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Evolution of Neotropical biodiversity : phylogeny, ecology, and biogeography of Mesoeucrocodylia (Vertebrata Crocodyliformes) from the Miocene of Peruvian Amazonia
Abstract
Under the influential role of the Andean range, western Amazonia developed distinctive environmental conditions that ultimately led to divergent, higher biodiversity within the Neotropics. Although this intimate geologic-biotic interaction might have produced similar phenomena in the past, our knowledge about the tropical biotic evolution occurring in close proximity to these rapid growing mountains is poorly documented in the deep time. A pivotal time interval for the emergence of the modern Amazonian ecosystems occurred during the Miocene, when major Andean uplift remodeled the landscape of the foreland basin and fostered the onset of the Amazon River System, at about 10.5 million years ago. Proto-Amazonian biotas just prior to this episode are integral to understanding origins of Neotropical biodiversity, yet vertebrate fossil evidence was extraordinarily rare thus far. By studying the evolution, ecology, and biogeography of fossil mesoeucrocodylians (caimans, gharials, and sebecids) documented in new rich paleontological sites of eastern Peru, this research provides a snapshot of the florishing Miocene life of western proto-Amazonia. The crocodylian assemblage of the Iquitos bonebeds (middle-late Miocene transition) is extraordinary in representing both the highest taxonomic diversity (with up to seven associated species) and the widest range of snout morphotypes ever recorded in any crocodyliform community, recent or extinct. The heterogeneity of snout shapes at the Peruvian Miocene localities covers most of the morphospace range known for the entire crocodyliform clade reflecting the combined influences of long-term evolution, resource abundance and variety, and niche partitioning in a complex ecosystem, with no recent equivalent. Besides the large-bodied Purussaurus and Mourasuchus, all other crocodylians in Iquitos are new taxa, including a stem caiman—Gnatusuchus pebasensis—bearing a massive shovel-shaped mandible, procumbent anterior and globular posterior teeth, and a mammal-like diastema. This unusual species is an extreme exemplar of a radiation of small caimans with crushing dentitions recording peculiar feeding strategies correlated with a peak in proto-Amazonian molluscan diversity and abundance, deep in the so-called Pebas Mega-Wetland System. The sole long-snouted crocodylian in this community is the basalmost gavialoid of the Amazonian basin, a critical taxon that offers evidence for accurately reconstructing the ancestral anatomy and ecology of this clade. Including this new species in phylogenetic-morphometric analyses suggests that the evolution of the similar rostral pattern between South American and Indian gavialoids results from parallel evolution in riverine habitats. As part of the same prevailing Pebas biome, Fitzcarrald localities correspond to coeval paleoenvironments closer to the Andean influence (13-12 Ma). This fauna includes deep-snouted sebecids (Sebecosuchia) and advanced gavialoids (Gavialoidea) with protruding eyes, associated with a wide array of caimans (Mourasuchus, Purussaurus, Paleosuchus et Eocaiman), and further suggesting the presence of terrestrial settings and fluvially-dominated ecosystems. On the other hand, the highly endemic Iquitos faunas evolved within the dysoxic marshes and swamps typical of the long-lived Pebas Mega-Wetland System (early–early late Miocene) and declined with the inception of the transcontinental Amazon drainage, favoring diversification of longirostrine crocodylians and more modern generalist-feeding caimans. Indeed, the end of the Pebas Mega-Wetland System notably resulted in the reduction of the phylogenetic and morphotypical mesoeucrocodylian proto-Amazonian diversity, designating the beginning of the modern Amazonian faunas. The rise and demise of distinctive, highly productive aquatic ecosystems substantially influenced evolution of Amazonian biodiversity hotspots of crocodylians and other organisms throughout the Neogene.
... The most detailed anatomical study of M. nativus cranial material to date, including the holotype MLP 73-IV-15-8, was given by Bona et al. (2013a). Scheyer et al. (2013) Gismondi, 2015) and the late Miocene Yecua Formation in Bolivia (Tineo et al., 2015). The record of Mourasuchus from the Pleistocene of Tarija, Bolivia (e.g., Bocquentin and Souza Filho, 1990;Aguilera, 2004), however, remains doubtful. ...
The late Miocene Urumaco Formation at Urumaco, Falcón state, Venezuela, is remarkably rich in extinct crocodylians, presenting a diversity hotspot in the Neotropics for the group. Herein, we revise the Caimaninae fauna by including novel fossil material as well as the previously described specimens assignable to this clade. In many instances the taxonomic status of species could be confirmed, which is the case in Caiman brevirostris, Globidentosuchus brevirostris, and Purussaurus mirandai, and novel osteological data is presented to corroborate previous anatomical descriptions. In other cases, specimens needed to be reassigned to different taxa; with material previously identified as Caiman lutescens now considered as belonging to either Caiman latirostris or Caiman wannlangstoni, and material of Melanosuchus fisheri reassigned to Caimaninae aff. Melanosuchus fisheri. Furthermore, Mourasuchus nativus is considered to be a junior synonym of Mourasuchus arendsi herein. This suggests that there are only three species of the duck-billed caimanine Mourasuchus present in the Miocene of South America, having colonised the continent from the northwest (Colombia and Peru) during the middle Miocene and moving to the east and southeast (Venezuela, Brazil and Argentina) in the late Miocene. Other specimens, which were previously identified as belonging to the genus Caiman, lack diagnostic features of the modern genus and are instead considered as Caimaninae indet. Besides improving the knowledge of the late Miocene crocodylians of South America, our results confirm the high taxonomic diversity of the fauna and the outstanding level of sympatry previously reported for the Urumaco Formation
Thirteen species of fossil molluscs are reported from the Solimões Formation of western Brazilian Amazonia. Based on mammalian chronology of the Solimões Formation and radiometric ages reported from coeval deposits in adjacent Peru, the age of the fauna is established as Late Miocene. The fauna includes five prosobranch gastropod species, seven pearly freshwater mussel species and one sphaeriid bivalve species. The supposed presence of Pachydon (Corbulidae: Bivalvia) in these deposits is rejected; Pachydon acreanum, whose status has long been uncertain, is transferred to the unionoid genus Callonaia. The Solimões mollusc fauna is entirely composed of obligate freshwater taxa, resembling species-poor modern Amazonian fluvial faunas. The presence of the fauna in outcrops covering large parts of western Amazonia indicates that by that time the preceding Pebas fauna (dominated by corbulid bivalves and cochliopid snails) must have been extinguished.
A gavialoid crocodylian from the Maastrichtian of the Oulad Abdoun phosphatic Basin (Morocco) is described, representing the oldest known crocodylian from Africa. The specimen consists of a skull that exhibits several features not found in other gavialoids, and a new genus and species is erected, Ocepesuchus eoafricanus. A phylogenetic analysis has been conducted including 201 characters and 71 taxa, where Ocepesuchus eoafricanus appears as the most basal African gavialoid, and the South American gavialoids are paraphyletic. This paraphyly has strong biogeographic implications, and the previous hypothesis of South American and Asian assemblages derived from African gavialoids should be reviewed. The historical biogeography of Gavialoidea is probably more complex than previously supposed. The phosphatic deposits of Morocco provide a unique opportunity to study the vertebrate faunal turnover across the Creta-ceous-Tertiary (KT) boundary. The crocodyliforms are very scarce in the Maastrichtian marine basins of Africa which are dominated by mosasaurid squamates. The latter became extinct by the KT boundary, while crocodyliforms survived and diversified in the Paleocene. Mosasaurids and crocodyliforms both lived in probably comparable marine environments during the Maastrichtian. The selectivity of the KT boundary extinctions remains to be explained; since freshwater environments are known for having been less affected by the KT crisis than marine ones, a freshwater lifestyle of the juveniles, like in extant marine crocodiles and unlike the fully marine mosasaurs, could explain this difference with regard to survivorship.
When and how non-‐volant vertebrates of South American origin became established in the Caribbean Archipelago is a matter of arduous debate. This phylogenetic disparate assembly includes extinct and/or extant caviomorph rodents, megalonychid sloths, platyrrhine primates, bufonid toads, cichlid fishes, and a gharial crocodylian that might have reached the Caribbean islands sometime during the Cenozoic. Although geological evidence reveals sea barriers isolating this Archipelago from South America since the Paleocene, the occurrence of short-‐term terrestrial paths has not been fully falsified. Unfortunately, the pre-‐Quaternary fossil record is scarce and it does not provide significant data about early Cenozoic life within the Caribbean Archipelago. Given the complexity of this scenario, biogeographical hypotheses range from Cretaceous vicariance models to Cenozoic dispersals by land bridges (e.g., GAARlandia) or rafting. Here, we would like to highlight the overlooked existence of an ancient South American drainage discharging into the Caribbean Sea by the eastern modern coast of Venezuela. This trunk river of northern flow was deeply rooted in western proto-‐ Amazonia, between the growing Andes and the Guyana Craton, from the Paleocene until the Middle-‐Late Miocene transition. Large rivers in tropical coastal areas have been acknowledged as key elements for the biotic dispersal (i.e., by rafting) through sea barriers, interestingly involving Caribbean clades such as caviomorph rodents and platyrrhine primates as models. In fact, the restricted higher-‐order taxonomic composition of the Caribbean fauna implies a selective pattern of dispersal within the biotic entities at the source, comprising also secondarily freshwater clades (gharials and cichlid fishes) and taxa with marine relatives (megalonychid sloths). In western Amazonia, most Caribbean groups have been recorded as fossils during the time interval this northern flow drainage might have acted as a dispersal catalyst. Relative to GAARlandia hypothesis, this model fits better with distinct and diachronous events of colonization throughout the Cenozoic, as has been suggested by molecular data analyses. Caribbean Sea currents running northwestward provide further support for the riverine model discussed herein and originally proposed by Hedges in 1996. Although this model is not new, there is now evidence of the South American river that would have promoted dispersals to the Caribbean Archipelago.
Aim
The subduction of the Nazca Plate and the eastward propagation of the Andean orogenic wedge in western Amazonia caused the formation of arches or ridges that have influenced the modern configuration of the upper Amazon drainage and the diversification of biota. We used a lineage of 15 palm species ( Astrocaryum sect. Huicungo , Arecaceae) to test two biogeographical hypotheses for lowland plants: (1) that vicariance resulted from tectonically mediated geographical barriers (population contraction), and (2) that recurrent dispersal events (population expansion) produced geographical isolation and subsequent speciation.
Location
Rain forests of South America.
Methods
A total of 78 palm individuals were collected in the field, from which five chloroplast and two nuclear DNA fragments were sequenced. We reconstructed a Bayesian dated phylogeny and inferred the demographic history. We used a Bayesian phylogeographical spatial diffusion approach to propose a model of colonization.
Results
We found a phylogeographical break at c . 5° S between two main clades with crown ages of c . 6.7 and 7.3 Ma located in the Fitzcarrald Arch ( FA ) and the subsiding northern Amazonian foreland basin ( NAFB ), respectively. These diversification times were close to the emergence of the FA in the late Miocene, and the coeval development of the transcontinental modern drainage and sedimentation plain of the NAFB . As expected for the recurrent‐dispersal hypothesis, lineage delimitations were spatially inconsistent with the location of rivers or ridges, and we found some evidence of past ancestral population expansion supported particularly by the chloroplast sequences.
Main conclusions
Our results support the biogeographical scenario whereby recurrent dispersal into western Amazonia produced spatial isolation of populations, followed by speciation within two areas of contrasting geological activity: tectonic uplift in the FA versus subsidence in the NAFB . We did not test and cannot rule out ecological speciation within western Amazonia or at finer spatial scales.
The Cocinetas Basin of Colombia provides a valuable window into the geological and paleontological history of northern South America during the Neogene. Two major findings provide new insights into the Neogene history of this Cocinetas Basin: (1) a formal re-description of the Jimol and Castilletes formations, including a revised contact; and (2) the description of a new lithostratigraphic unit, the Ware Formation (Late Pliocene). We conducted extensive fieldwork to develop a basin-scale stratigraphy, made exhaustive paleontological collections, and performed 87Sr/86Sr geochronology to document the transition from the fully marine environment of the Jimol Formation (ca. 17.9–16.7 Ma) to the fluvio-deltaic environment of the Castilletes (ca. 16.7–14.2 Ma) and Ware (ca. 3.5–2.8 Ma) formations. We also describe evidence for short-term periodic changes in depositional environments in the Jimol and Castilletes formations. The marine invertebrate fauna of the Jimol and Castilletes formations are among the richest yet recorded from Colombia during the Neogene. The Castilletes and Ware formations have also yielded diverse and biogeographically significant fossil vertebrate assemblages. The revised lithostratigraphy and chronostratigraphy presented here provides the necessary background information to explore the complete evolutionary and biogeographic significance of the excellent fossil record of the Cocinetas Basin.
Crocodilian remains are among the most abundant fossils in the best known Miocene deposits from northern South America (Acre, Urumaco and La Venta). Bony dermal armor plates (osteoderms), teeth and vertebrae are a common find, among reptile fossils second only to turtle shell bones. Rarer, more complete remains, such as skull and mandible fragments, limb bones, and even partial skeletons can be found. These remains show that most Miocene South American crocodilians exhibited a remarkable diversity, including some of the weirdest known crocodilians of all time. Several groups of crocodiles inhabited South America during Miocene times; some of them belong to groups still alive today, while other groups are now extinct or can be found only outside South America.
In this chapter we summarize the most salient aspects of the evolution of fossil crocodiles from Venezuela, where the greatest diversity is found in the Urumaco sequence of Falcón State. The Urumaco sequence, which includes the middle Miocene Socorro Formation, the late Miocene Urumaco Formation, and the upper Miocene-lower Pliocene Codore Formation, is the most fossiliferous in Venezuela, including at least twelve described species of fossil crocodilians (Aguilera, 2004; Sánchez-Villagra & Aguilera, 2006). Stratigraphically older than the Urumaco sequence is the lower Miocene Castillo Formation, which includes the Cerro la Cruz and Siquisique localities (e.g., Sánchez et al., 2004; Sánchez-Villagra & Aguilera, 2006). The crocodilian fossils, especially the skulls and mandibles, recovered from these Formations are usually preserved three-dimensionally; however the state of preservation ranges considerably from excellent to poor (see also section on the bone microstructure of the crocodilian armor below). The fossil diversity of Venezuelan crocodiles presents a remarkable contrast with the living species found in South America today, which have a much more restricted ecomorphological diversity.
Amazonia contains one of the world's richest biotas, but origins of this diversity remain obscure. Onset of the Amazon River drainage at approximately 10.5 Ma represented a major shift in Neotropical ecosystems, and proto-Amazonian biotas just prior to this pivotal episode are integral to understanding origins of Amazonian biodiversity, yet vertebrate fossil evidence is extraordinarily rare. Two new species-rich bonebeds from late Middle Miocene proto-Amazonian deposits of northeastern Peru document the same hyperdiverse assemblage of seven co-occurring crocodylian species. Besides the large-bodied Purussaurus and Mourasuchus, all other crocodylians are new taxa, including a stem caiman-Gnatusuchus pebasensis-bearing a massive shovel-shaped mandible, procumbent anterior and globular posterior teeth, and a mammal-like diastema. This unusual species is an extreme exemplar of a radiation of small caimans with crushing dentitions recording peculiar feeding strategies correlated with a peak in proto-Amazonian molluscan diversity and abundance. These faunas evolved within dysoxic marshes and swamps of the long-lived Pebas Mega-Wetland System and declined with inception of the transcontinental Amazon drainage, favouring diversification of longirostrine crocodylians and more modern generalist-feeding caimans. The rise and demise of distinctive, highly productive aquatic ecosystems substantially influenced evolution of Amazonian biodiversity hotspots of crocodylians and other organisms throughout the Neogene.
© 2015 The Author(s) Published by the Royal Society. All rights reserved.
Purussaurus brasiliensis thrived in the northwestern portion of South America during the Late Miocene. Although substantial material has been recovered since its early discovery, this fossil crocodilian can still be considered as very poorly understood. In the present work, we used regression equations based on modern crocodilians to present novel details about the morphometry, bite-force and paleobiology of this species. According to our results, an adult Purussaurus brasiliensis was estimated to reach around 12.5 m in length, weighing around 8.4 metric tons, with a mean daily food intake of 40.6 kg. It was capable of generating sustained bite forces of 69,000 N (around 7 metric tons-force). The extreme size and strength reached by this animal seems to have allowed it to include a wide range of prey in its diet, making it a top predator in its ecosystem. As an adult, it would have preyed upon large to very large vertebrates, and, being unmatched by any other carnivore, it avoided competition. The evolution of a large body size granted P. brasiliensis many advantages, but it may also have led to its vulnerability. The constantly changing environment on a large geological scale may have reduced its long-term survival, favoring smaller species more resilient to ecological shifts.
In the evolution of crocodylomorphs, there were at least three giant-dimension genera: Deinosuchus from late Cretaceous of North America, Sarcosuchus from middle Cretaceous of Africa and South America, and Purussaurus from Miocene of South America. It has been suggested that these predators could have fed on very large prey as dinosaurs and megamammals. The ‘death roll’ is a spinning manoeuver executed to subdue and dismember large prey; therefore, it has been previously suggested that giant cocrodylomorphs may have used this manoeuver. However, this manoeuver can generate torsional stresses in the skull. We propose a biomechanical model to estimate the capability of a crocodylomorphs for withstanding this torsional stresses. Our results show a good correlation between a ‘death roll’ capability indicator and the feeding categories related with the actual use of ‘death roll’ in 16 living species. Here, for the first time, we propose a biomechanical approach of the implications of ‘death roll’ in fossil crocodylomorphs. We suggest that Deinosuchus and Purussaurus were able to execute death roll over dinosaurs and large mammals, respectively, but Sarcosuchus probably was not. We also found some allometry effects and, finally, we discuss palaeobiological implications based on our results.
The Middle Miocene has been identified as a time of great diversification in modern lineages now distrib-uted in tropical South America, and when basic archetypal traits defining Amazonia appear, including climatic humid conditions, basic floral physiognomy and phylogenetic com-position of modern rainforests. Nonetheless, Middle Miocene localities in South America are poorly known, especially at low latitudes where only one species-rich locality, La Venta in Colombia, has been extensively studied. The present con-tribution describes the mammal fauna of Fitzcarrald, a new Middle Miocene local fauna from western Amazonia in Peru. Fitzcarrald is correlated with the Laventan South American Land Mammal Age based on the presence of taxa defining the 'Miocochilius assemblage zone' in La Venta. The mamma-lian fauna of Fitzcarrald comprises 24 taxa among cingulates, folivores, astrapotheres, notoungulates, litopterns, rodents, odontocetes and a possible marsupial. At this time, tropical South America was characterized by the presence of the Pebas megawetland, a huge lacustrine complex that provided unique ecological and environmental conditions most likely isolating northern South America from southern South America. These isolating conditions might have come to an end with its disappearance in the Late Miocene and the establishment of the subsequent Acre system, the predecessor fluvial system of modern Amazonia. Results of faunistic simi-larity between Fitzcarrald and other Miocene faunas through-out South America support these scenarios. The Fitzcarrald mammal fauna exhibits first appearance datums and last appearance datums of various taxa, showing that tropical South America has played a crucial role in the evolutionary history and biogeography of major clades, and revealing a more complex biological history than previously proposed, based on the record from the southern cone of the continent.
Study of the evolution of crocodile-line archosaurs (Pseudosuchia) has intensified recently. Along with this increased attention have come dramatic changes in our understanding of the evolutionary relationships of the group. Modern crocodylians are generally similar-looking with most of their morphological disparity expressed in their skulls. However, phylogenetic studies have demonstrated that skull shape is highly convergent, with similar skull forms arising in distantly related lineages. Yet, crocodylians and their relatives (crocodyliforms) have traditionally been divided into taxonomic and ecomorphological groups based on skull morphology. Many of the crocodyliform groups with the most controversial affinities also share the same skull shape (long, slender snouts). This issue was assessed from multiple angles including geometric morphometrics, detailed assessment of anatomy, and expanded phylogenetic analyses. Previously proposed skull shape categories were based largely on assessments of snout length and width but ignored important features in the rest of the skull. I applied two-dimensional geometric morphometrics including landmarks and sliding semilandmarks to characterize the skull shape of 131 extant and extinct crocodyliforms. Newly developed shape categories based on normal mixture analysis show a weak correlation to phylogeny corroborating hypotheses of convergent evolution. Different skull shapes are not evenly distributed through time and the results presented here indicate that crocodyliform disparity peaked in the Late Cretaceous followed by a dramatic decline in the Maastrichtian (preceding the end-Cretaceous extinction). The emptied region of morphospace, exemplified by a short and narrow snout, was never explored by crown-group crocodylians. Modern crocodylian disparity is much lower than in the Cretaceous, but is similar to Jurassic levels. The recent increases in knowledge of pseudosuchian relationships have important implications for phylogenetic studies of crocodylomorphs. Previous studies of crocodylomorph and crocodyliform relationships have rooted trees on outgroups that in many cases are either quite distantly related to the group under study (e.g. Gracilisuchus stipanicicorum, a basal suchian), or could actually belong within the ingroup. Thalattosuchia, one of the earliest occurring groups of crocodyliforms (Early Jurassic, Sinemurian), has a controversial phylogenetic position relative to other crocodyliforms — they are recovered in either a basal position, nested high up in the tree, or sister to Crocodyliformes. Thalattosuchians lack several crocodyliform apomorphies, but share a longirostrine skull shape with highly derived neosuchian groups. These groups share a similar ecological habit, suggesting the derived position of thalattosuchians may be the result of convergent evolution. Several of the "shared" characters uniting these groups are due to ambiguously worded character state definitions - structures that are superficially similar (though anatomically different in detail) are scored the same. A new analysis of crocodylomorphs recovers the thalattosuchians as the sister-group to Crocodyliformes outside of the "protosuchians" and all other crocodyliforms. It is also demonstrated that the addition of the rauisuchid Postosuchus kirkpatricki to the outgroup of previously published analyses draws thalattosuchians down the tree as the sister-group of Crocodyliformes. These results demonstrate the importance of careful outgroup sampling, and highlight issues with current descriptions of morphological characters.
Landmark-based geometric morphometrics is a powerful approach to quantifying biological shape, shape variation, and covariation of shape with other biotic or abiotic variables or factors. The resulting graphical representations of shape differences are visually appealing and intuitive. This paper serves as an introduction to common exploratory and confirmatory techniques in landmark-based geometric morphometrics. The issues most frequently faced by (paleo)biologists conducting studies of comparative morphology are covered. Acquisition of landmark and semilandmark data is discussed. There are several methods for superimposing landmark configurations, differing in how and in the degree to which among-configuration differences in location, scale, and size are removed. Partial Procrustes superimposition is the most widely used superimposition method and forms the basis for many subsequent operations in geometric morphometrics. Shape variation among superimposed configurations can be visualized as a scatter plot of landmark coordinates, as vectors of landmark displacement, as a thin-plate spline deformation grid, or through a principal components analysis of landmark coordinates or warp scores. The amount of difference in shape between two configurations can be quantified as the partial Procrustes distance; and shape variation within a sample can be quantified as the average partial Procrustes distance from the sample mean. Statistical testing of difference in mean shape between samples using warp scores as variables can be achieved through a standard Hotelling's T ² test, MANOVA, or canonical variates analysis (CVA). A nonparametric equivalent to MANOVA or Goodall's F-test can be used in analysis of Procrustes coordinates or Procrustes distance, respectively. CVA can also be used to determine the confidence with which a priori specimen classification is supported by shape data, and to assign unclassified specimens to pre-defined groups (assuming that the specimen actually belongs in one of the pre-defined groups).
Examples involving Cambrian olenelloid trilobites are used to illustrate how the various techniques work and their practical application to data. Mathematical details of the techniques are provided as supplemental online material. A guide to conducting the analyses in the free Integrated Morphometrics Package software is provided in the appendix.
Non-marine Tertiary molluscan faunas of Colombia, Ecuador and the "Pebasian' or the Upper Amazon Basin of eastern Peru and adjoining parts of Brazil are reviewed, and compared with the living fauna of South America. New Colombian fossil faunas from La Tagua, on Rio Caqueta, and Puerto Narino, on Rio Maranon, are described: the latter is Pebasian. The new La Tagua fauna provides a valuable link, permitting correlation between the Santa Teresa (San Juan de Rio Seco of Anderson, 1928), La Cira (both Magdalena Valley) and Pebasian faunas. Evidence is presented that during the mid-Tertiary a brackish water connection existed between the Upper Amazon Valley and the Caribbean by a north-south trough, lying parallel to the still rising Andes and occupied by a continually shifting pattern of streams, swamps and lakes of varying salinity. Similarities between the La Cira and Santa Teresa faunas of the Magdalena Valley and those of La Tagua demonstrate that they were all deposited before the Andean orogeny at the close of the Miocene raised the Cordillera Oriental to form a mountain barrier impenetrable to aquatic molluscs between the Magdalena and Upper Amazon Valleys. Taxonomic conclusions are described. -from Author
Gondwana, though extant for approximately one-half billion years, is now present as fragments across much of the globe. Following an assembly during the latest Proterozoic into the early Phanerozoic, the megacontinent has gradually fragmented to its current dispersed pattern. Paleozoic fragmentation, primarily on its north and west margins, formed a series of ribbon-shaped continents that collided with southern Laurasia and generated major orogenic events. Meanwhile, much of its southern and eastern margin was the site of subduction and associated Cordilleran-style tectonics. Mesozoic and Paleogene rifting completed the fragmentation, sending continents northward to generate the Alpine-Himalayan mountain chain from Spain to China. Much of Gondwana flirted with the South Pole throughout the Paleozoic, and several major glacial episodes resulted. The largest and most extensive of these was the late Paleozoic ice age; the consequences of this event dominated global geology for nearly 100 m.y. and orchestrated the greatest cyclic stratigraphic record in Phanerozoic history.
Outgroup sampling is a central issue in phylogenetic analysis. However, good justification is rarely given for outgroup selection in published analyses. Recent advances in our understanding of archosaur phylogeny suggest that many previous studies of crocodylomorph and crocodyliform relationships have rooted trees on outgroup taxa that are only very distantly related to the ingroup (e.g., Gracilisuchus stipanicicorum), or might actually belong within the ingroup. Thalattosuchia, a group of Mesozoic marine crocodylomorphs, has a controversial phylogenetic position - they are recovered as either the sister group to Crocodyliformes, in a basal position within Crocodyliformes, or nested high in the crocodyliform tree. Thalattosuchians lack several crocodyliform apomorphies, but share several character states with derived long-snouted forms with a similar ecological habit, suggesting their derived position may be a result of convergent evolution. Several of these “shared” characters may result from ambiguously worded character state definitions - structures that are superficially similar but anatomically different in detail are identically coded. A new analysis of crocodylomorphs with increased outgroup sampling recovers Thalattosuchia as the sister group to Crocodyliformes, distantly related to long-snouted crocodyliforms. I also demonstrate that expanding the outgroup sampling of previously published matrices results in the recovery of thalattosuchians as sister to Crocodyliformes. The exclusion of thalattosuchians from Crocodyliformes has numerous implications for large-scale evolutionary trends within the group, including extensive convergence in the evolution of the secondary palate characteristic of the group. These results demonstrate the importance of careful outgroup sampling and character construction, and their profound effect on the position of labile clades.
Recent investigations into the evolution of the living Crocodilia, belonging to the suborder Eusuchia, have revealed that the genus Gavialis may be its most primitive living member. New morphological studies have shown that the braincase structure, neural pocket, air sinus systems, jaw adductor mechanisms, pelvic and hindlimb morphology and epaxial musculature of the caudal region of Gavialis gangeticus do not correspond to the rest of the living Eusuchia. Contrary to the morphological findings, recent biochemical studies suggest a sister group relationship between Gavialis gangeticus and Tomistoma schlegelii, another longirostrine eusuchian. Judged by its morphology, Tomistoma is merely another member of the genus Crocodylus within the Eusuchia. This conflict in data either means that not enough of the genome of both Gavialis and Tomistoma is known, the shared genome represents the primitive states for these genes or that similar genotypes can give rise to rather different morphologies. As Gavialis resembles in some ways a Mesozoic level of organization it is considered to be a surviving eusuchian relict.
The relationships of the three living groups of crocodilians (crocodylids, alligatorids and gavialids) are poorly understood. Recent molecular results favor a sister group relationship between the crocodylid genus Tomistoma and Gavialis, with this as the sister group to the crocodylids (Densmore, 1983). Buffetaut (1985) has reinterpreted some morphologic evidence as supportive of this viewpoint. This morphologic evidence is examined here using outgroup analysis; it fails to support this hypothesis. Few if any morphological features unambiguously support a Gavialis + crocodylid or Gavialis + Tomistoma relationship. Instead the classic pre-Darwinian phylogeny of Duméril (1806) is corroborated by anatomical evidence. This phylogeny supports a monophyletic crocodylid + alligatorid clade as a monophyletic sister group to Gavialis.
Modifications of the jaw mechanism of the lizard, Dracaena guianensis (family Teiidae), for durophagy (feeding upon hard-shelled foods) are described on the basis of comparisons with its close relative Tupinambis. There is an enlargement of sites for jaw muscle attachments and strengthening of palatal architecture for the crushing mode of feeding. The distinctively flared quadrate allows for enlargement of the Add. mand. externus muscle, resulting in improvement of the mechanical advantage of the muscle as well as reinforcement of the jaw articulation. Study of the dentition in relation to size reveals that enlarged and flattened teeth are present even in smaller lizards and are not the result of ontogenetic alteration. The state of the constrictor dorsalis muscles and the intracranial joints indicate that cranial kinetic motions may occur during feeding. These findings indicate the need for re-evaluation of theories of the adaptive significance of cranial kinesis in lizards.
