospecies are inconsistently applied between workers, leading to loss of information and incomparable datasets. We present a taxonomic appraisal of each morphospecies, including detailed taxonomic histories and refinement of their morphological concepts, using a combined population-based and typological approach. Morphometric data and scanning electron microscopy are used to illustrate morphological intergradation in the Trilobatus sacculifer plexus. The distinctive morphology of Globigerinoidesella fistulosa is shown to develop from T. sacculifer (as previously documented), but also from the other morphospecies in the plexus, providing the first fossil evidence demonstrating that the four morphospecies of the T. sacculifer plexus are the same species. Our new analyses support culturing and molecular genetic evidence from extant specimens that suggests the four T. sacculifer plexus morphospecies are the same biological species. However, we advocate using the four morphospecies concepts (T. sacculifer, T. quadrilobatus, T. immaturus and T. trilobus) and G. fistulosa, here refined, to increase their palaeoecological and biostratigraphical value.
... Points along the chamber surface could not be deemed homologous because of the incremental growth in foraminifera and because chamber shape can change throughout growth (see Caromel et al., [2016]). Chamber centroids were thus chosen as homologous reference points from which to calculate measurements as prescribed by Arnold (1983). The chamber centroids were determined as the region centers of the internal cavities and their coordinates extracted via a specific automatic function in the Avizo software. ...
Developmental processes represent one of the main constraints on the generation of adult form. Determining how constructional and energetic demands operate throughout growth is essential to understanding fundamental growth rules and trade-offs that define the framework within which new species originate. In organisms producing spiral shells, coiling patterns can inform on the constructional constraints acting throughout development that dictated the diversification of forms within a group. Here, we use Synchrotron radiation X-Ray tomographic microscopy (SRXTM) reconstructions of eight planktic foraminifera representative of the major morphotypic groups to determine disparity of coiling patterns by measuring Raupian parameters. The results show that foraminifera are a morphologically highly conservative group, exploiting a limited range of potential coiling patterns. Very similar coiling patterns during early ontogeny, regardless of species, point toward strong constraints in early ontogeny and to common developmental processes acting across all morphogroups. Dispersion and lateral displacement of taxa in morphospace are limited to the adult stage. Accretion with low translation down the coiling axis in juveniles may maximize lateral growth and metabolic efficiency in light of costly calcification. Increased translation in the adult stages allows growth to accommodate new chamber shapes, mediated by changes in aperture location and the site of accretion over ontogeny. These constructional constraints, and the accretion of a small number of discrete chambers, limit the potential for novel forms within the foraminifera compared to other groups of coiling organisms and may explain the repeated evolution of similar morphotypes throughout the evolutionary history of the group.
... The group that has been most commonly analysed morphometrically is the planktonic foraminifers, largely because of their well-established taxonomy and their optimal fossil record (Arnold, 1983). A number of detailed studies focusing on fine-scale variation and microevolutionary processes have been carried out on species of planktonic foraminifers (e.g., Lazarus et al., 1995;Norris et al., 1996;Huber et al., 1997;Spencer-Cervato and Thierstein, 1997;Darling et al., 1999Darling et al., , 2000De Vargas et al., 1999). ...
Culturing experiments of the intermediate morphotype of the cosmopolitan coccolithophore Calcidiscus leptoporus , indicate that the size of its coccosphere and of its coccoliths are affected only in a minor way by temperature. The changes observed in clones growing under different temperature and light conditions are within the range defined for this morphotype in the plankton and Holocene sediments. This outcome suggests that the three morphotypes of living C. leptoporus may be reproductively isolated species rather than stages in a life cycle of a single species or ecophenotypic adaptations of a single species with considerable morphological plasticity. Numerous extinct morphotypes of C. leptoporus have been recorded from marine sediments deposited during the last approximately 25 Ma. In the light of our experiments, these may in fact represent genetically distinct species, which experienced rapid evolution.
... shape information from parameters of Fourier curves fitted to close-spaced coordinates around specimen outlines. The eigenshape algorithm of Lohmann (1983), Arnold (1983) and Hull and Norris (2009) is similarly based on these data which, although regularly located around the edge from a fixed location, may not be homologous, point to point, among specimens. Landmark data are considered to have this property and have been used for specimen alignment in shape studies on globorotaliids by Tabachnick and Bookstein (1990) and Scott et al. (2007). ...
... Most studies of evolutionary size changes in planktic foraminifers focused on single species or lineages (e.g. Malmgren and Kennett, 1981;Arnold, 1983;Malmgren et al., 1983;Spencer-Cervato and Thierstein, 1997;Kucera and Malmgren, 1998;MacLeod et al., 2000). Single species or lineages have provided regional ecological and stratigraphic information on size change. ...
... The relative frequency and evolutionary significance of phyletic gradualism and punctuated equilibrium as two alternative speciation models have been intensively questioned ever since both terms were put forward by Eldredge and Gould (1972). It is generally conceived that most of the speciation on land and in the benthic realm proceeds rapidly (Lynch 1989;Gould and Eldredge 1993), while many examples of slow, gradual transformations of entire populations into new species are known from the fossil record of free-living marine microplankton (Kellogg 1975(Kellogg , 1983Malmgren and Kennett 1981;Arnold 1983;Lazarus 1986;Hunter et al. 1988). Several explanations of this phenomenon have been suggested, most of them concentrating on the fact that the open-ocean environment is exceptionally devoid of geographical barriers (Johnson 1982;Sheldon 1990Sheldon , 1996. ...
... It is now clear that among microscopic protistans, gradualism does seem to prevail (Hayami and Ozawa, 1975;Scott, 1982;Arnold, 1983;Malmgren and Kennett, 1981;Malmgren et al., 1983;Wei and Kennett, 1988, on foraminiferans; Kellogg and Hays, 1975;Kellogg, 1983;Lazarus et al., 1985;Lazarus, 1986, on radiolarians, andSorhannus et al., 1988;Fenner et al., 1989;Sorhannus, 1990, on diatoms). As discussed by Gould and Eldredge (1977) and Lazarus (1983), this may be due to the fact that most of these organisms are either asexual clones, or show alternation of of sexual and asexual generations. ...
... The group that has been most commonly analysed morphometrically is the planktonic foraminifers, largely because of their well-established taxonomy and their optimal fossil record (Arnold, 1983). A number of detailed studies focusing on fine-scale variation and microevolutionary processes have been carried out on species of planktonic foraminifers (e.g., Lazarus et al., 1995;Norris et al., 1996;Huber et al., 1997;Spencer-Cervato and Thierstein, 1997;Darling et al., 1999Darling et al., , 2000De Vargas et al., 1999). ...
Culturing experiments of the intermediate morphotype of the cosmopolitan coccolithophore Calcidiscus leptoporus, indicate that the size of its coccosphere and of its coccoliths are affected only in a minor way by temperature. The changes observed in clones growing under different temperature and light conditions are within the range defined for this morphotype in the plankton and Holocene sediments. This outcome suggests that the three morphotypes of living C. leptoporus may be reproductively isolated species rather than stages in a life cycle of a single species or ecophenotypic adaptations of a single species with considerable morphological plasticity. Numerous extinct morphotypes of C. leptoporus have been recorded from marine sediments deposited during the last approximately 25 Ma. In the light of our experiments, these may in fact represent genetically distinct species, which experienced rapid evolution.
... Both the G. tosaensis and G. inflata FAD levels require that the transition from their respective descendents, Globorotalia crassa[ormis and Globorotalia puncticulata, are consistently recognized (Stainforth et al., 1975;Kennett and Srinivasan, 1983;Bolli and Saunders, 1985). Recent work on the morphological evolution of several planktonic foraminiferal lineages has demonstrated that evolutionary transitions are not very rapid within this group (Malmgren and Kennett, 1981;Arnold, 1983;Malmgren et al., 1983), suggesting that such datum levels may be inherently difficult for a number of different workers to asign with any consistency. ...
A reliability assessment of 20 commonly employed Late Pliocene and Pleistocene calcareous plankton biostratigraphic datums published for 30 DSDP holes has revealed a subset of ten which appear to be reasonably isochronous and consistently recognizable by a large number of biostratigraphers. The assessment is based on an analysis of datum rank order reliability, and is supplemented by an examination of the absolute age estimates for the same 20 datums which were derived using the paleomagnetic stratigraphy presented for 19 of the 30 holes. The results indicate only four of the initial eleven datums based on planktonic foraminifers can be considered reliable, compared to seven reliable calcareous nannofossil datums of an initial nine. Compared to calcareous nannofossils, planktonic foraminifera thus show greater provincialism and more fluid ecophenotypic and evolutionary changes, implying that they are more interesting subjects for evolutionary studies but relatively less suitable tools for biostratigraphy.An important datum rejected by this assessment is the first appearance datum (FAD) of the planktonic foraminiferGloborotalia truncatulinoides, once considered a reliable indicator of the Plio-Pleistocene boundary. While the FAD for this species occurs at its commonly accepted level of approximately 1.9 Ma in the equatorial Pacific and throughout the temperate and tropical Atlantic, its age in the Indo-Pacific between about 15°S and 40°S is strongly diachronous. The earliest FAD is dated at a maximum of 2.6–2.7 Ma between about 20°S and 35°S in the Indo-Pacific. A global extremum in the average size of Holocene populations has been previously recognized within this same latitudinal zone.
... Remarks: This species has distinct morphotypes that have been classified as " subspecies " by some workers (e.g., Bolli and Saunders, 1985; Arnold, 1983). Kennett and Srinivasan (1983) inferred " a possible ancestry " between Truncorotalia crassula and T. crassaformis. ...
Planktonic foraminifers were examined in at least three samples per core at Site 999 in the western Caribbean Sea (12°45'N, 78°44'W; 2829 m water depth) through sediments representing the last ~18 m.y. An age model for Hole 999A was constructed using the available magnetic reversal record (down to the top of the Gilbert Chron, 3.58 Ma) and selected plank-tonic foraminifer datum ages. Near 10 Ma an interval of extremely slow accumulation (5 m/m.y.) corresponds to the "carbonate crash" detected in other Leg 165 studies. Planktonic foraminifer datum ages, as calculated with the Hole 999A age model, are compared to the astrochronological ages assigned to datums at Ceara Rise (Leg 154) and to other published ages. Although there is general agreement, some significant differences are found that may be attributed to either regional paleoceanographic conditions or to shortcomings of the age model for this site. In the middle Miocene temperate latitude globoconellids (Glob-oconella praescitula, Gc. panda, and Gc. miozea) and in the upper Miocene Neogloboquadrina pachyderma (s) are found reg-ularly at Site 999, suggesting the existence of an influx of cool Pacific surface water and/or regional seasonal upwelling before the emergence of the Central American Isthmus. Menardellid species (Menardella miocenica, M. pertenuis, and M. exilis) endemic to the tropical Atlantic are all encountered at this Caribbean site, although not as regularly or in as large numbers as they were found in the western tropical Atlantic (Leg 154). Several species that were absent from the tropical Atlantic for much of the Pliocene were also found to be missing from the Caribbean record during similar intervals.
Culturing experiments of the intermediate morphotype of the cosmopolitan coccolithophore Calcidiscus leptoporus, indicate that the size of its coccosphere and of its coccoliths are affected only in a minor way by temperature. The changes observed in clones growing under different temperature and light conditions are within the range defined for this morphotype in the plankton and Holocene sediments. This outcome suggests that the three morphotypes of living C. leptoporus may be reproductively isolated species rather than stages in a life cycle of a single species or ecophenotypic adaptations of a single species with considerable morphological plasticity. Numerous extinct morphotypes of C. leptoporus have been recorded from marine sediments deposited during the last approximately 25 Ma. In the light of our experiments, these may in fact represent genetically distinct species, which experienced rapid evolution.
Microfossils are of prime importance in documenting patterns of evolution due to their great abundance (often tens of thousands to millions of specimens in a hand sample) and widespread distribution (in both time and space) in the fossil record. The term “microfossil” is often used for paleontological material that requires a microscope for its study, no matter what its biological affinities. For the purposes of this article we will be looking at the remains of protists (single-celled organisms). The several examples I discuss in this chapter are of three groups of planktonic (floating) protists, the calcareous nannoplankton (tiny plant-like protists whose single cell is covered in minute calcitic scales), the radiolaria (animal-like protists with siliceous shells) and the planktonic foraminifera (animal-like protists with calcitic shells). These organisms have been the subject of extensive study because the material from which they are often extracted, cores of deep-sea sediments, are usually comprised of a more complete sedimentological record ( i.e. , fewer breaks) than shallow shelf deposits. Hypotheses of evolutionary history have been constructed for many groups (lineages) of microfossils using specimens from deep-sea cores. Ancestor-descendent relationships have been recognized by tracking shape and form (morphologic) changes through time. This approach to reconstruction of evolutionary history provides an empirical record of morphologic evolution; that is, a record based on observations.
Microfossils are of prime importance in documenting patterns of evolution due to their great abundance (often tens of thousands to millions of specimens in a hand sample) and widespread distribution (in both time and space) in the fossil record. The term “microfossil” is often used for paleontological material that requires a microscope for its study, no matter what its biological affinities. For the purposes of this article we will be looking at the remains of protists (single-celled organisms). The several examples I discuss in this chapter are of three groups of planktonic (floating) protists: the calcareous nannoplankton (tiny plant-like protists whose single cell is covered in minute calcitic scales), the radiolaria (animal-like protists with siliceous shells), and the planktonic foraminifera (animal-like protists with calcitic shells). These organisms have been the subject of extensive study because the material from which they are often extracted, cores of deep-sea sediments, are usually comprised of a more complete sedimentological record (i.e., have fewer breaks) than shallow shelf deposits. Hypotheses of evolutionary history have been constructed for many groups (lineages) of microfossils using specimens from deep-sea cores. Ancestor-descendent relationships have been recognized by tracking shape and form (morphologic) changes through time. This approach to reconstruction of evolutionary history provides an empirical record of morphologic evolution; that is, a record based on observations.
The molecular and morphological shreds of evidence of the origin of foraminifera are discussed in this chapter. The trends of evolution in planktic foraminifera, larger benthic foraminifera, and deep-sea benthic foraminifera are also discussed. Photosymbiosis has been a driving force in the evolution of foraminifera. It is discussed how it helped planktic and larger benthic foraminifera evolve. The major extinction events, including the end-Permian, Cretaceous/Paleogene boundary extinction, and Paleocene/Eocene boundary extinctions, are described.
Pelagic (open-ocean) species have enormous population sizes and broad, even global, distributions. These characteristics should damp rates of speciation in allopatric and vicariant evolutionary models since dispersal should swamp diverging populations and prevent divergence. Yet the fossil record suggests that rates of evolutionary turnover in pelagic organisms are often quite rapid, comparable to rates observed in much more highly fragmented terrestrial and shallow-marine environments. Furthermore, genetic and ecological studies increasingly suggest that species diversity is considerably higher in the pelagic realm than inferred from many morphological taxonomies.
Zoogeographic evidence suggests that ranges of many pelagic groups are much more limited by their ability to maintain viable populations than by any inability to disperse past tectonic and hydrographic barriers to population exchange. Freely dispersing pelagic taxa resemble airborne spores or wind-dispersed seeds that can drift almost anywhere but complete the entire life cycle only in favorable habitats. It seems likely that vicariant and allopatric models for speciation are far less important in pelagic evolution than sympatric or parapatric speciation in which dispersal is not limiting. Nevertheless, speciation can be quite rapid and involve cladogenesis even in cases where morphological data suggest gradual species transitions. Indeed, recent paleoecological and molecular studies increasingly suggest that classic examples of “phyletic gradualism” involve multiple, cryptic speciation events.
Paleoceanographic and climatic change seem to influence rates of turnover by modifying surface water masses and environmental gradients between them to create new habitats rather than by preventing dispersal. Changes in the vertical structure and seasonality of water masses may be particularly important since these can lead to changes in the depth and timing of reproduction. Long-distance dispersal may actually promote evolution by regularly carrying variants of a species across major oceanic fronts and exposing them to very different selection pressures than occur in their home range. High dispersal in pelagic taxa also implies that extinction should be difficult to achieve except though global perturbations that prevent populations from reestablishing themselves following local extinction. High rates of extinction in some pelagic groups suggests either that global perturbations are common, or that the species are much more narrowly adapted than we would infer from current taxonomies.
Recent studies have shown that modes of evolution, namely directional trend, random walk, and stasis, vary across morphologic traits and over the geographic range of a taxon. If so, is it possible that our interpretation of evolutionary modes is actually driven by our selection of traits in a study? In an attempt to answer this question, we have restudied the middle Miocene planktonic foraminifera Fohsella lineage, an iconic example of gradual morphologic evolution. In contrast to previous studies that have focused on the gross morphology as embodied by the edge view of tests, we analyze here multiple phenotypic traits chosen because their biologic and ecologic significance is well understood in living populations. We find that traits in the lineage did not evolve in concert. The timing and geographic pattern of changes in shape, coiling direction, size, and ecology were different. The evolution of this lineage is a mosaic combination of different evolutionary modes for different traits. We suggest that overemphasis on the evolution of some single trait, such as the edge-view outline, from narrow geographic ranges has significantly underestimated the dynamic evolutionary history of this group.
Classification of modern and fossil planktic foraminifers is based on a morphological species concept (i.e. morphotypes) for practical reasons, i.e. a non-destructive enumeration from strew-mounted samples, and economical (i.e. time-saving) analyses. Detailed classification of each test, for example, using scanning electron microscopy or analysis of the molecular genetics in case of live specimens would be too costly.
Several very large, taxonomically standardized data sets have been compiled and utilized to investigate biogeographic and evolutionary patterns of continental margin benthic foraminifera. Mean partial species durations for 87 frequently occurring and 180 rarely occurring species on the Atlantic continental margin of North America are the same, namely 21 m.y. The global fossil record of these species indicates no center or centers of origin and indicates very rapid dispersal. The Miocene had the greatest number of first occurrences with 46%, followed by the Pleistocene, Pliocene and Oligocene with approximately 13% each. The remaining 14% first occur in the Eocene, Paleocene, and Cretaceous. Species with a wide geographic distribution often exhibit longer species durations than those with narrow geographic ranges. The vast majority of endemic species (150 of 175) occur rarely and have no fossil record. 1989 The Paleontological Society.
The study of evolution by natural selection is difficult because, by definition, it occurs under uncontrolled conditions. All biological organisms are highly complex entities and their interactions with the environment and each other are unpredictable, except in the most general terms. Countless chance events impact cumulatively on the genetic composition of a descendant lineage, resulting in substantial evolutionary change over very long periods of time. Understanding how evolution works in practice is a matter for basic science, but it is attended by unique and difficult problems.
The climatic history of the Arctic has been a matter of debate ever since the systematic sampling and study of seafloor sediments commenced several decades ago. The early Soviet investigators (Sacks, Belov, and Lapina, 1955), using radium distribution in sedimentary cores, estimated that rates of sediment accumulation in the entire basin were 1.2–2 cm/103 yr. These values are an order of magnitude higher than rates based on uranium series isotope dates (Ku and Broecker, 1967; Herman and Osmond, 1984; Chapter 22 of this volume). Linkova (1965) was the first to determine the magnetic polarity of Arctic basin sedimentary cores. Her studies demonstrated conclusively that sediment accumulation rates on topographic highs, such as the Lomonosov Ridge, are extremely slow, ~1–3 mm/103 yr. Similar results were obtained for the Alpha-Mendeleev Rise (Hunkins et al., 1971; Aksu, 1985a; Aksu and Mudie, 1985).
The evolutionary origin of Morozovella angulata from its immediate ancestor, Praemurica uncinata, is preserved in Paleocene sediments from the Gulf of Mexico. This event represents the beginning of the morozovellid radiation and marks the first appearance of keeled planktonic foraminifera after the Cretaceous/Tertiary extinction. Parallel biometric and isotopic analyses were performed on size-segregated specimens from a succession of stratigraphic horizons. The biometric data reveal a temporal pattern of variation consistent with paedomorphosis. The appearance of angulose juvenile chambers in the otherwise rounded ancestral form (Praemurica uncinata) results in an allometry that becomes more pronounced upsection. At the origin of M. angulata, the juvenile morphology of the ancestor is retained throughout the entire ontogeny. Isotopic analysis of this sequence reveals the gradual acquisition of an increasingly heavy adult δ13C signal relative to that of the juvenile, while the δ18O data display no temporal or size-related trends. The temporal increase seen in the slope of the δ13C/size relationship may reflect the evolution of an increased dependency on photosymbionts.
Praemurica uncinata displays pronounced allometry during its evolutionary transformation into Morozovella angulata, an important species in Paleocene planktic foraminiferal biostratigraphy. The allometry is such that smaller specimens of P. uncinata resemble large specimens of the descendant, M. angulata. Biometric analysis of populations spanning this paedomorphic evolutionary transition reveals that the event is recorded over an interval of 10.69 meters in sediments from the DeSoto Canyon, Gulf of Mexico, and involves specimens ranging in size from less than 100 to more than 300 μm. Over this size range, failure to recognize the interdependence of size and shape can lead to discordant zonations when biostratigraphers identify the first appearance datum of Morozovella angulata after using different sieve sizes during standard laboratory treatment of their samples.
In addition to providing information about ancient environments and macroevolution, microfossils can be used to correlate the absolute ages of rocks. Following the development of biostratigraphy from classical origins into petroleum exploration and deep-ocean drilling, this survey explores in depth the surprisingly wide application of biostratigraphic methods. The book will be essential reading for students and researchers working in basin analysis, sequence stratigraphy, palaeoceanography, palaeobiology and related fields.
Evolutionary trends in planktonic foraminifera across the K/T boundary are very well documented in numerous surface sections of the Basque Country (Western Pyrenees). After the massive extinction at the K/T boundary, the polyphyletic radiation of the Paleogene planktonic foraminifera starts from a few survivors which include Heterohelix spp., Guembelitria cretacea, G. trifolia, Hedbergella hillebrandti and H. holmdelensis. A total of nine different evolutionary lineages are recognized within the early Paleocene planktonic foraminifera. The new genus Civisina with its type-species C. euskalherriensis n. sp. is characterised by its aperture wich extends onto the peripheral margin of the youngest chamber. Besides its type-species, it includes also C. longiapertura which is kept separate from Parvularugoglobigerina eugubina.
Analysis of the evolution of the Globorotalia ( Fohsella ) lineage of planktic foraminifera suggests that reproductive ecology and shell shape have evolved independently in this group. The silhouette of fohsellid shells displays a nearly unbroken anagenetic trend, yet isotopic data show that the fohsellids changed their depth of reproduction during the anagenetic evolution of their skeletons. Remarkably, there are no correlations between anagenesis in skeletal shape and the establishment of reproductive isolation. Apparently, anagenesis masks at least one speciation event that is apparent only in the isotopic evidence for a change in reproductive ecology. Although anagenetic trends have been widely cited as evidence for gradual speciation in planktic foraminifera and other microfossil groups, our data suggest that they should not always be considered to record either the tempo or mode of speciation.
Speciation was apparently uncoupled from morphological evolution in fohsellids because these evolutionary phenomena occurred in different phases of ontogeny. Gradual morphological changes were associated with the main phase of shell growth of both the ancestor and descendant species in the near-surface ocean. Reproductive isolation occurred when ancestral and descendant populations became established at different depths near the end of the life cycle. Morphological evolution may also be uncoupled from reproductive isolation in other organisms that experience very different selection pressures over the duration of their ontogenies, such as parasites with many hosts, species with multiple phases of metamorphosis, and organisms that broadcast their gametes.
This paper concerns paleontological phylogenies that have a "budding" configuration, wherein "ancestral" species persist through branching events to coexist with their "descendants." Two principal tests are proposed for detecting patterns within such trees. The first test, called the "ancestor-descendant extinction test," compares the number of cases in which, after a split, the ancestral species became extinct before its descendant with the number of cases in which the descendant became extinct before its ancestor. The second test, called the "ancestor-descendant speciation test," compares the number of cases in which, after a split, the ancestral species gave rise to a further species with the number of cases in which the descendant species gave rise to a further species. The null hypothesis in each case is that the frequencies are equal, as predicted by a random Markovian branching model of evolution. Five stratophenetic species-level phylogenies of three taxonomic groups, planktonic foraminifera, nannofossils, and graptoloids, are examined using these tests, including one (Paleogene planktonic foraminifera) that is presented for the first time. In all cases, the phylogenetic trees are found to be strongly nonrandom. The general pattern, although by no means expressed perfectly in every case, corresponds to a Simpsonian "step-series," in which ancestor taxa are simultaneously more likely to become extinct and less likely to speciate than their coexisting descendants. It is shown that this pattern cannot simply be the result of simple age-dependent factors such as an increasing extinction risk in older taxa. Instead, the very fact that a species has given rise to another appears to increase its future extinction risk and decrease its likelihood of further speciation. Many possible biases may affect the shape of paleontological phylogenies, which are as yet poorly understood and unquantified. One potentially important effect follows from the taxonomic subdivision of gradual chronoclines into artificial morphospecies, such as might conceivably induce a step-series pattern in the phylogeny. Even if this is the partial or entire reason for the observed patterns, it would appear to imply directional evolution in phyletic gradualism. Other possible artifacts are discussed, but they are regarded as probably too weak to produce the observed patterns. If the pattern is not artificial, the fact that three of the best known fossil groups exhibit substantial asymmetries in speciation and extinction argues against the currently popular "nonprogressive" view of evolution. Instead, the evolutionary step-series pattern is consistent with the classical Darwinian concept of the general competitive superiority of newly evolved species over their ancestors and supports the idea of evolutionary progress.
Morpholocial continuity in the fossil record is the principal evidence favoring evolution as a historical explanation for the diversity of life. Continuity is usually discussed on scales broader than the species level. Patterns of morphological variation characteristic of living species are useful in recognizing species on time planes in the fossil record, but the fossil record is rarely complete enough temporally or geographically to preserve more than a fraction of species living in a given interval. Transitions between known species are even rarer. Where transitions are preserved, new species appear to arise through anagenesis and cladogenesis. Evolutionary species are often necessarily bounded arbitrarily in the dimension of time. Orthogenesis and punctuated equilibrium lie at opposite poles in a spectrum of speciation modes. Orthogenesis, highly constrained anagenesis, is probably rare. Cladogenesis appears to differ little from anagenesis once ancestral stocks are segregated. Limited evidence suggests that morphological differentiation during cladogenesis postdates genetic isolation. Hence, punctuated equilibrium may be rare as well. Rates of evolution vary and rate distributions are truncated and biased by limits of stratigraphic completeness and time resolution: moderate to high rates of morphological evolution and species turnover are rarely recorded by fossils. Species durations are poorly characterized, but they appear to be so variable that there is no suggestion of periodicity. Species longevity is unpredictable. The episodic nature of faunal turnover suggests that extrinsic environmental factors rather than intrinsic homeostatic factors govern evolution at the species level.-from Author
The enormous population size and cosmopolitan distributions of many pelagic species in everything from protists to vertebrates suggest that many of them should be relatively resistant to speciation and extinction. Yet evolutionary rates in many pelagic taxa are fast compared to shallow marine benthos whose populations and geographic distributions are smaller by several orders of magnitude. Genetic and Zoogeographic analysis suggest that dispersal is relatively easy for many groups of pelagic fish, pteropods, salps, and planktic foraminifera, compounding the apparent problem of the relatively high rates of evolutionary turnover in many pelagic taxa. Nonetheless, the geographic distributions of these pelagic groups are closely correlated with surface watermasses suggesting that although individuals can cross major hydrographic boundaries, they can grow and reproduce within a much more restricted variety of habitats. In this respect, freely dispersing pelagic taxa resemble wind-dispersed seeds that can go almost anywhere but grow only in favorable environments.
The Paleocene Subbotina pseudobulloides lineage is shown to evolve gradually through the intermediate species Planorotalites compressa and Planorotalites ehrenbergi to Planorotalites pseudomenardii and Planorotalites troelseni. The latter two forms are found to be endmembers in a morphological continuum, and the transitional ancestry of the lineage is found to contain no statistically definable cladogenetic event. Rates of evolution calculated for the lineage on the basis of different variables are slow and relatively invariant when compared to other foraminiferal lineages. Planorotalites pseudomenardii was compared to its Neogene "homeomorph", Globorotalia margaritae and found to be readily distinguished by discriminant analysis, and biometric evidence suggests that G. margaritae is derived from G. scitula.
Introdução Fósseis são restos de organismos ou vestígios de sua atividade preservados nas rochas. Dependendo do processo de fossilização, os fósseis podem consistir de partes originais ou substituídas do organismo (como partes do esqueleto) ou simplesmente aparecem como impressões fixadas nos sedimentos. Marcas deixadas nos sedimentos e produzidas durante a alimentação, repouso ou movimento (tais como pegadas de dinossauros ou escavações feitas por invertebrados) são conhecidas pelo nome técnico de icnofósseis. Os fósseis são relativamente comuns nas rochas sedimentares e variam grandemente em tamanho, desde carapaças microscópicas de organismos plantônicos encontrados nas argilas marinhas até ossos de tamanho métrico de dinossauros saurópodes gigantes. Em certos casos específicos (tais como camadas de ossos, camadas de conchas e carbonatos esqueletais) os fósseis podem mesmo constituir toda a formação rochosa. A importância científica dos fósseis é verdadeiramente extraordinária porque eles representam o único arquivo disponível de formas de vida do passado. Através dos fósseis podem ser reconstituídos não somente a morfologia dos seres extintos, como também se podem inferir aspectos de sua ecologia e comportamento. Além disso, os fósseis sempre retêm informações sobre o ambiente no qual eles viveram ou morreram e podem ser, portanto, usados para investigar sítios deposicionais antigos e parâmetros ecológicos. Os fósseis também são muito relevantes para a discussão de modelos de história da vida na Terra. De fato, eles apresentam dados importantes contra os quais teorias como a da evolução ou cenários derivados da Bíblia podem ser testados. Nesse artigo revisaremos alguns dos padrões que emergem de uma visão geral do registro fóssil e avaliaremos sua compatibilidade com o modelo secular da evolução gradual da vida em longos períodos e o modelo bíblico baseado na criação, no dilúvio universal e numa cronologia curta.
Planktonic and benthic foraminifera are the most significant providers of information on the state of surface and deep oceans in the past. Many foraminiferal proxies rely on the knowledge of ecological preferences of individual species and the assumption that these remained similar through time. Consequently, the applicability of such proxies is limited in time by the extent of the modern fauna. By analysing extensive datasets of species occurrences, we show that the modern oceanic foraminifer fauna originated during the Neogene. This occurred during two distinct diversification pulses: one in the Middle Miocene (17-14 Ma) and the second at the Miocene/Pliocene transition (7-4 Ma). The first diversification coincides with the time of a major change in the frequency of the dominant climate cycles during the Miocene Climatic Optimum. The environmental driver of the second diversificatio could be related to an increased provincialism induced by the closure of the Panama Seaway, but the exact link is not clear, particularly for the plankton. Surprisingly, major changes of ocean circulation due to the growth of Antarctic ice-sheet and closure of low-latitude seaways appear to have caused mainly extinctions. Given the age of the latest diversification and extinction pulses that shaped the modern foraminiferal fauna, we conclude that calibrated proxies based on assemblage properties should not be interpreted quantitatively in sediments older than the late Pliocene.
Body size is one of the most significant organismal characteristics because of its strong association with nearly all important ecological and physiological characteristics. While direct body mass measure- ment (or estimation from other size metrics) is not feasible with most extinct taxa, body volume is a measurable and general proxy for fossil size. This study explores the reliability of several metrics that can be used to estimate the body volume of Paleozoic invertebrates of various sizes, shapes, taxonomic affinities, and ecological habits. The ATD model, based on the product of lengths of the three major body axes (anteroposterior, transverse, and dorsoventral), is simple and widely applicable. Models specific to particular morphological and taxonomic groups are slightly more accurate than this ATD mod- el, but the advantages are minor. The ATD model is consistent with previous studies demonstrating widespread shape allometry—that is, small taxa tend to have globose geometries while large ones tend to be conical, even within the same taxonomic group. The ATD model successfully predicts the volume of 10 validation samples that were excluded from development of the original model. Because the linear measurements used to estimate volume are easy to obtain from spec- imens in the field or from published work, estimates of body volume can be incorporated into paleontological analyses, even those span- ning multiple phyla.
Statistical analysis of a literature-based compilation of latitudinal ranges, phylogenetic relationships and biometric measurements of 135 Neogene planktonic foraminiferal species yielded several well-defined relationships between paleobiogeographic distribution and morphology. When cosmopolitan species originate, they tend to be relatively small and morphologically conservative. As keeled forms develop and the Neogene radiation invades other areas of shape space, the cosmopolitan species also shift in the same direction, but continue to lag behind and remain at the conservative end of the shape and size distribution. A significant positive correlation exists between species longevity and geographic range, and significantly greater longevities in species that range into the subpolar and polar habitats when compared with those that don't. A similar relationship was found in speciation rates: lineages that range into the polar and subpolar habitats have lower speciation rates than those that do not. This relationship holds true when keeled forms are removed from the analysis. Furthermore, when species ranging into the polar/subpolar habitats are removed from the data, the difference in speciation rate between keeled and unkeeled forms is not significant. It appears that a species' extension into or beyond the subpolar realm has a stronger statistical relationship with its probability of speciation and its longevity than its total latitudinal range. Comparison of latitudinal ranges for ancestor–descendant pairs reveals a significant trend toward reduction of range for new species.
Morphological evolution of the planktonic foraminifer Globorotalia truncatulinoides in the southern Indian Ocean accompanied changes in the species' paleobiogeographic range during the late Quaternary. The first sustained migrational appearance of G. truncatulinoides in subantarctic waters during oxygen isotopic stage 13 (∼500,000 years ago) involved peripheral populations dominated by juvenile-sized kummerform types. The stage 13 colonization of subantarctic waters coincided with a southward migration of the Subtropical Convergence (STC). The first sustained appearance of the species in northern Antarctic waters during isotopic stage 7 (∼200,000 years ago) is followed by fluctuations in the species' paleobiogeographic range due to glacial-interglacial water mass changes and increasing tolerance to the colder waters of the subantarctic and Antarctic regions. Examination of allometric shape changes during the ontogenetic development of G. truncatulinoides reveals a statistically significant shape change from elongate, axially compressed juveniles to conical, axially extended adults in samples from isotopic stages 1 and 2 in core E48-28. The results provide evidence that the morphotype of a species which originally invades a new biogeographic area may rapidly take on shape characteristics which differ significantly from the original colonizing population.
Geographical size distribution within entire Holocene foraminiferal
assemblages is related to global environmental gradients such as
temperature, primary productivity, and environmental variability. This
study demonstrates that these correlations are also recognizable in late
Quaternary assemblages from three locations in the South Atlantic on
temporal and latitudinal scales. The size response to temporal
paleoenvironmental changes during glacial-interglacial cycles mimics the
geographic Holocene size variability. The amplitude of size variability
is directly related to the amplitude of the climatic fluctuations as
shown by the stable size-temperature relationship over time. The
documented changes in the assemblage size are caused by species
replacement and intraspecific size variability. The relative importance
of these processes depends on the environmental setting. Species have
been shown to reach their maximum size and abundance under certain
optimum conditions and decrease in size if environmental conditions
differ from these optima. We confirm that late Quaternary species sizes
were largest at paleotemperatures identical to Holocene ones.
The punctuated equilibrium hypothesis is one of many possible paleontologic patterns of speciation which may be tested against the deep-sea fossil record of planktonic microfossils. The nature of reproduction and variation within a species has a profound effect on its expected tempo and mode of evolutionary change. Biologic data on pelagic plankton and on protistan genetics and reproduction suggest that speciation in pelagic holoplanktonic protists may also be parapatric, 'equal' allopatric, or the result of hybridization. Each of these models makes testable predictions of paleontologic pattern. Published records of speciation in planktonic deep-sea microfossil data are compatible with these alternative models. Existing data sets are not yet sufficiently complete to provide strong tests. -from Author
Morphometric examination of cladogenesis and plyletic evolution in two late Neogene sister lineages of marine microfossils (Pterocanium prismatium and P. charybdeum, Radiolaria) from two equatorial Pacific sediment cores was undertaken to better understand the rate of cladogenesis and its relation to subsequent phyletic change. The origin of P. prismatium from P. charybdeum approx 4 Ma ago has been estimated to take place over an interval of approx 500 000 yr. Results show that the speciation event consists of two distinct phases. The first phase, cladogenesis, occurred relatively rapidly (on the order of 50 000 yr). A second phase of relatively rapid divergent phyletic evolution away from the commmon ancestral state followed in both descendant branches and continued for at least 50 000 yr after completion of the cladogenetic event. Net evolutionary rates over the next 2 ma appear to be much lower. Individual characters change by as much as two population standard deviations during cladogenesis, and by a total of approximately three standard deviations over 2.5 ma of phyletic evolution. Up to five population standard deviations of change during = or <50 000 yr of cladogenesis, and seven additional standard deviations of phyletic change over 500 000 yr are observed in multivariate (discriminant function) indices of morphologic difference. The measured pattern does not appear to be either strictly 'punctuated' or strictly 'gradual', but instead show features of both hypotheses.-Author
Speciation processes are only rarely studied with fossil materials, even though in principle hypotheses of speciation patterns are most directly testable in the fossil record. We quantitatively document in two widely separated South Pacific DSDP holes the mid-Pliocene speciation of the planktonic foraminifer Globorotalia truncatulinoides. Speciation, with continuous geographic co-occurrence of ancestor and descendant forms, occurred simultaneously at both localities over a period of ~500,000 years. This suggests a sympatric speciation process that involved a large, geographically extensive population. Globorotalia truncatulinoides underwent its most rapid and extensive evolutionary change between ~2.8 and 2.5 Ma. This time interval corresponds to the development of northern hemisphere glaciation, suggesting that climate-controlled paleoceanographic change may have played a significant role in the evolution of G. truncatulinoides.
This study presents a detailed, quantitative account of morphological evolution in the deep‐sea benthic foraminifer Parkiella through the last 10 million years of the Cretaceous (late Campanian to Maastrichtian). Cores taken at DSDP Sites 465 and 171, in the low‐latitude central Pacific, yielded a total of 437 specimens, from 53 samples. The shape of the apical outline of the foraminifer, its chamber arrangement, shell globularity, shell area and proloculus diameter were quantified. Significant, unidirectional changes in shell morphology were observed between sampling levels, indicating that the lineage was not in a state of morphological stasis. The occurrence of morphologically intermediate paleopopulations and the generally normal distribution of morphotypes at any given sampling level suggest that the observed pattern of evolution represents an anagenetic trend in a single, non‐branching lineage. These results confirm predictions of the plus ça change evolutionary model. Gradual evolution of this lineage may represent a reaction to changed oceanographic conditions that were established after the Maastrichtian deep‐water reversal.
Pelagic (open-ocean) species have enormous population sizes and broad, even global, distributions. These characteristics should damp rates of speciation in allopatric and vicariant evolutionary models since dispersal should swamp diverging populations and prevent divergence. Yet the fossil record suggests that rates of evolutionary turnover in pelagic organisms are often quite rapid, comparable to rates observed in much more highly fragmented terrestrial and shallow-marine environments. Furthermore, genetic and ecological studies increasingly suggest that species diversity is considerably higher in the pelagic realm than inferred from many morphological taxonomies. Zoogeographic evidence suggests that ranges of many pelagic groups are much more limited by their ability to maintain viable populations than by any inability to disperse past tectonic and hydrographic barriers to population exchange. Freely dispersing pelagic taxa resemble airborne spores or wind-dispersed seeds that can drift almost anywhere but complete the entire life cycle only in favorable habitats. It seems likely that vicariant and allopatric models for speciation are far less important in pelagic evolution than sympatric or parapatric speciation in which dispersal is not limiting. Nevertheless, speciation can be quite rapid and involve cladogenesis even in cases where morphological data suggest gradual species transitions. Indeed, recent paleoecological and molecular studies increasingly suggest that classic examples of “phyletic gradualism” involve multiple, cryptic speciation events. Paleoceanographic and climatic change seem to influence rates of turnover by modifying surface water masses and environmental gradients between them to create new habitats rather than by preventing dispersal. Changes in the vertical structure and seasonality of water masses may be particularly important since these can lead to changes in the depth and timing of reproduction. Long-distance dispersal may actually promote evolution by regularly carrying variants of a species across major oceanic fronts and exposing them to very different selection pressures than occur in their home range. High dispersal in pelagic taxa also implies that extinction should be difficult to achieve except though global perturbations that prevent populations from reestablishing themselves following local extinction. High rates of extinction in some pelagic groups suggests either that global perturbations are common, or that the species are much more narrowly adapted than we would infer from current taxonomies.
The process of the initiation, formation and establishment of descendent species from existing ancestral species is called speciation. Several aspects of this process can be studied using the fossil record, including how and why rates of speciation have changed through geologic history and how the morphology of lineages change over time as descendents gain phylogenetic independence from their ancestors. The fossil record also provides ancient deoxyribonucleic acid (DNA) which allows genetic analyses of extinct populations and species that give insight into genetic differentiation, among other processes. Organisms are neither all equally likely to be preserved in the fossil record, nor do they have similar rates of achieving reproductive isolation and morphological differentiation from their ancestors. We know more about the fossil record of speciation in groups such as marine bivalves, gastropods, plankton and bryozoans, which have comparatively better fossil records, than groups such as mammals or plants.
The timing of evolutionary changes in some late Neogene lineages of planktonic foraminifers and major paleoceanographic events are evaluated to assess possible correspondences between paleoclimatic change and evolutionary innovations. Previously published data on evolutionary changes in the Globorotalia inflata lineage, the Globorotalia tumida lineage, and the Globorotalia truncatulinoides lineage, and new data on the Sphaeroidinella dehiscens lineage, are used in the study. Most evolutionary rate changes in these lineages are shown to occur near times of major paleoceanographic events. Although evolutionary change exhibits striking temporal correlation with climatic change, some evolutionary time series do not show any obvious relationship to paleoclimate: chamber number in the final whorl in the G. inflata lineage and conicality in G. truncatulinoides.-from Authors
In the animal kingdom evolutionary size changes involved increasing, decreasing and stationary patterns. Planktic and benthic Foraminifera chiefly increased their size during evolution. This increase, however, did not always occur gradually, but could be interrupted by periods when the animals maintained or even decreased in size. The rate of the size increase is different for the various species examined, some benthic forms grew only 10% during the Oligocene-Pleistocene interval, while for others this figure was up to 96%. Some benthic species increased in size in certain areas, but not in others. It is not improbable that some phylogenetic trends of planktic Foraminifera representing, according to stratigraphers, the evolution of one species into another, represent in reality, from the biological point of view, specimens of the same species which changed their size and in addition some minor morphological traits which are encompassed by the normal span of intraspecific variability. A comprehensive understanding and explanation of the size change of Foraminifera needs much additional research. ▭Foraminifera, size change.
Shape measurements have been made on planktonic foraminifera from a South Pacific Late Miocene to Recent temperate evolutionary lineage ( Globorotalia conoidea through intermediate forms to G. inflata in DSDP Site 284). The sampling interval is about 0.1 Myr over nearly 8 Myr. Gradual evolution (phyletic gradualism) clearly occurs in all but one measured parameter. No clear evidence exists for abrupt evolutionary steps (punctuated equilibria) within the bioseries. If they occur, they are the exception rather than the rule. The number of chambers in the final whorl decreases almost linearly, despite known paleoceanographic oscillations within the temperate water mass. Mean size and apertural shape variations seem to correlate with paleoceanographic change. It is speculated that certain major morphological changes that took place within this evolutionary bioseries (i.e. loss of keel, rounding of periphery) developed in response to a major latest Miocene cooling, associated with instability in the water column and resulting adjustments of the test structure to water density changes. Changes exhibited in shape measurements may offer a precise method of stratigraphic correlation between temperate South Pacific Late Cenozoic sequences. Four species and two subspecies, long recognized to form the basis of this lineage, are redefined biometrically.
Using a geometric model of shell morphology, it is demonstrated that biconvex brachiopods occupy only a small region of the potential geometric space available to organisms with planispiral exoskeletons composed of two articulated valves. Measurements taken for a sample of 324 genera of the articulate orders Pentamerida, Rhynchonellida, Spiriferida, and Terebratulida were analyzed using a simple geometric model of shell form and ontogeny. The frequency distribution of brachiopod shell morphologies exhibited by the four orders represents the biological optimization of the spatial relationships between area and volume. Biconvex brachiopods develop shells which are designed to minimize shell surface area while maximizing internal shell volume. The means by which optimization is achieved is related directly to the effects of increase in absolute size during ontogeny. The boundaries upon shell geometries utilizable by biconvex brachiopods are determined by (1) limitations of articulation, and (2) limitations of surface and volume.
Morphologic and taxonomic rates of evolution within some groups of Cenozoic planktonic foraminifera have been calculated. Taxonomic rates of evolution vary from a low of 0.11 species per million years in the genus Orbulina to 1.44 species per million years in the Neogene globorotaliids. The rate of change in the total fauna (rate of origination minus rate of extinction) shows negative values in the Late Eocene and Early Oligocene and in the Late Miocene to Pleistocene, reflecting, in all likelihood, two crises which the planktonic foraminifera experienced during the Cenozoic. Taxonomic frequency data are plotted against a paleotemperature curve for the Middle Eocene-Pleistocene interval (Devereux, 1967). Some striking similarities are seen, and possible relationships are discussed.
Micropaleontology is a peculiar subject: it is not easily defined, it focuses on geologic problems, and it ignores fundamental paleontologic and evolutionary questions it could best attack. As a result of its historic development, micropaleontology is directed to the solution of stratigraphic, paleoceanographic and paleoclimatologic problems, but it has seldom addressed paleobiologic or evolutionary ideas. It is a tradition rather than a discipline. The term “micropaleontology” and all it signifies should be abandoned, for it obscures natural relationships, attracts people with geologic rather than biologic approaches, isolates its practitioners in a blanket of systematics, biostratigraphies, and terminologies, and, as a result, discourages outsiders with other viewpoints or contributions from utilizing its fine fossil record. The growth of the field has been exponential in people-power and literature but not in the development of fundamentally new ideas. Micropaleontology has therefore contributed little to recent paleobiologic or evolutionary hypotheses, in spite of the possession by the many organisms relegated to it of biological properties and fossil records which have much potential for the generation and testing of such hypotheses. Microfossil studies have served geology powerfully and they should continue in that role, but they should also be used to fulfill their promise in the interpretation of paleobiogeography, paleoecology, morphology, evolutionary processes and the origin of new groups and ground plans.
Late Neogene planktonic foraminiferal biostratigraphy of eight piston cores atop the Rio Grande Rise (2100-2300 m water depth) reveals the presence of Late Pleistocene, and intra-Pliocene, erosional unconformities. These unconformities span an aggregate time interval ranging from 1 to 12 m.y. The biostratigraphic-biochronologic framework suggests Late Neogene (pre-Pleistocene) average sedimentation rates of ca. 0.3 cm/1000 years and a continuous stratigraphic record over the interval 7-3 Ma and 14-12 Ma. Phylogenetic relationships within the Globorotalia miozea-conoidea and G. puncticulata-crassaformis lineages allow a refined 3- and 5-fold zonation of the Late Miocene (11-5 Ma) and Early Pliocene (5-3 Ma), respectively, of the mid-latitude Atlantic. The virtually simultaneous evolutionary appearance of G. conomiozea and G. mediterranea (from G. conoidea and G. miozea, respectively) at ca. 6.5 Ma and 6.3 Ma, respectively, and a brief (< 0.5 m.y.) overlap in the ranges of G. margaritae and Globoquadrina dehiscens corroborate recent findings elsewhere. The former events serve to identify the approximate position of the Tortonian/Messinian boundary, the latter to define a new, short zone at the top of the Miocene. The Miocene/Pliocene boundary is recognized in three piston cores and Site 357 on the basis of several multiple reinforcing criteria of regional (i.a., LAD of Globoquadrina dehiscens and Globorotalia mediterranea) and local (i.a., FAD of Globorotalia scitula and Globigerinoides conglobatus) significance.
A general morphometric procedure is described that organizes collections of microfossil outlines according to their shape. It involves representing the greatest proportion of variation observed among a collection of shapes by the least number of different shapes. Since these are determined as empirical orthogonal shape functions—eigenfunctions—of the observed shapes, the procedure is termed eigenshape analysis. Observed shapes are arranged and their shape differences systemized by reference to these determined eigenshape functions. The well-known ecophenotypic shape variation with latitude exhibited by the Pleistocene planktonic foraminifer Globorotalia truncatulinoides (d'Orbigny)serves as an example.
Techniques for biometric analysis of foraminifera
- Arnold
The growth spiral in some planktonic foraminifera from the Eocene of Denmark
- A Dinesen
The Globorotalia crassaformis bioseries in N
- Kennett
X-ray microscopy of Recent planktonic foraminifera
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