Crown Turtles Research Articles

A systematic comparative description of extant turtle humeri, with comments on humerus disparity and evolution based on fossil comparisons.

The humerus is central for locomotion in turtles as quadrupedal animals. Osteological variation across testudine clades remains poorly documented. Here, we systematically describe the humerus anatomy for all major extant turtle clades based on 38 species representing the phylogenetic and ecological diversity of crown turtles. Three Late Triassic species of shelled stem turtles (Testudindata) are included to establish the plesiomorphic humerus morphology. Our work is based on 3D models, establishing a publicly available digital database. Previously defined terms for anatomical sides of the humerus (e.g., dorsal, ventral) are often not aligned with the respective body sides in turtles and other quadrupedal animals with sprawling gait. We propose alternative anatomical directional terms to simplify communication: radial and ulnar (the sides articulating with the radius/ulna), capitular (the side bearing the humeral head), and intertubercular (opposite to capitular surface). Turtle humeri show low morphological variation with exceptions concentrated in locomotory specialists. We propose 15 discrete characters to summarize osteological variation for future phylogenetic studies. Disparity analyses comparing non-shelled and shelled turtles indicate that the presence of the shell constrains humerus variation. Flippered aquatic turtles are released from this constraint and significantly increase overall disparity. Ontogenetic changes of turtle humeri are related to increased ossification and pronunciation of the proximal processes, the distal articulation areas, and the closure of the ectepicondylar groove to a foramen. Some turtle species retain juvenile features into adulthood and provide evidence for paedomorphic evolution. We review major changes of turtle humerus morphology throughout the evolution of its stem group.

The topological organization of the turtle cranium is constrained and conserved over long evolutionary timescales.

The cranium of turtles (Testudines) is characterized by the secondary reduction of temporal fenestrae and loss of cranial joints (i.e., characteristics of anapsid, akinetic skulls). Evolution and ontogeny of the turtle cranium are associated with shape changes. Cranial shape variation among Testudines can partially be explained by dietary and functional adaptations (neck retraction), but it is unclear if cranial topology shows similar ecomorphological signal, or if it is decoupled from shape evolution. We assess the topological arrangement of cranial bones (i.e., number, relative positioning, connections), using anatomical network analysis. Non-shelled stem turtles have similar cranial arrangements to archosauromorph outgroups. Shelled turtles (Testudinata) evolve a unique cranial organization that is associated with bone losses (e.g., supratemporal, lacrimal, ectopterygoid) and an increase in complexity (i.e., densely and highly interconnected skulls with low path lengths between bones), resulting from the closure of skull openings and establishment of unusual connections such as a parietal-pterygoid contact in the secondary braincase. Topological changes evolutionarily predate many shape changes. Topological variation and taxonomic morphospace discrimination among crown turtles are low, indicating that cranial topology may be constrained. Observed variation results from repeated losses of nonintegral bones (i.e., premaxilla, nasal, epipterygoid, quadratojugal), and changes in temporal emarginations and palate construction. We observe only minor ontogenetic changes. Topology is not influenced by diet and habitat, contrasting cranial shape. Our results indicate that turtles have a unique cranial topology among reptiles that is conserved after its initial establishment, and shows that cranial topology and shape have different evolutionary histories.

Cranial morphology of Heckerochelys romani Sukhanov, 2006, a stem turtle from the Middle Jurassic of European Russia, with implications for the paleoecology of stem turtles

ABSTRACT The detailed morphological description of the skull of the stem turtle Heckerochelys romani from the Middle Jurassic (Bathonian) of the Peski locality, Moscow Region, Russia, is provided herein. All cranial structures of Heckerochelys romani prove to be more advanced than in the Triassic and Early Jurassic stem turtles, but more primitive than in crown turtles; its morphology can be basic for both Cryptodira and Pleurodira. Only a few insignificant differences in skull morphology could be found between Heckerochelys romani and Eileanchelys waldmani, except for the size. Several cranial characters of Heckerochelys romani, such as architecture of the feeding apparatus and geometry of the cavum tympani, may indicate the aquatic habitat preference for this turtle, which is supported by taphonomic conditions.

Hyperphalangy in a new sinemydid turtle from the Early Cretaceous Jehol Biota.

Hyperphalangy is a rare condition in extant aquatic turtles, and mainly limited to soft-shelled turtles. Here we report a new freshwater turtle, Jeholochelys lingyuanensis gen. et sp. nov. from the Early Cretaceous Jehol Biota of western Liaoning, China. This new turtle is characterized by a hyperphalangy condition with one additional phalanx in pedal digit V, rather than the primitive condition (phalangeal formula: 2-3-3-3-3) of crown turtles. J. lingyuanensis is recovered with other coexisting turtles in the family Sinemydidae in the phylogenetic analysis. This discovery further confirms that hyperphalangy occurred multiple times in the early evolutionary history of the crown turtles. Hyperphalangy is possibly a homoplasy in Jeholochelys and the soft-shelled turtles to adapt to the aquatic environments.

Phylogenomic analyses of 539 highly informative loci dates a fully resolved time tree for the major clades of living turtles (Testudines)

Accurate time-calibrated phylogenies are the centerpiece of many macroevolutionary studies, and the relationship between the size and scale of molecular data sets and the density and accuracy of fossil calibrations is a key element of time tree studies. Here, we develop a target capture array specifically for living turtles, compare its efficiency to an ultraconserved element (UCE) dataset, and present a time-calibrated molecular phylogeny based on 539 nuclear loci sequenced from 26 species representing the breadth of living turtle diversity plus outgroups. Our gene array, based on three fully sequenced turtle genomes, is 2.4 times more variable across turtles than a recently published UCE data set for an identical subset of 13 species, confirming that taxon-specific arrays return more informative data per sequencing effort than UCEs. We used our genomic data to estimate the ages of living turtle clades including a mid-late Triassic origin for crown turtles and a mid-Carboniferous split of turtles from their sister group, Archosauria. By specifically excluding several of the earliest potential crown turtle fossils and limiting the age of fossil calibration points to the unambiguous crown lineage Caribemys oxfordiensis from the Late Jurassic (Oxfordian, 163.5–157.3Ma) we corroborate a relatively ancient age for living turtles. We also provide novel age estimates for five of the ten testudine families containing more than a single species, as well as several intrafamilial clades. Most of the diversity of crown turtles appears to date to the Paleogene, well after the Cretaceous-Paleogene mass extinction 66mya.

Multilocus phylogeny and statistical biogeography clarify the evolutionary history of major lineages of turtles

Despite their complex evolutionary history and the rich fossil record, the higher level phylogeny and historical biogeography of living turtles have not been investigated in a comprehensive and statistical framework. To tackle these issues, we assembled a large molecular dataset, maximizing both taxonomic and gene sampling. As different models provide alternative biogeographical scenarios, we have explicitly tested such hypotheses in order to reconstruct a robust biogeographical history of Testudines. We scanned publicly available databases for nucleotide sequences and composed a dataset comprising 13 loci for 294 living species of Testudines, which accounts for all living genera and 85% of their extant species diversity. Phylogenetic relationships and species divergence times were estimated using a thorough evaluation of fossil information as calibration priors. We then carried out the analysis of historical biogeography of Testudines in a fully statistical framework. Our study recovered the first large-scale phylogeny of turtles with well-supported relationships following the topology proposed by phylogenomic works. Our dating result consistently indicated that the origin of the main clades, Pleurodira and Cryptodira, occurred in the early Jurassic. The phylogenetic and historical biogeographical inferences permitted us to clarify how geological events affected the evolutionary dynamics of crown turtles. For instance, our analyses support the hypothesis that the breakup of Pangaea would have driven the divergence between the cryptodiran and pleurodiran lineages. The reticulated pattern in the ancestral distribution of the cryptodiran lineage suggests a complex biogeographic history for the clade, which was supposedly related to the complex paleogeographic history of Laurasia. On the other hand, the biogeographical history of Pleurodira indicated a tight correlation with the paleogeography of the Gondwanan landmasses.

A Review of the Fossil Record of Basal Mesozoic Turtles

Abstract Turtles (Testudinata) are the clade of amniotes characterized by a complete turtle shell. New insights into the phylogeny of the group have revealed that a diverse assemblage of fossil turtles populate the stem lineage that lead to the turtle crown (Testudines). To aid communication, the terms Mesochelydia and Perichelydia are herein defined for two internested clades more inclusive than Testudines but less inclusive than Testudinata. The earliest representatives of Testudinata are found globally in Late Triassic (Norian) to Middle Jurassic deposits. In concert with the vicariant split of crown Testudines into three primary clades (i.e., Paracryptodira, Pan-Pleurodira, and Pan-Cryptodira), basal perichelydians diversify into three additional clades with overlapping geographic distributions: Helochelydridae in Euramerica, Sichuanchelyidae in Asia, and Meiolaniformes in southern Gondwana. Sedimentological, anatomical, and histological data universally hint at terrestrial habitat preference among th...

A toothed turtle from the Late Jurassic of China and the global biogeographic history of turtles.

BackgroundTurtles (Testudinata) are a successful lineage of vertebrates with about 350 extant species that inhabit all major oceans and landmasses with tropical to temperate climates. The rich fossil record of turtles documents the adaptation of various sub-lineages to a broad range of habitat preferences, but a synthetic biogeographic model is still lacking for the group.ResultsWe herein describe a new species of fossil turtle from the Late Jurassic of Xinjiang, China, Sichuanchelys palatodentata sp. nov., that is highly unusual by plesiomorphically exhibiting palatal teeth. Phylogenetic analysis places the Late Jurassic Sichuanchelys palatodentata in a clade with the Late Cretaceous Mongolochelys efremovi outside crown group Testudines thereby establishing the prolonged presence of a previously unrecognized clade of turtles in Asia, herein named Sichuanchelyidae. In contrast to previous hypotheses, M. efremovi and Kallokibotion bajazidi are not found within Meiolaniformes, a clade that is here reinterpreted as being restricted to Gondwana.ConclusionsA revision of the global distribution of fossil and recent turtle reveals that the three primary lineages of derived, aquatic turtles, including the crown, Paracryptodira, Pan-Pleurodira, and Pan-Cryptodira can be traced back to the Middle Jurassic of Euramerica, Gondwana, and Asia, respectively, which resulted from the primary break up of Pangaea at that time. The two primary lineages of Pleurodira, Pan-Pelomedusoides and Pan-Chelidae, can similarly be traced back to the Cretaceous of northern and southern Gondwana, respectively, which were separated from one another by a large desert zone during that time. The primary divergence of crown turtles was therefore driven by vicariance to the primary freshwater aquatic habitat of these lineages. The temporally persistent lineages of basal turtles, Helochelydridae, Meiolaniformes, Sichuanchelyidae, can similarly be traced back to the Late Mesozoic of Euramerica, southern Gondwana, and Asia. Given the ambiguous phylogenetic relationships of these three lineages, it is unclear if their diversification was driven by vicariance as well, or if they display a vicariance-like pattern. The clean, primary signal apparent among early turtles is secondarily obliterated throughout the Late Cretaceous to Recent by extensive dispersal of continental turtles and by multiple invasions of marine habitats.Electronic supplementary materialThe online version of this article (doi:10.1186/s12862-016-0762-5) contains supplementary material, which is available to authorized users.

A sinemydid turtle from the Jehol Biota provides insights into the basal divergence of crown turtles.

Morphological phylogenies stand in a major conflict with molecular hypotheses regarding the phylogeny of Cryptodira, the most diverse and widely distributed clade of extant turtles. However, molecular hypotheses are often considered a better estimate of phylogeny given that it is more consistent with the stratigraphic and geographic distribution of extinct taxa. That morphology fails to reproduce the molecular topology partly originates from problematic character polarization due to yet another contradiction around the composition of the cryptodiran stem lineage. Extinct sinemydids are one of these problematic clades: they have been either placed among stem-cryptodires, stem-chelonioid sea turtles, or even stem-turtles. A new sinemydid from the Early Cretaceous Jehol Biota (Yixian Formation, Barremian-Early Aptian) of China, Xiaochelys ningchengensis gen. et sp. nov., allows for a reassessment of the phylogenetic position of Sinemydidae. Our analysis indicates that sinemydids mostly share symplesiomorphies with sea turtles and their purported placement outside the crown-group of turtles is an artefact of previous datasets. The best current phylogenetic estimate is therefore that sinemydids are part of the stem lineage of Cryptodira together with an array of other Jurassic to Cretaceous taxa. Our study further emphasises the importance of using molecular scaffolds in global turtle analyses.

Neck motion in turtles and its relation to the shape of the temporal skull region

Extant turtles are characterized by diverse marginal reductions in their temporal skull region. Among other minor factors, their modes of neck retraction were hypothesized to have a key influence for shaping that region through evolution. A recent study on the mobility of the turtle neck highlighted the possibility that stem turtles, such as Proganochelys quenstedti were able to easily retract their necks by laterally tucking the skull under the anterior edge of the shell. A small emargination in the “cheek” of this species could be correlated to its mode of neck retraction. In the present study, by using a geometric morphometric approach, I correlated the curve shapes of retracted necks and other neck positions with the expansion of marginal reductions in turtle skulls. I hypothesize based on morphospace distributions that neck retraction evolved only once within turtle evolution and that pleurodiran and cryptodiran turtle retraction are directly and independently derived from ancestral neck tucking. Pleurodires evolved a middle kink in their elongated neck for lateral retraction. At the dawn of turtle evolution, associated to the ancestrally retracted (laterally rotated) neck, the cervicals were less specialized than in extant taxa. For cryptodires, that condition may have permitted a transitional, intervertebral rotation towards the vertical neck orientation found in that group during retraction. It retained the ancestral characteristically oriented curvature of the cervical column. I found that the cryptodiran mode of retraction and the ventral neck flexion in all turtles are strongly correlated to the expansion of the occiput emargination. Pleurodiran retraction, however, does not influence skull shape to such a degree. The “cheek” emargination is correlated with the expansion of the “occiput” emargination and appears to occur in correlation to the fixation of the palatoquadrate to the braincase in crown turtles. Neck related forces acting on the skull and ventral neck flexion are also hypothesized to be crucial factors for the reduction of a potential temporal fenestration inherited from a potentially fenestrated turtle ancestor.

Shell bone histology of solemydid turtles (stem Testudines): palaeoecological implications

Lately, solemydid turtles have been repeatedly recovered as stem Testudines, indicating that they belong to neither one of the two major branches of crown turtles, the Pancryptodira and Panpleurodira. Despite their wide temporal (Late Jurassic to Late Cretaceous) and spatial (North America and Europe) distributions, solemydid turtles are not particularly well known, as exemplified by the fact that only a single skull has been described for the whole group so far. Furthermore, the palaeoecology of solemydid turtles is still contested with hypotheses ranging from semi-aquatic to terrestrial lifestyles. However, the habitat preference of stem Testudines, such as solemydids, is important to understand the evolution and early radiation of the turtle crown, which is primitively aquatic. Here we describe the shell bone microanatomy and histological microstructures of solemydid turtles using a broad sample of taxa of different ages and localities, as well as review previous histological accounts, to elucidate the palaeoecology of the group independent of the geological setting and gross anatomy of the fossil finds. Our results indicate that Solemydidae share unique histological features pertaining to their strongly ornamented shell bones, which a) in cases allow taxonomic identification of even small shell fragments and b) unambiguously corroborate a terrestrial lifestyle of its members. The latter further supports a terrestrial lifestyle preference of most representatives of the turtle stem.

Evolution of neck vertebral shape and neck retraction at the transition to modern turtles: an integrated geometric morphometric approach.

The unique ability of modern turtles to retract their head and neck into the shell through a side-necked (pleurodiran) or hidden-necked (cryptodiran) motion is thought to have evolved independently in crown turtles. The anatomical changes that led to the vertebral shapes of modern turtles, however, are still poorly understood. Here we present comprehensive geometric morphometric analyses that trace turtle vertebral evolution and reconstruct disparity across phylogeny. Disparity of vertebral shape was high at the dawn of turtle evolution and decreased after the modern groups evolved, reflecting a stabilization of morphotypes that correspond to the two retraction modes. Stem turtles, which had a very simple mode of retraction, the lateral head tuck, show increasing flexibility of the neck through evolution towards a pleurodiran-like morphotype. The latter was the precondition for evolving pleurodiran and cryptodiran vertebrae. There is no correlation between the construction of formed articulations in the cervical centra and neck mobility. An increasing mobility between vertebrae, associated with changes in vertebral shape, resulted in a more advanced ability to retract the neck. In this regard, we hypothesize that the lateral tucking retraction of stem turtles was not only the precondition for pleurodiran but also of cryptodiran retraction. For the former, a kink in the middle third of the neck needed to be acquired, whereas for the latter modification was necessary between the eighth cervical vertebra and first thoracic vertebra. Our paper highlights the utility of 3D shape data, analyzed in a phylogenetic framework, to examine the magnitude and mode of evolutionary modifications to vertebral morphology. By reconstructing and visualizing ancestral anatomical shapes, we provide insight into the anatomical features underlying neck retraction mode, which is a salient component of extant turtle classification.

Toward consilience in reptile phylogeny: miRNAs support an archosaur, not lepidosaur, affinity for turtles.

Understanding the phylogenetic position of crown turtles (Testudines) among amniotes has been a source of particular contention. Recent morphological analyses suggest that turtles are sister to all other reptiles, whereas the vast majority of gene sequence analyses support turtles as being inside Diapsida, and usually as sister to crown Archosauria (birds and crocodilians). Previously, a study using microRNAs (miRNAs) placed turtles inside diapsids, but as sister to lepidosaurs (lizards and Sphenodon) rather than archosaurs. Here, we test this hypothesis with an expanded miRNA presence/absence dataset, and employ more rigorous criteria for miRNA annotation. Significantly, we find no support for a turtle + lepidosaur sister-relationship; instead, we recover strong support for turtles sharing a more recent common ancestor with archosaurs. We further test this result by analyzing a super-alignment of precursor miRNA sequences for every miRNA inferred to have been present in the most recent common ancestor of tetrapods. This analysis yields a topology that is fully congruent with our presence/absence analysis; our results are therefore in accordance with most gene sequence studies, providing strong, consilient molecular evidence from diverse independent datasets regarding the phylogenetic position of turtles.

A divergence dating analysis of turtles using fossil calibrations: an example of best practices

Turtles have served as a model system for molecular divergence dating studies using fossil calibrations. However, because some parts of the fossil record of turtles are very well known, divergence age estimates from molecular phylogenies often do not differ greatly from those observed directly from the fossil record alone. Also, the phylogenetic position and age of turtle fossil calibrations used in previous studies have not been adequately justified. We provide the first explicitly justified minimum and soft maximum age constraints on 22 clades of turtles following best practice protocols. Using these data we undertook a Bayesian relaxed molecular clock analysis establishing a timescale for the evolution of crown Testudines that we exploit in attempting to address evolutionary questions that cannot be resolved with fossils alone. Some of these questions, such as whether the turtle crown originated in the Triassic or Jurassic, cannot be resolved by our analysis. However, our results generate novel age-of-origination estimates for clades within crown Testudines. Finally, we compare our fossil calibrations and posterior age estimates to those from other studies, revealing substantial differences in results and interpretation.

The girdles of the oldest fossil turtle, Proterochersis robusta, and the age of the turtle crown

BackgroundProterochersis robusta from the Late Triassic (Middle Norian) of Germany is the oldest known fossil turtle (i.e. amniote with a fully formed turtle shell), but little is known about its anatomy. A newly prepared, historic specimen provides novel insights into the morphology of the girdles and vertebral column of this taxon and the opportunity to reassess its phylogenetic position.ResultsThe anatomy of the pectoral girdle of P. robusta is similar to that of other primitive turtles, including the Late Triassic (Carnian) Proganochelys quenstedti, in having a vertically oriented scapula, a large coracoid foramen, a short acromion process, and bony ridges that connect the acromion process with the dorsal process, glenoid, and coracoid, and by being able to rotate along a vertical axis. The pelvic elements are expanded distally and suturally attached to the shell, but in contrast to modern pleurodiran turtles the pelvis is associated with the sacral ribs.ConclusionsThe primary homology of the character “sutured pelvis” is unproblematic between P. robusta and extant pleurodires. However, integration of all new observations into the most complete phylogenetic analysis that support the pleurodiran nature of P. robusta reveals that this taxon is more parsimoniously placed along the phylogenetic stem of crown Testudines. All current phylogenetic hypotheses therefore support the basal placement of this taxon, imply that the sutured pelvis of this taxon developed independently from that of pleurodires, and conclude that the age of the turtle crown is Middle Jurassic.

Cretaceous tetrapod fossil record sampling and faunal turnover: Implications for biogeography and the rise of modern clades

We use newly compiled data on global occurrences of Cretaceous lepidosaurs, mammals and crocodylomorphs, and existing data on dinosaurs, to investigate faunal turnover and fossil record heterogeneity. Statistically significant relationships between many clade-specific fossil record sampling proxies within major continental areas (e.g. European mammal-bearing formations) suggest that temporal patterns of fossil record sampling intensity of the most abundant tetrapod clades are similar to each other, with a few exceptions that might reflect clade-specific facies preferences or differences in worker effort (especially in poorly-sampled regions such as Gondwana). However, the absence of strong statistical relationships between tetrapod sampling proxies from different continental areas suggests that there is no unified ‘global’ sampling signal for terrestrial tetrapods. The Cretaceous witnessed substantial faunal turnover and the rise of many ‘advanced’ clades that today dominate terrestrial faunas. Despite strong spatiotemporal heterogeneity in sampling of the Cretaceous tetrapod record, it is clear that this transition occurred in a spatiotemporally staggered fashion. Thus, it cannot be attributed to a temporally localised early Late Cretaceous (Cenomanian–Turonian) extinction event. Many advanced clades, including eutherian mammals, iguanian and gekkotan squamates, and the main cryptodiran turtle crown clades first appeared, or first attained high fossil diversities, in Asia. However, relatively poor sampling and dating of ‘middle’ Cretaceous terrestrial deposits means that hypotheses of Asian, rather than Laurasian, diversifications remain tentative. Differences between Gondwanan and Laurasian faunas became progressively greater during the Cretaceous, and although many Gondwanan clades survived the end-Cretaceous extinction event, these only survive to the present as relictual populations with narrow geographic ranges (e.g. monotremes and rhynchocephalians).

Phylogenetic relationships among extinct and extant turtles: the position of Pleurodira and the effects of the fossils on rooting crown-group turtles

The origin and evolution of the crown-group of turtles (Cryptodira + Pleurodira) is one of the most interesting topics in turtle evolution, second perhaps only to the phylogenetic position of turtles among amniotes. The present contribution focuses on the former problem, exploring the phylogenetic relationships of extant and extinct turtles based on the most comprehensive phylogenetic dataset of morphological and molecular data analyzed to date. Parsimony analyses were conducted for different partitions of data (molecular and morphological) and for the combined dataset. In the present analysis, separate analyses of the molecular data always retrieve Pleurodira allied to Trionychia. Separate analysis of the morphological dataset, by contrast, depicts a more traditional arrangement of taxa, with Pleurodira as the sister group of Cryptodira, being Chelonioidea the most basal cryptodiran clade. The simultaneous analysis of all available data retrieves all major extant clades as monophyletic, except for Cryptodira given that Pleurodira is retrieved as the sister group of Trionychia. The paraphyly of Cryptodira is an unorthodox result, and is mainly caused by the combination of two factors. First, the molecular signal allies Pleurodira and Trionychia. Second, the morphological data with extinct taxa locates the position of the root of crown-group Testudines in the branch leading to Chelonioidea. This study highlights major but poorly explored topics of turtle evolution: the alternate position of Pleurodira and the root of crown turtles. The diversification of crown turtles is characterized by the presence of long external branches and short internal branches (with low support for the internal nodes separating the major clades of crown turtles), suggesting a rapid radiation of this clade. This rapid radiation is also supported by the fossil record, because soon after the appearance of the oldest crown-group turtles (Middle-Late Jurassic of Asia) the number and diversity of turtles increases remarkably. This evolutionary scenario of a rapid diversification of modern turtles into the major modern lineages is likely the reason for the difficulty in determining the interrelationships and the position of the root of crown-group turtles.

Turtle carotid circulation: a character analysis case study

The bony canals permeating the turtle skull associated with the cranial circulation have long been considered integral to an understanding of extinct and extant turtle systematics. Recent phylogenetic analyses, employing a variety of data sets, suggest alternatives to the traditional arrangement of crown turtles. Of particular note is the recent failure of investigations employing molecular techniques to retrieve a monophyletic Trionychoidea, a clade supported largely by shared circulatory features. These alternative phylogenies may also have implications for extinct forms. The turtle cranial arterial circulation therefore represents an ideal system upon which to conduct a detailed, multipronged analysis of a systematically influential character. In the present study, the theoretical underpinnings of character analysis are critically evaluated, and a new interpretation of circulatory variation in turtles is offered, including a revised phylogenetic character suite. This assessment indicates an autapomorphic circulatory pattern in Trionychia, whereas Trionychoidea is not supported. © 2008 The Linnean Society of London, Biological Journal of the Linnean Society, 2008, 93, 239–256.

Phylogenetic Relationships of Mesozoic Turtles

Presented herein is the first morphological analysis of turtle relationships to examine the monophyly of many turtle groups by using only single species as terminals and by integrating a large number of primitive fossil taxa. The data matrix consists of 136 osteological parsimony informative characters with 169 derived character states for 45 fossil and 22 living species of the clade Testudinata. The results corroborate the monophyly of a large number of previously hypothesized clades, but refute the accepted hypothesis regarding the basal split of living turtles. In particular, the primitive turtles Proterochersis robusta, Kayentachelys aprix, Mongolochelys efremovi, Meiolania platyceps, and Kallokibotion bajazidi are removed from their current position as crown turtles and placed along the phylogenetic stem of this clade. The age of the turtle crown is thereby adjusted from the Late Triassic to the Late Jurassic, which is relevant to testing molecular clock hypotheses. This revised topology has important implications for the evolution of several character complexes, because it implies that the common ancestor of all living turtles must have had a partially braced brain case and a primitive trochlear mechanism. Other noteworthy conclusions include the tentative exclusion of protostegids from Chelonioidea, the placement of Platysternon megacephalum outside of Chelydridae, and the tentative interpretation of Sandownia harrisi as a basal eucryptodire.