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Lehrbuch der Vergleichenden Zootomie. Ernst Fleischer, Leipzig p. Cebra-Thomas J. How the turtle forms its shell: A paracrine hypothesis of carapace formation. B, Mol. Cherepanov G. On the nature of the plastron anterior elements in turtles. Ontogenetic development of shell in Trionyx sinensis Trionychidae, Testudinata and some questions on nomenclature of bone plates.

Rus J Herpetol. The origin of the bony shell of turtles as a unique evolutionary model in reptiles. Chiari Y. Phylogenomic analyses support the position of turtles as the sister group of birds and crocodiles Archosauria. BMC Biology. Colbert M. Ontogenetic sequence analysis: Using parsimony to characterize developmental sequences and sequence polymorphism. Crawford N. More than ultraconserved elements provide evidence that turtles are the sister group of archosaurs Biol. Cuvier G. Danni T. Revta Bras. Davis D. Clearing and staining skeleton of small vertebrates.

Evolutionary relationships, osteology, morphology and zoogeography of Kemp's ridley sea turtle.

Field Mus. Digenkus G. Differential staining of bone and cartilage in cleared and stained fish using alcian blue to stain cartilage and enzymes for clearing fish. Stain Technol. Ewert M. Embryology of turtles, p. In: CGans C. Eds , Biology of the Reptilia. John Wiley and Sons, New York. Fabrezi M. Developmental basis of limb homology in Peurodiran turtles, and the identity of the hooked element in the chelonian tartus. Geodiversitas Gaffney E. Comparative cranial morphology of recent and fossil turtles.

A phylogeny of turtles, p. In: Benton M. Clarendon, Oxforf. Gardner J. Carapacial variation among softshelled turtles Testudines: Trionychidae and its relevance to taxonomic and systematic studies of fossil taxa. Gaspar A. Food Technol. Gilbert S. The contribution of neural crest cells to the nuchal bone and plastron of the turtle shell.

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In: Wyneken J. Eds , Biology of Turtles. Hill R. Integration of morphological data sets for phylogenetic analysis of Amniota: the importance of integumentary characters and increased taxonomic sampling. Hoffmann C. Archiv Zoo. Joyce W. Phylogenetic relationships of Mesozoic turtles. Peabody Mus. Acta Anat.

Kordikova E. Heterochrony in the evolution of the shell of Chelonia. Part 1. Lee M. Correlated progression and the origin of turtles. Nature An ancestral turtle from the Late Triassic of southwestern China. Lima F. Ontogeny of the shell bones of embryos of Podocnemis unifilis Troschel, Testudines, Podocnemididae. Lyson T. Transitional fossils and the origin of turtles. Bio Letters Biol Lett.

Evolution of the Turtle Shell

Luz V. Pau Brasil Meyer A. Recent advances in the molecular phylogeny of vertebrates. Meylan P. The phylogenetic relationships of soft-shelled turtles Family Trionychidae. Molina F. Biotemas Early loss and multiple return of the lower temporal arcade in diapsid reptiles. Naturwissenschaften Nagashima H. On the carapacial ridge in turtle embryos: Its developmental origin, function and the chelonian body plan. Development Evolution of the turtle body plan by the folding and creation of new muscle connections.

Turtle-chicken chimera: An experimental approach to understanding evolutionary innovation in the turtle. Ogushi K. Ohya Y. Unique features of Myf-5 in turtles: Nucleotide deletion, alternative splicing, and unusual expression pattern. Owen R. On the development and homologies of the carapace and plastron of the chelonian reptiles. B, Biol. Presnell J. Animal tissue techniques. Freeman, San Francisco. Pritchard P. The Turtles of Venezuela. Society for the Study of Amphibians and Reptiles. Rathke H.

Morphology and Evolution of Turtles (new book)

Rieppel O. Studies on skeleton formation in reptiles: Patterns of ossification in the skeleton of Chelydra serpentina Reptilia, Testudines. Studies on skeleton formation in reptiles: implications for turtle relationships. Turtles as diapsid reptiles. Sitzungsberichte der Gesellschaft Naturforschender Freunde zu Berlin Romer A. Osteology of the Reptiles. Chicago University Press, Chicago. Saint-Hilaire G. Skeletal development in the Chinese softshelled turtle Pelodiscus sinensis Testidines: Trionychidae. Autopodial skeleton in side-necked turtles Pleurodira.

Scheyer T. The ontogeny of the shell in side-necked turtles, with emphasis on the homologies of costal and neural bones. Carapace bone histology in the giant pleurodiran turtle Stupendemys geographicus : Phylogeny and function. Acta Palaeontol. Neural II is approximately two thirds the length of neural I. Neural III is only partially preserved.

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Given that this element has a short anterolateral contact with costal II and tapers posteriorly, we presume that it had a hexagonal outline. It lacks a posterolateral contact with costal V and therefore had a hexagonal outline. Neural V is the most posterior element to have an elongate hexagonal outline with short anterior sides.

They therefore form isometric hexagons. Neural VIII has a broad posterior contact with the most anterior suprapygal element. The neural formula can be summarized as The anterior costals were likely oriented to the anterior, but the exaggerated anterior orientation seen in IVPP V is due to plastic deformation.

As in most turtles, the posterior costals have a slight orientation to the posterior, as is apparent from IVPP V All remaining costals contact two neurals medially. The detailed lateral contacts of the remaining costals with the peripherals are obscured by deformation, but it is apparent that the costals articulate with the peripherals via free ribs and that small costal fontanelles were retained in smaller individuals, but were closed in larger individuals.

The free rib ends are better developed in IVPP V, a smaller specimen, indicating that the costal fontanelles were larger in juveniles. The number of peripherals is therefore unknown. Peripheral I is wedge-shaped, but nevertheless retains a broad, posterior contact with costal I.

The ventral view of the bridge region is not sufficiently preserved in any specimen and the ventral contacts of the peripherals with the plastron are unclear. The contacts with the costals are discussed above. The pygal region is poorly preserved in all specimens and no significant details can be discerned.

The surface of IVPP V is decorated by wide and distinct carapacial scutes that allow asserting the presence of at least four vertebral scutes, four pleural scutes, and ten marginal scutes Fig. The likely presence of a cervical cannot be confirmed. The vertebral series consists of at least five elements, of which the anterior four are approximately equal in width.

All vertebrals are about twice as wide as the pleurals. Vertebral I has a lenticular to octagonal shape and is therefore anteroposteriorly longer along the midline than at its lateral margins. Vertebral I has a broad anterior contact with marginal I, a short anterolateral contact with marginal II, a broad lateral contact with pleural I, and a broad posterior contact with vertebral II.

An anterior contact with the cervical was likely present as well. Vertebrals II—IV are roughly hexagonal elements that contact two pleurals each laterally. Vertebral II has the outline of a butterfly that thereby partially surrounds vertebral I. Each pleurals contact two vertebrals medially Fig. It otherwise contacts marginals II—V laterally.

The remaining contacts of the marginals with the plastral scutes are unclear. The plastron of IVPP V is near complete, but there is some damage to the anterior margin and the right bridge Fig. The plastron of IVPP V preserves the entoplastron best, but otherwise only consists of part of the anterior plastron lobe Fig. The plastra of V and V Fig. There are no meaningful visceral views of the plastron. Most plastral scutes are relatively indistinct. The scale is metric.

The epiplastra are relatively large elements that form the margins of the anterior half of the anterior plastral lobe Fig. The anterior, rectangular part of the epiplastra contacts the entoplastron posteromedially, and the hyoplastron posteriorly, and has a short midline contact with its counterpart. The posterior half of the epiplastron is a notably elongate, triangular posterior process that frames the anterolateral portions of the hyoplastra, similar to the process seen in Mongolochelys efremovi.

The contacts with the hyoplastra are blunt and the epiplastra therefore easily dislocate from the anterior plastral lobe after decomposition. The anterior margin of the plastron is oriented transversely, but it is decorated by four broad lobes that correspond to the gular and extragular scutes. The margin, however, is not thickened. A distinct articular scar along the anterolateral margin of the hyoplastron in partial specimens confirms presence of a small contact between the epiplastra and hyoplastra.

The anterior portion of the entoplastron contacts the epiplastra anterolaterally but does not contribute to the anterior plastral margins Fig. The posterior portion is broadly covered by the hyoplastra in ventral view and the full extension of this element therefore remains unclear.

The remaining part of the plastron is formed by a large pair of hyoplastra, a pair of mesoplastra, a pair of hypoplastra, and a pair of xiphiplastra Figs. The mesoplastra are well-developed, rectangular in shape, show no sign of narrowing medially, but do not contact one another due to the presence of a medial plastral fontanelle in all subadult specimens.

The plastron is not preserved in the largest, presumable adult specimens and it therefore remains unclear if this fontanelle closes during ontogeny. The posterior plastral lobe is similar in dimensions to the anterior plastral lobe and does not exhibit an anal notch. The sutural margins of the hyo- hypo-, and xiphiplastra are finely digitated Figs. The detailed quality of the bridge articulation is unclear, but the lack of blunt sutures combined with the presence of finely fingered margins indicates that the bridge appears to have been ligamentous.

The lateral margins of the plastron are too irregular or damaged to allow identifying the presence of musk duct foramina. The hyoplastron and hypoplastron form well-developed axillary and inguinal buttresses, respectively. The distal ends of the buttresses are not preserved in any specimen, but it is apparent that the anterior buttress ended anterior to peripheral IV and therefore may have inserted in peripherals I, II, or III and that the posterior buttresses ended posterior to peripheral VII and therefore may have inserted in peripheral VIII to XI.

The hyoplastra meet broadly along their posterior half thereby leaving a narrow, triangular gap for the entoplastron. A clear central plastron fontanelle is formed by the hyo-, meso-, and hypoplastra that fully separates the mesoplastra along the midline, but it remains unclear if this is a juvenile feature, as the plastron is not known for any of the skeletally mature individuals.

A second midline plastral fontanelle is also present between the hypo- and xiphiplastra. The gulars are triangular scutes that produce clear lobes from the anterior margin of the plastron. The gulars contact the extragulars laterally, the humerals posterolaterally, and one another along the midline and cover the anterior half of the entoplastron. The extragulars are mediolaterally elongate elements that cap the anterolateral margin of the plastron. The extragulars contact the gulars medially and the humerals posteriorly, but do not contact one another medially and are restricted to the epiplastra.

If the remaining portion of the sulcus were to continue transversely, it would not intersect with the entoplastron. Scute sulci are poorly preserved in the bridge region of all specimens and it is therefore unclear if and how many inframarginals are present. Sichuanchelys chowi has four pairs of inframarginals [ 16 , 17 ]. A number of disarticulated cervicals are preserved associated with specimens IVPP V — V, but preservation is generally poor Fig. The cervicals are typical of basal turtles in being relatively short, but high. It is unclear if cervical ribs are present, as no cervical ribs were found and the centra are too damaged to preserve parapophyses.

Only a single, concave articular facet is preserved among the centra and it is therefore unclear if formed cervical articulations were present. Transverse processes are relatively long and centrally located. The dorsal and sacral vertebrae and ribs are either covered by sediment or too poorly preserved to allow any meaningful observations. Transverse processes are distinct along the anterior processes, but become increasingly smaller towards the posterior and are absent in the posterior half of the caudal column.

The entire caudal column appears to have chevrons, as is evidenced by clear articular sites along the anterior half of the column and minute chevrons in articulation with the posterior caudals of IVPP V The articular surfaces of only a few caudals are visible, but some appear to be amphicoelous, whereas others are slightly opisthocoelous.

The great size of the basal caudals is consistent with the tail having been long i. It is nevertheless apparent that the scapulocoracoid is a slender triradiate complex that lacks a coracoid foramen and that the glenoid is not fused in any specimen.

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The scapular process is rounded distally and is only slightly longer than the acromion process. Only a minor bony lamina is developed between the dorsal process and the acromion, but it is unclear if a bony lamina or ridge runs to the glenoid, as this region is not preserved in any specimen. The distal end of the acromion is not preserved in all views and it is therefore uncertain if it is rounded distally or decorated by ridges.

The scapula has a distinct neck that offsets the processes from the glenoid. The coracoid is shorter than the acromion and distally expanded to a broad fan.

Only two isolated pelvic elements are preserved, indicating that the acetabulum was not fused in subadult specimens. The pubes have a broad midline contact with one another, the thyroid fenestrae are large, perhaps even confluent, and the epipubic process was not ossified. The ischia have a broad midline contact and the ischial process is relatively indistinct. The posterior margin of the ischium agrees with that of Mongolochelys efremovi in being poorly emarginated but differs by being much smaller [ 32 ].

A number of disarticulated elements are preserved that can be attributed to the limbs, but all material is encased in sediments making it impossible to observe most details Fig. The humerus is more than twice as long as wide and has a slightly sigmoidal shaft. The medial process is flared outwards and better developed than the ventrolaterally oriented lateral process. The head is damaged in all specimens and it is therefore uncertain if a shoulder is present. The ectepicondylar canal is open, at least in the subadult specimens that preserve this bone.

The ulna and radius could not be identified among the remains. A collection of bone is associated in the anterior region of IVPP V that may represented a disarticulated hand, but it is not possible to identify any particular digit and the digital formula therefore remains unknown. The phalanges are nevertheless short and robust. The only preserved femur, tibia, and fibula are too poorly preserved to allow discerning any details, beyond the observation that the femur has a slightly sigmoidal shaft Fig. The Late Triassic Proganochelys quenstedti possesses a full set of palatal teeth that adorn the ventral surfaces of the paired vomers, palatines, and pterygoids [ 37 ].

Palatal teeth are otherwise known from the Permian proto turtle Eunotosaurus africanus [ 38 ] and the Middle Triassic proto turtle Odontochelys semitestacea [ 39 ]. This is the basal amniotic condition [ 40 ]. The skull of all other known Triassic turtles is either missing or too poorly preserved to allow rigorously assessing the presence of palatal teeth. The gradual loss of teeth was previously documented only by the Early Jurassic Kayentachelys aprix , which clearly lacks vomerine and palatine teeth, but retain a reduced count of pterygoid teeth.

All more derived and younger turtles were thought to lack palatal teeth [ 22 ]. The presence of pterygoid teeth in S. Our parsimony analysis see Methods below resulted in most parsimonious trees with steps. Heckerochelys romani , Eileanchelys waldmani , Indochelys spatulata , Patagoniaemys gasparinae , and Xinjiangchelys junggarensis act as wild-card taxa and were therefore pruned from the consensus tree Fig. Sichuanchelys palatodentata was retrieved in a polytomy with Sichuanchelys chowi and Mongolochelys efremovi within the clade Sichuanchelyidae along the stem lineage of turtles.

Helochelydridae and Kallokibotion bajazidi are placed in successively more crownward positions relative to Sichuanchelyidae. Turtles with the paracryptodiran carotid circulation are retrieved as monophyletic as the immediate sister of crown Testudines. Meiolaniformes is here restricted to Meiolaniidae and Peligrochelys walshae and are placed in a more basal position than Sichuanchelyidae. Spoochelys ormondea and Chubutemys copelloi form a polytomy with Meiolaniformes and the clade consisting of all other more derived taxa.

Morphological support for the placement of M. A strict consensus tree of most parsimonious trees with steps resulting from phylogenetic analysis. The fossil record of turtles is relatively good and multiple attempts have therefore been made to discern global [ 6 , 13 ] or regional biogeographic patterns [ 26 , 31 , 44 — 46 ].

However, new fossils, insights into the paleoecology of fossil turtles, and novel phylogenetic hypotheses allow us to synthesize a global biogeographic model that reveals that the diversification of turtles was primarily driven by vicariance caused by the breakup of Pangaea. We demonstrate below that this pattern is apparent at two successive phylogenetic levels.

Morphological Evolution of the Turtle Shell and its Mechanical Implications, Part 2: Theoretical

Given that some parts of the turtle tree remain controversial, in particular the inclusiveness of Pan-Cryptodira and the interrelationships of sichuanchelyids, helochelydrids, and meiolaniforms [ 6 — 12 , 25 , 26 , 41 — 43 , 47 ], we attempt to present a model that is relatively immune to future changes in the understanding of phylogenetic patterns by highlighting the distinct evolutionary history of seven clades of turtles. These conflicting signals are reflected in the composite topology we utilize herein Fig. The monophyly of each clade is discussed below and phylogenetic ambiguities are highlighted.

A composite phylogenetic consensus of turtle relationships highlighting the most important clades discussed in the text and their stratigraphic distribution as derived from the inclusion of fragmentary material. To aid understanding the text, internal relationships and nodes are only provided within Pan-Cryptodira. Support for all clades or polytomies is provided in the text see Biogeographic analysis.

The taxonomic identity of the fossils utilized herein is not controversial, as we only employ specimens that exhibit clear, apomorphic characters. The vast majority of informative fossils is fragmentary, however, and has therefore not yet been integrated into global phylogenetic analyses. We therefore refrain for the moment from providing a probabilistic model of historical biogeography and rather present a narrative account based on all available data. All species-level phylogenies of the last decade [ 6 — 12 , 25 , 26 , 41 — 43 , 47 ] have converged upon the novel conclusion that the stem lineage leading to the crown is populated by a diverse assemblage of fossil turtles that inhabited all continents from the Triassic to the Pleistocene.

All conflicting hypotheses [ 4 — 7 ] have been shown to converge upon this result through minor modifications, in particular the addition of characters, taxa, or new specimens [ 48 , 49 ]. Although a certain amount of ecological plasticity is apparent, the basal stem turtle lineage is dominated by terrestrial forms, whereas crown turtles and their immediate sister groups are dominated by freshwater aquatic forms [ 50 ].

We herein discuss the parallel diversification of derived, aquatic turtles and basal, terrestrial turtles separately for convenience and highlight important developments that occurred in parallel. It is important to emphasize that the strong biogeographic signal we discuss herein only emerges once all littoral to marine clades are omitted from consideration, as these obscure the continental pattern that otherwise emerges based on freshwater aquatic and terrestrial forms alone.

The littoral to marine groups we identify are listed further below under dispersal. The vast majority of recent molecular and morphological studies see [ 14 , 47 ] for most recent summary support the monophyly of the primary clades that make up crown Testudines: Pan-Cryptodira and Pan-Pleurodira, which in turn is comprised of Pan-Chelidae and Pan-Pelomedusoides. The fossil record furthermore reveals the presence of another clade that diverged near the base of the crown group: Paracryptodira e.

Our review of the fossil record indicates that these four clades can be traced back to four distinct biogeographic areas in the Late Jurassic to Early Cretaceous. The monophyly of each group and their biographic distribution is discussed below. The monophyly of crown Chelidae has never been controversial. Chelids are freshwater aquatic turtles that today occur throughout South America and Australasia [ 2 ] Fig.

The known distribution of pan-chelids predicts the former presence and extinction of the group on Antarctica, as a transoceanic dispersal event is highly unlikely between South America and Australia. The early history of the group is consistent with a vicariant origin of South American versus Australian chelids in the Early Cretaceous, as predicted by molecular phylogenies [ 55 , 56 ] and molecular calibration studies [ 57 ], but contrast with morphological data [ 58 ].

Rigorous phylogenetic analysis of all Cretaceous representatives is needed to further test this hypothesis. The vicariant origin of the primary clades of turtles. In contrast to chelids, pelomedusoids today occur throughout Africa, Madagascar, and the northern half of South America [ 2 ] Fig. Although various littoral to marine representatives helped this group of turtles to achieve a near-global distribution during much of the Cretaceous and Tertiary [ 59 , 60 ] through a dizzying array of marine dispersal events see below , the freshwater aquatic representatives of this clade were consistently restricted to northern South America, Africa, Madagascar, and India throughout their evolutionary history [ 59 , 60 ] Fig.

There currently is no evidence that the primary split of Pelomedusoides into the pelomedusid and podocnemidid lineages was caused by vicariance, as the fossil record of freshwater aquatic pan-podocnemidids is broadly distributed across Northern Gondwana [ 60 ]. Unambiguous pan-pelomedusids, by contrast, are only known from the Neogene of Africa [ 57 ].

The biogeographic history of derived turtles following their primary origin through dispersal. Shaded areas highlight the distribution of turtle clades at the beginning of a particular time bin as inferred from the fossil record see Fig. Arrows highlight paths of freshwater aquatic or terrestrial dispersal. For simplicity, all island taxa and groups adapted to coastal and marine settings are disregarded.

There has been much debate whether the unusual distribution of extant podocnemidids in South America and Madagascar [ 2 ] is, among others, the result of vicariance [ 63 ], differential extinction within a formerly widespread group [ 64 , 65 ], dispersal from Africa to Madagascar [ 66 ], or a mixture of vicariance and dispersal across Antarctica [ 67 ]. One reason why this conundrum remains unresolved is because there is no agreement as to the phylogenetic relationships among the three primary lineages of extant podocnemidids i.

This problem is only further compounded by the lack of fossil forms that unambiguously represent the stem lineages of the Malagasy Erymnochelys madagascariensis and the South American Peltocephalus dumerilianus , which may perhaps reach back into the Cretaceous. Until the fossil record provides a more definitive answer, we here refrain from supporting any particular biogeographic scenario. However, we feel that the complete lack of fossil podocnemidid turtles in Southern Gondwana [ 60 , 68 ] make dispersal from South America to Madagascar via Southern Gondwana highly unlikely.

The primary distribution of pan-pelomedusoids in Northern Gondwana and pan-chelids in Southern Gondwana is best interpreted as the result of a vicariance event, as previously proposed [ 44 ], and this event must have occurred prior to the Barremian [ 69 ]. A previous study [ 69 ] speculated that vicariance was driven by a volcanic event that is documented by large volcanic fields in southern Brazil, but we do not think this to be likely, as this volcanic event only lasted about one million years [ 70 ]. As an alternative, we speculate that the subtropical desert zone that crossed the southern portion of Gondwana during much of the late Mesozoic [ 71 — 73 ] persistently divided the freshwater habitats of early pleurodires into a larger northern and a smaller southern range.

This desert zone apparently influenced the biogeographic distribution of other groups of organisms, including dinosaurs [ 74 ]. Although the phylogenetic relationships of Paracryptodira relative to Pleurodira and Cryptodira remains unresolved, there is broad agreement that the group is monophyletic, not situated within crown Cryptodira or crown Pleurodira, and that their freshwater aquatic habitat preferences are a derived character shared with crown turtles [ 7 , 8 , 10 , 11 , 25 , 26 , 42 , 47 , 75 ]. The oldest known paracryptodires are known from Upper Jurassic deposits in both North America and Europe [ 31 , 46 , 76 ], but isolated finds extend the range to the Middle Jurassic of Europe [ 77 ].

Paracryptodires were particularly diverse throughout the Late Cretaceous and Paleogene [ 76 , 79 — 81 ], but the group went extinct prior to the Oligocene [ 82 ]. The paracryptodiran clade Baenidae is restricted to the Early Cretaceous to Paleogene of western North America Laramidia , but there is no reason to interpret this as evidence for vicariance contra [ 6 ] , as Baenidae lacks a sister group on a nearby landmass.

The currently accepted sister of Baenidae, Pleurosternidae, instead shows a broad distribution across Euramerica. The composition of the total group of Cryptodira, i. Although the broad sample of basal, terrestrial forms discussed below has been removed from the cryptodiran stem group with confidence see above , there is still much uncertainty regarding a similarly broad sample of freshwater aquatic forms [ 7 , 8 , 10 , 11 , 25 , 26 , 42 , 47 , 75 ], in particular xinjiangchelyids, sinemydids, and macrobaenids sensu [ 11 ].

The character evidence that places xinjiangchelyids and sinemydids along the phylogenetic stem of Cryptodira is quite convincing, because these taxa document the step-wise acquisition of cryptodiran characters throughout the Middle to Late Jurassic. The character evidence is particularly strong in the basicranial region and these turtles have therefore been collectively united with crown cryptodires in the clade Eucryptodira [ 83 ]. Yet, pleurodires have been routinely recovered deep within Eucryptodira e. The extremely rich Middle Jurassic to Late Jurassic fossil record of freshwater aquatic pan-cryptodires is restricted to Asia [ 29 , 84 , 85 ], with the exception of marine plesiochelyids and eurysternids found in the Late Jurassic of Europe [ 31 , 46 ] Fig.

It was not before the Early Cretaceous that isolated freshwater eucryptodire taxa dispersed into Europe [ 86 ]. The early record of unambiguous crown cryptodires is fully restricted to Asia until the Early Cretaceous [ 84 — 88 ] Fig. The currently available fossil record does not support the hypothesis that the basal split within crown Cryptodira i. It is notable, however, that the primary clades of Durocryptodira i. It will only be possible to establish whether this distribution is due to vicariance as suggested for Testudinoidea [ 6 ] or dispersal as previously suggested [ 13 ] through the discovery of unambiguous stem-americhelydian turtles.

The early presence of such taxa in Asia positively would imply dispersal of the americhelydian ancestor to North America, whereas their absence in Asia would corroborate vicariance. A previous analysis [ 6 ] suggested that Chelydridae, one of the primary clades of Americhelydia, originated through vicariance, but given that most Late Cretaceous chelydrid localities also contain fossils of the sister group Kinosternoidea e.

The exclusive presence of fresh-water aquatic pleurodires in Gondwana, paracryptodires in Euramerica during the late Middle Jurassic to Early Cretaceous, and the restriction of pan-cryptodires to Asia during the Middle to Late Jurassic Fig. Based on the distribution of fossils Fig. The freshwater aquatic habitat preferences of the common ancestor of this clade of turtles [ 50 ] either suggest barriers created by salt water or terrestrial deserts. The marine barriers that appear to have driven these vicariance events are the opening of the North Atlantic, which originated from the initial breakup of Pangaea into Gondwana and Laurasia, and the establishment of the Turgai Strait, which split Asia from the remaining northern continents during the Jurassic and Cretaceous.

Independent geological evidence places the origin of these barriers at the Middle Jurassic [ 78 , 92 ]. The subsequent split of pleurodires, by contrast, appears to have been driven by a terrestrial barrier see above , which likely originated after the Middle Jurassic, but prior to the late Early Cretaceous. This leads us to the novel conclusion that the primary divergence of derived turtles into four clades i. There currently is no evidence for further vicariance within these clades, although we note possible examples above.

The extensive destruction of the vicariant signal through secondary dispersal events is discussed below. This stem lineage includes the expected sequence of basal forms that help span the morphological gap between the most turtle-like proto turtle, the Middle Triassic Odontochelys semitestacea [ 39 ], and the turtle crown, such as the Late Triassic Proterochersis robusta Fraas, [ 93 ], Proganochelys quenstedti , and Palaeochersis talampayensis Rougier et al. In addition, the stem lineage includes a number of lineages that diversified throughout the Mesozoic and Cenozoic, in parallel with crown turtles, and of which the last representative died out as recently as the late Pleistocene [ 5 , 97 ].

We herein recognize three post-Jurassic lineages that can be traced back to geographic areas that coincide with those already established above for more derived aquatic turtles: Meiolaniformes, Helochelydridae, and Sichuanchelyidae for phylogenetic definitions of all three clades see above.

In current phylogenies, these three lineages typically are retrieved as a clade [ 8 — 11 , 26 , 42 ], but also occasionally as a paraphyletic grade [ 6 ], or even a polyphyletic assemblage [ 7 ]. The analysis we present herein, the most comprehensive to date in regards to this taxa, retrieves a paraphyletic arrangement see above , but we remain cautious as the phylogeny of basal turtles is in a state of flux. To a certain degree, this phylogenetic ambiguity is not problematic, because the monophyly of each of these three lineages appears to be unambiguous and because these three lineages can be inferred to have been present by the Middle Jurassic.

To remain conservative for the moment, we consider these three clades to have unresolved relationships with respect to each other outside of crown Testudines Fig. To highlight that additional turtles exist with disputed phylogenetic relationships and at the same evolutionary level, we also include the Cretaceous turtles Kallokibotion bajazidi and Spoochelys ormondea into this polytomy Fig. Further analyses may reveal these turtles to be attributable to Meiolaniformes, Helochelydridae, and Sichuanchelyidae or to represent additional lineages of basal turtles that survived long past the origin of crown turtles. A number of recent finds have been important for the understanding of this clade, as they bridge the morphology chasm that exists between classic meiolaniid turtles and the turtle stem lineage. The most important taxon is Chubutemys copelloi [ 7 ], which is based on a partial skeleton from the Early Cretaceous Aptian of Argentina also see [ 43 ]. A series of additional taxa based on more fragmentary material may further document the persistent presence of the lineage in this biogeographic area, including Patagoniaemys gasparinae Sterli and de la Fuente, [ 41 ] and Trapalcochelys sulcata Sterli et al.

The complete biogeographic distribution of this clade of turtles fully coincides with the core area of chelid turtles and also predicts the presence of the group on Antarctica at some time, as previously already noted [ 26 , 43 ]. The realization that this clade of turtles is restricted to South America and Australia is a novel result obtained through the addition of Sichuanchelys palatodentata to the analysis, as the addition of this taxon pulls the Asian Mongolochelys efremovi and the European Kallokibotion bajazidi out of Meiolaniformes.

Although the presence of this transcontinental grouping had already been proposed during the first half of the 20th century [ 33 ], it was not well accepted until recently, though under the name Solemydidae [ 6 , 28 , 34 ]. The best-known representatives of this clade are Naomichelys speciosa Hay, [ 76 ] which is known from a complete skeleton from the Early Cretaceous Aptian—Aptian of Texas [ ] and Helochelydra nopcsai Lapparent de Broin and Murelaga, [ 28 ], known from a partial skeleton [ 33 ] and a well-preserved skull from the Barremian of England [ ].

The highly distinct surface sculpture of helochelydrid shells allows confident referral of fragmentary remains and the current record of the group spans from the Late Jurassic Tithonian to the Late Cretaceous Maastrichtian of Euramerica [ , ]. The geographic distribution of Helochelydridae and Paracryptodira fully overlap with one another over their entire known history Fig.

Mongolochelys efremovi from the Late Cretaceous Maastrichtian of Mongolia had previously been a paleobiogeographic enigma, because it was the only stem turtle known from the Cretaceous of Asia and therefore biogeographically isolated from all potential sister groups Fig. This taxon has repeatedly been discussed as a primitive relict within the Asian turtle fauna [ 26 , 31 , 43 , ], but the absence of fossils to the contrary made it impossible to exclude that this pattern was due to dispersal, a possibility made plausible by the late occurrence and highly nested position of M.

Our conclusion that the Late Jurassic Sichuanchelys palatodentata and, by extension, the Middle Jurassic Sichuanchelys chowi are sister to the Late Cretaceous Mongolochelys efremovi is highly significant. Even though S. Although a basal position had previously been inferred for S. Mongolochelys efremovi is therefore no longer an out of place turtle and further fossil discoveries in Asia will likely further fill the record of Sichuanchelyidae.

While the herein proposed phylogenetic relationships of Sichuanchelyidae relative to Helochelydridae, Meiolaniformes, and crown Testudines may change through the addition of further data in the future, the unambiguous presence of sichuanchelyids in the Middle Jurassic of Asia [ 16 ] and of helochelydrids in the Late Jurassic of Europe [ ] highlights that the three groups of basal turtles discussed herein i. Meiolaniformes, Sichuanchelyidae, and Helochelydridae had split from one another no later than the Middle Jurassic.

Previous authors have already noted the overlapping distribution of Helochelydridae with Paracryptodira [ 6 ] and of Meiolaniformes with Pan-Chelidae [ 26 ] and we here are able to highlight an equivalent overlapping distribution between the newly established clade Sichuanchelyidae and Pan-Cryptodira. Interestingly, the two clades of basal turtles from the northern hemisphere not only correspond with their crown-ward counterparts in their geographic distribution, but also in their temporal distribution, by originating in the Middle to Late Jurassic.

The Jurassic terrestrial record of Gondwana is still too poor to allow tracing groups of turtles with confidence on this landmass. Although it remains unclear if the three groups of basal turtles discussed herein form a clade, the close temporal and spatial association with derived aquatic turtles makes it likely that these groups were separated from one another by the same processes that vicariantly separated Pan-Pleurodira, Pan-Cryptodira, and Paracryptodira i.

However, whereas it would be possible to postulate vicariance as the cause for the breakup of a monophyletic group of basal turtles, a complicated pattern of regional extinctions would have to be postulated if these turtles form a paraphyletic grade. Regardless of the outcome of this debate, the non-overlapping distribution of these three clades makes it apparent that each diversified following the Middle Jurassic from a different ancestor stranded on a different part of the globe. It is notable that Pan-Pelomedusoides is the only clade of derived aquatic turtles that lacks a basal counterpart.

If the phylogeny of basal turtles was driven by the same processes as crown turtles, our model would predict the presence of a hereto undiscovered clade of terrestrial, basal turtles that originated in Northern Gondwana no later than the Early Cretaceous and that went extinct at some time prior to the Recent. The three clades of basal turtles outlined herein i. Some amount of internal movement can only be posited with confidence for Meiolaniidae, as the discovery of highly derived meiolaniids on islands off the coast of continental Australia is best explained by dispersal [ 5 ].

The fossil record of pan-chelid turtles is restricted to the southern portions of South America prior to the Neogene Fig. Molecular phylogenies retrieve monophyletic clades of South American and Australian chelids [ 55 , 56 ] and therefore imply the vicariant origin of modern chelids.

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By contrast, competing morphological hypotheses [ 58 ] either demand the diversification of chelids prior to the breakup of Southern Gondwana with select extinction on both continents, or diversification after the breakup of Southern Gondwana with multiple subsequent dispersal events, neither of which is currently supported by the fossil record.

Multiple lineages of chelid turtles successfully invaded the Amazon Basin during the Neogene [ 44 ]. Our model predicts that chelids must have been present on Antarctica during the Early Cretaceous, but have since gone extinct Fig. The representatives of two clades of pan-pelomedusoid turtles, Bothremydinae and Stereogenyina, are regularly retrieved from littoral to marine sediments [ 59 , 60 , , ] and we therefore reconstruct these clades as being ancestrally adapted to near-shore conditions, even if derived representatives within these clades are occasionally found in freshwater deposits [ 59 ].

If these two clades are disregarded from consideration, the entire pan-pelomedusoid fossil record is restricted to Northern Gondwana i. The question whether the current distribution of podocnemidids in South America and Madagascar is due to vicariance or dispersal remains unresolved see discussion above. The entire fossil record of paracryptodires is restricted to the original land area of the clade Fig. Although initially connected, North America and Europe were slowly fragmented during the Jurassic and Cretaceous by the opening of the North Atlantic Ocean and the Labrador Sea and by the incursion of epicontinental seaways that criss-crossed Euramerica.

As currently preserved, the paracryptodiran record contracted from its maximum in the Late Jurassic i. In contrast to all other clades of turtles, pan-cryptodires dispersed prolifically throughout their evolutionary history and have come to dominate turtle faunas on most continents [ 2 ] Fig. By the Early Cretaceous the Turgai Strait became leaky as demonstrated by an eclectic assemblage of unrelated fresh water taxa in the European fossil record [ 86 , ]. Among freshwater aquatic and terrestrial lineages, trionychids, adocids Adocus , nanhsiungchelyids Basilemys , and macrobaenids successfully invaded North America from Asia via the Bering Land Bridge during the Cretaceous [ 45 ], and carettochelyids Anosteira , emydids, geoemydids, and testudinids followed during warm periods of the Paleogene [ 14 , ] Fig.

The unidentified ancestral lineage of Americhelydia must have followed this path as well. Given that western North America was separated from eastern North America by the intercontinental seaway, it is possible that various taxa migrated to eastern North America via the Arctic [ ]. The Bering Land Bridge was utilized by chelydrids and the Emys orbicularis -lineage to disperse to Eurasia during the Paleogene [ , ] and Neogene [ ], respectively Fig.

Isolated remains of trionychids in South America document a failed attempt to follow this path as well [ ]. Interestingly, no cryptodires are known to have dispersed from South America to North America during the entire Cenozoic e. The northernmost distribution that South American testudinids Chelonoidis achieved was into the Caribbean [ ] Fig.

Carettochelyids Allaeochelys , geoemydids, testudinids, and trionychids invaded Europe [ 31 , 46 ] and India [ 87 , , ] from Asia during the Eocene. Although it is not clear if they dispersed from Asia or from Europe, testudinids arrived in Africa during the Eocene [ ] and carettochelyids, geoemydids, and trionychids followed in the Miocene [ 66 ] Fig. The dispersal of testudinids from Africa to South America is currently dated at the Oligocene [ 68 ] whereas their arrival on Madagascar is calculated to have occurred in the Neogene based on molecular data [ ] Fig. For simplicity, we omitted all dispersal events to small islands in our summary Fig.

Our study differs from previous biogeographic analyses by revealing that the early diversification of turtles from the Middle Jurassic to Early Cretaceous was driven by vicariance, but that this pattern is secondarily obliterated through extensive dispersal throughout the Late Cretaceous and Tertiary. Although many important aspects of this pattern had previously been recognized, in particular in regards to post-Jurassic dispersal [ 6 , 13 , 31 , 45 , 46 , 82 , , ], little vicariance had previously been proposed [ 14 , 44 ].

We identify three factors that helped us discover the primary vicariance pattern. Fossil turtles comprise a significant portion of the fossil vertebrate fauna globally, but little attention had been accorded to the group throughout the 20th century. Much therefore remains to be learned about the fossil record of many groups of turtles [ 57 ]. Affordable international travel and photography furthermore have made it easier to compare directly material from different regions. These aspects have had a significant impact upon our study, as many groups of turtles and transcontinental relationships were only recognized in the last decades.

In contrast to all previous studies, we are able to retrieve clean biogeographic patterns by omitting all groups of easily dispersing littoral to marine turtles from consideration and thereby concentrating our efforts on discerning patterns among slowly dispersing, terrestrial to freshwater aquatic turtles. We would expect the shapes of the plastron and carapace to be integrated in hinged turtles so that a tight fit can be maintained when the shell is closed, but hingeless less turtles may show little or no integration between their plastron and carapace because they do not have this constraint.

Research methods and techniques : In this project we will first collect shell shape data from hingeless and hinged turtle species using the collections of The Field Museum. Then, in collaboration with Dr. Peter Roopnarine California Academy of Sciences , we will use a new method for detecting patterns of integration in the turtle data set. The intern will be trained in the collection and analysis of geometric morphometric data, analysis of morphological integration, and the use of phylogeny as a framework for analyzing comparative data. Stephanie Ware is currently a research assistant in the Division of Insects currently working with Dr.

Petra Sierwald. Skip to main content. The Field Museum. Tickets Membership The Field Museum fuels a journey of discovery across time to enable solutions for a brighter future rich in nature and culture. Category: Blog Tags Articles.