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[dinosaur] Basal Mesozoic Turtles + Pan-Trionychids + oldest known softshell turtles from Japan




Ben Creisler
bcreisler@gmail.com

Some recent fossil turtle papers:


Walter G. Joyce (2017)
A Review of the Fossil Record of Basal Mesozoic Turtles.
Bulletin of the Peabody Museum of Natural History 58(1):65-113
doi: http://dx.doi.org/10.3374/014.058.0105
http://www.bioone.org/doi/abs/10.3374/014.058.0105


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.

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Georgios L. Georgalis and Walter G. Joyce (2017)
A Review of the Fossil Record of Old World Turtles of the Clade Pan-Trionychidae.
Bulletin of the Peabody Museum of Natural History 58(1):115-208. 
doi: http://dx.doi.org/10.3374/014.058.0106
http://www.bioone.org/doi/abs/10.3374/014.058.0106


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.


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Yasuhisa Nakajima, Igor G. Danilov, Ren Hirayama, Teppei Sonoda & Torsten M. Scheyer (2017)
Morphological and histological evidence for the oldest known softshell turtles from Japan.
Journal of Vertebrate Paleontology  e1278606 
doi:  http://dx.doi.org/10.1080/02724634.2017.1278606   
http://www.tandfonline.com/doi/full/10.1080/02724634.2017.1278606

Herein we report morphologically and histologically diagnostic trionychid specimens from the Barremian–Aptian (129.4–113.0 Ma) of Japan. One specimen (FPDM-V9487) is an associated skeleton that consists of a scapula, a humerus, an ischium, and hypoplastra. The limb and girdle bones are similar in morphology to those of modern trionychids. The hypoplastron also resembles that of modern trionychids; however, it is distinctive in that it completely lacks callosities. The other three new specimens are fragmentary costals that show a reduction in the shell bones and an absence of scute sulci. Two of the newly reported costals and FPDM-V0127 were examined histologically. Bone fiber bundles organized in a plywood-like structure, which is unique to trionychids, were identified. Fossil occurrence data indicate that morphologically and histologically typical trionychids already inhabited the coastal region of Asia (e.g., western Japan) as early as the Aptian. In contrast, the Hauterivian–Aptian stem trionychid Kappachelys okurai from Japan does not show a plywood-like shell microstructure, suggesting that K. okurai could be the earliest-branching taxon of known stem trionychids or a carettochelyid. Our paleobiogeographic compilation suggests that the spread of wetlands in the Northern Hemisphere and the high global temperature during the middle Cretaceous (Aptian–Turonian, 125.0–89.8 Ma) might have contributed to the dispersal of trionychids from Asia, allowing for the establishment of this modern freshwater reptilian fauna.

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