A thin-shelled reptile from the Late Triassic of North America and the origin of the turtle shell

Abstract

A new, thin-shelled fossil from the Upper Triassic (Revueltian: Norian) Chinle Group of New Mexico, Chinlechelys tenertesta, is one of the most primitive known unambiguous members of the turtle stem lineage. The thin-shelled nature of the new turtle combined with its likely terrestrial habitat preference hint at taphonomic filters that basal turtles had to overcome before entering the fossil record. Chinlechelys tenertesta possesses neck spines formed by multiple osteoderms, indicating that the earliest known turtles were covered with rows of dermal armour. More importantly, the primitive, vertically oriented dorsal ribs of the new turtle are only poorly associated with the overlying costal bones, indicating that these two structures are independent ossifications in basal turtles. These novel observations lend support to the hypothesis that the turtle shell was originally a complex composite in which dermal armour fused with the endoskeletal ribs and vertebrae of an ancestral lineage instead of forming de novo. The critical shell elements (i.e. costals and neurals) are thus not simple outgrowths of the bone of the endoskeletal elements as has been hypothesized from some embryological observations.

Reassessment of the phylogenetic interrelationships of basal turtles (Testudinata)

“…In Odontochelys semitestacea (Li et al 2008) and the majority of other reptiles (e.g., Romer 1956), the transverse process of each dorsal vertebra is located around the middle of the centrum. According to Joyce et al (2009), the contact of dorsal ribs with two successive centra is only incipient in the Norian turtle Chinlechelys tenertesta (not included in the present analysis given the fragmentary nature of the material). According to Gaffney (1990: 119) this articulation between dorsal ribs and two successive centra "occurs in all turtles [known at that time] to some extent but usually the posterior thoracics tend to lose the articulation with the posterior ribs and maintain a complete articulation with their own rib anteriorly" (see also Hoffstetter & Gasc, 1969).…”

“…6). Contact between dorsal vertebrae and dorsal ribs: 0 = transverse processes of all dorsal vertebrae in the middle or anterior part of the vertebral centra; 1 = dorsal ribs in contact with two successive vertebral centra (new character; derived from Rieppel & Reisz 1999;Li et al 2008;Joyce et al 2009). All turtles, with the notable exception of Odontochelys semitestacea, are characterised by a peculiar relationship between the dorsal vertebral centra and ribs: each dorsal rib contacts the vertebrae on the boundary between two successive centra (Hoffstetter & Gasc 1969;Rieppel & Reisz 1999: 15;Joyce et al 2009: 509).…”

“…The last two decades have witnessed the discovery of few additional early turtles such as Palaeochersis talampayansis (Late Triassic, Argentina), Australochelys africanus (Early Jurassic, South Africa), and Kayentachelys aprix (Early Jurassic, USA), which allowed to complete in some ways the turtle phylogenetic tree. However, it is the past few years which have been most exciting for specialists with the discovery of Late Triassic turtles more primitive than Proganochelys quenstedti (Li et al 2008;Joyce et al 2009), as well as of several species from the Middle Jurassic (Sukhanov 2006;Sterli 2008;Anquetin et al 2009), a period from which the turtle fossil record is particularly sparse. These discoveries coincided with a major reappraisal of phylogenetic relationships within turtles by Joyce (2007).…”

Turtles and Tortoises of the World During the Rise and Global Spread of Humanity: First Checklist and Review of Extinct Pleistocene and Holocene Chelonians.

“…Major contributions to this topic include Gaffney (1984), Gaffney and Meylan (1988), Joyce (2007), Gaffney et al (2007), Danilov and Parham (2008), Sterli (2008Sterli ( , 2010, Joyce et al (2009Joyce et al ( , 2013, Barley et al (2010), Gaffney and Jenkins (2010), Sterli and de la Fuente (2013), and Crawford et al (2015).…”

A new stem turtle from the Middle Jurassic of Scotland: new insights into the evolution and palaeoecology of basal turtles

“…New discoveries have helped to narrow the major temporal and morphological gaps that existed between the earliest known turtles, the Late Triassic Proganochelys quenstedti and Proterochersis robusta, and the turtle crown group, which diversified during the Late Jurassic. These discoveries have included the recognition of additional taxa from the Late Triassic and Early Jurassic (Gaffney et al 1987;Gaffney & Kitching 1994;Rougier et al 1995;Joyce et al 2009): Palaeochersis talampayensis (Late Triassic, Argentina); Chinlechelys tenertesta (Late Triassic, USA); Australochelys africanus (Early Jurassic, South Africa); and Kayentachelys aprix ( Early Jurassic, USA). Perhaps more importantly, these discoveries have prompted major reappraisals of basal turtle relationships (Rougier et al 1995;Joyce 2007) that have challenged what may be termed the 'traditional' interpretation of turtle phylogeny (e.g.…”