The origin of turtles: A paleontological perspective

The origin of turtles and their unusual body plan has fascinated scientists for the last two centuries. Over the course of the last decades, a broad sample of molecular analyses have favored a sister group relationship of turtles with archosaurs, but recent studies reveal that this signal may be the result of systematic biases affecting molecular approaches, in particular sampling, non-randomly distributed rate heterogeneity among taxa, and the use of concatenated data sets. Morphological studies, by contrast, disfavor archosaurian relationships for turtles, but the proposed alternative topologies are poorly supported as well. The recently revived paleontological hypothesis that the Middle Permian Eunotosaurus africanus is an intermediate stem turtle is now robustly supported by numerous characters that were previously thought to be unique to turtles and that are now shown to have originated over the course of tens of millions of years unrelated to the origin of the turtle shell. Although E. africanus does not solve the placement of turtles within Amniota, it successfully extends the stem lineage of turtles to the Permian and helps resolve some questions associated with the origin of turtles, in particular the non-composite origin of the shell, the slow origin of the shell, and the terrestrial setting for the origin of turtles

A review of the fossil record of turtles of the clade Pan-Chelydridae

Turtles of the total clade Pan-Chelydridae have a relatively sparse fossil record that reaches back to the Late Cretaceous (Santonian). The clade was only present in North America during the Cretaceous but spread along unclear routes to Asia and Europe during the Paleocene, only to go extinct on those continents by the end of the Pliocene. Final dispersal to South America took place at some time during the late Neogene. The ecology of stem chelydrids seems to have been similar to that of the extant Chelydra serpentina, although more primitive representatives were more molluscivorous as inferred from their broader triturating surfaces. Current phylogenies only recognize five internested clades: Pan-Chelydridae, Chelydridae, Chelydropsis, Chelydra and Macrochelys. A taxonomic review of the group concludes that of 31 named fossil taxa, 8 are nomina valida, 10 are nomina invalida, 9 are nomina dubia, 1 is a nomen nudum and 1 is a regular, unavailable name

A review of the fossil record of basal mesozoic turtles

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 the earliest stem turtles, but a more mixed, though unambiguously continental signal is apparent further towards the crown. A taxonomic review of Mesozoic stem turtles, excluding representatives of the Gondwanan Meiolaniformes, concludes that of 48 named taxa, 26 are nomina valida, 18 are nomina invalida, 4 are nomina dubia, 1 is a nomen nudum, and that 9 do not represent turtles

A Review of the Fossil Record of Turtles of the Clades Platychelyidae and Dortokidae

The fossil record of platychelyid turtles expands from the Late Jurassic (Oxfordian) of Cuba to the Early Cretaceous (Valanginian) of Colombia. Platychelyids were adapted to freshwater to coastal environments. Current phylogenies confidently suggest that platychelyids are situated along the stem lineage of crown Pleurodira. A taxonomic review of the group concludes that of six named “platychelyid” taxa, four are valid and two are nomina nuda. Dortokids are a poorly understood group of freshwater aquatic turtles that are restricted to the Early Cretaceous (Barremian) to Eocene (Lutetian) of Europe. The phylogenetic position of the group is still under debate, but there is some evidence that these turtles are positioned along the stem lineage of crown Pleurodira as well. A taxonomic review of the group concludes that of four named dortokid taxa, two are valid, one is a nomen invalidum and one a nomen nudum

A reassessment of the late jurassic turtle Eurysternum wagleri (Eucryptodira, Eurysternidae)

Eurysternum wagleri is one of the first named, yet most poorly understood turtles from the Late Jurassic of Europe. Over the years, many specimens have been referred to and many species synonymized with E. wagleri, but little consensus is apparent, and the taxonomy is therefore highly confusing. Based on the rare, only known illustration of the lost holotype and on the reassessment of select individuals, the species E. wagleri is recharacterized herein. Eurysternum wagleri is diagnosed by a deep pygal notch, a carapace with a pentagonal outline, a contribution of vertebral 5 to the posterior carapace margin, three cervical scales, very wide vertebral scales with a well-developed radiating pattern, well-developed costoperipheral fontanelles in medium-sized individuals, a plastron connected to the carapace by ligaments, gracile, peg-like bony projections of the hyo- and hypoplastra, and large, oval-to-quadrangular lateral plastral fontanelles. A lectotype is designated for Acichelys redenbacheri, and this taxon is interpreted as the junior subjective synonym of Eurysternum wagleri. All other, previously proposed synonymies are rejected, because they lack characters that would allow diagnosing them as E. wagleri

A review of the fossil record of old world turtles of the clade pan-trionychidae

Turtles of the clade Pan-Trionychidae have a rich fossil record in the Old World, ranging from the Early Cretaceous (Hauterivian) to the Holocene. The clade most probably originated in Asia during the Early Cretaceous but spread from there to the Americas and Europe by the Late Cretaceous, to India and Australia by the Eocene, and to Afro-Arabia by the Neogene. The presence of a single pan-cyclanorbine in the Late Cretaceous (Maastrichtian) of Asia provides a minimum estimate for the age of the trionychid crown. As preserved, diversity was relatively high in Asia during the Late Cretaceous, but the subsequent, strong decline is likely a preservational bias, as extant faunas are relatively rich, especially throughout Asia. The range of trionychids contracted southward in Europe over the course of the Neogene, and the group is now locally extirpated. The group is now similarly absent from Arabia and Australia. A taxonomic review of the 180 named Old World taxa finds 42 nomina valida, 38 nomina invalida, 88 nomina dubia, 11 nomina nuda, and 1 nomen suppressum

A Review of the fossil record of turtles of the clade Pan-Kinosternoidea

Turtles of the total clade Pan-Kinosternoidea have a relatively poor fossil record that extends back to the Late Cretaceous (Campanian). The clade is found only in North America during its early history, but dispersed to Central America no later than the Miocene and to South America no later than the Pleistocene. Ancestral pan-kinosternoids were likely aquatic, bottom-walking omnivores or carnivores that preferred low-energy freshwater habitats. The Pan-Dermatemys lineage is often recovered in more fluvial habitats, and some are specialized to feed on aquatic vegetation. Alternatively, many representatives of Kinosternon evolved specializations (e.g., plastral lobe kinesis) that allowed them to successfully inhabit and disperse across more terrestrial habitats such as savannas and floodplains. A taxonomic review of the group concludes that of 42 named taxa, 27 are nomina valida (including two species of the controversial taxon Planetochelys), 14 are nomina invalida and only one a nomen dubium

The first soft-shelled turtle from the Jehol Biota of China

A new turtle from the Early Cretaceous (Aptian) Jiufotang Formation of western Liaoning, China, Perochelys lamadongensis, gen. et sp. nov., represents the first species of soft-shelled turtle from the Jehol Biota. The new taxon is diagnosed by the combination of the following characters: nuchal bone about five times wider than long; preneural absent; reversal of the orientation in the neural series at neural V; neural series fully separates costal series; costal VIIIs reduced; plastral callosities poorly developed and poorly sculpted; postorbital bar narrow, around one-fourth of orbit diameter; jugal contacting squamosal; foramen jugulare posterius separated from fenestra postotica; neural spines weakly developed on anterior cervicals; and phalangeal formula for pes 2-3-3-4-?. High levels of homoplasy make the phylogenetic relationships of the new taxon difficult to assess, and the possibility therefore exists that Perochelys lamadongensis either represents a stem or a crown trionychid. This phylogenetic uncertainty confirms that the skeletal morphology of trionychids has remained virtually unchanged for the last 120 million years

A review of the fossil record of turtles of the clade Thalassochelydia

The Late Jurassic (Oxfordian to Tithonian) fossil record of Europe and South America has yielded a particularly rich assemblage of aquatic pan-cryptodiran turtles that are herein tentatively hypothesized to form a monophyletic group named Thalassochelydia. Thalassochelydians were traditionally referred to three families, Eurysternidae, Plesiochelyidae, and Thalassemydidae, but the current understanding of phylogenetic relationships is insufficient to support the monophyly of either group. Given their pervasive usage in the literature, however, these three names are herein retained informally. Relationships with marine turtles from the Cretaceous have been suggested in the past, but these hypotheses still lack strong character support. Thalassochelydians are universally found in near-shore marine sediments and show adaptations to aquatic habitats, but isotopic evidence hints at a broad spectrum of specializations ranging from freshwater aquatic to fully marine. A taxonomic review of the group concludes that of 68 named taxa, 27 are nomina valida, 18 are nomina invalida, 18 are nomina dubia, and 5 nomina oblita

A new baenid turtle from the early Paleocene (Torrejonian) of New Mexico and a species-level phylogenetic analysis of Baenidae

New cranial and postcranial material of the baenid turtle Neurankylus from the Paleocene Nacimiento Formation (Torrejonian NALMA) of northwestern New Mexico represents a new species, Neurankylus torrejonensis. The material consists of a fragmented but mostly complete skull, a partial carapace and plastron, portions of both humeri, a partial pelvis, a complete right femur, and a distal phalanx. The small, undivided cervical scale, wide vertebrals, complete ring of marginals, and large size (carapace length 520 mm) diagnose the new taxon as belonging to Neurankylus. The narrow fifth vertebral scale and scalloped posterior shell margin reveal affinities with Neurankylus baueri Gilmore, 1916, which is known from Campanian sediments in New Mexico and Utah. The holotype of Neurankylus torrejonensis is the youngest known specimen of the Neurankylus lineage, which is known to reach at least back to the Late Cretaceous (Santonian). A nearly complete species-level analysis of baenids confirms the basal placement of Neurankylus outside of Baenodda and the split of Baenodda into two primary subclades, herein named Palatobaeninae and Eubaeninae