A Middle Triassic stem-turtle and the evolution of the turtle body plan

Abstract

The origin and early evolution of turtles have long been major contentious issues in vertebrate zoology. This is due to conflicting character evidence from molecules and morphology and a lack of transitional fossils from the critical time interval. The ∼220-million-year-old stem-turtle Odontochelys from China has a partly formed shell and many turtle-like features in its postcranial skeleton. Unlike the 214-million-year-old Proganochelys from Germany and Thailand, it retains marginal teeth and lacks a carapace. Odontochelys is separated by a large temporal gap from the ∼260-million-year-old Eunotosaurus from South Africa, which has been hypothesized as the earliest stem-turtle. Here we report a new reptile, Pappochelys, that is structurally and chronologically intermediate between Eunotosaurus and Odontochelys and dates from the Middle Triassic period (∼240 million years ago). The three taxa share anteroposteriorly broad trunk ribs that are T-shaped in cross-section and bear sculpturing, elongate dorsal vertebrae, and modified limb girdles. Pappochelys closely resembles Odontochelys in various features of the limb girdles. Unlike Odontochelys, it has a cuirass of robust paired gastralia in place of a plastron. Pappochelys provides new evidence that the plastron partly formed through serial fusion of gastralia. Its skull has small upper and ventrally open lower temporal fenestrae, supporting the hypothesis of diapsid affinities of turtles.

Sensory Evolution and Ecology of Early Turtles Revealed by Digital Endocranial Reconstructions

“…The scant fossil record of stem-turtles (i.e., non-Testudines Testudinata) has further obscured the evolutionary origin of this group. Recent discoveries of new species and reanalysis of existing specimens with novel methodological approaches (e.g., computed tomography and digital visualization) have provided new data to the debate of turtle ancestry (Li et al, 2008;Bever et al, 2015;Schoch and Sues, 2015). These studies found support for the diapsid origin of turtles and produced potential evidence for closure of the temporal fenestrae early in their evolutionary history (Schoch and Sues, 2015;Werneburg, 2015;Lyson et al, 2016).…”

“…In contrast, the morphology of the ribs, as well as the more rigid body wall, powerful forelimbs and triangular skull, have been considered to represent adaptations to fossoriality in E. africanus (Lyson et al, 2016). On the other hand, the type localities of both P. rosinae and O. semitestacea have also yielded terrestrial taxa (Joyce, 2015;Schoch and Sues, 2017) and, in fact, terrestrial diapsid remains are dominant at the type locality of the former (Schoch and Sues, 2015). Additionally, Joyce (2015) argued in favor of Odontochelys semitestacea as a terrestrial proto-turtle, based on its phalangeal formula.…”

“…220 Ma) were retrieved from lacustrine and deltaic deposits and were considered to have been semi-aquatic (Li et al, 2008;Rieppel, 2013;Schoch and Sues, 2015). In the last two taxa, the dorsoventrally flattened, expanded ribs, and thickened gastralia have been interpreted as adaptations for buoyancy control in an aquatic environment (Schoch and Sues, 2015). In contrast, the morphology of the ribs, as well as the more rigid body wall, powerful forelimbs and triangular skull, have been considered to represent adaptations to fossoriality in E. africanus (Lyson et al, 2016).…”

“…240 Ma) and Odontochelys semitestacea (ca. 220 Ma) were retrieved from lacustrine and deltaic deposits and were considered to have been semi-aquatic (Li et al, 2008;Rieppel, 2013;Schoch and Sues, 2015). In the last two taxa, the dorsoventrally flattened, expanded ribs, and thickened gastralia have been interpreted as adaptations for buoyancy control in an aquatic environment (Schoch and Sues, 2015).…”

“On the Fence” versus “All in”: Insights from Turtles for the Evolution of Aquatic Locomotor Specializations and Habitat Transitions in Tetrapod Vertebrates

“…The oldest, fully-shelled turtles were likely terrestrial (Joyce and Gauthier 2004;Scheyer and Sander 2007;Schoch and Sues 2015). However, extensive use of aquatic habitats became pervasive soon thereafter, to the extent that, among living turtles, all terrestrial lineages are nested within primarily aquatic clades (Gosnell et al 2009).…”