An ancestral turtle from the Late Triassic of southwestern China

Abstract

The origin of the turtle body plan remains one of the great mysteries of reptile evolution. The anatomy of turtles is highly derived, which renders it difficult to establish the relationships of turtles with other groups of reptiles. The oldest known turtle, Proganochelys from the Late Triassic period of Germany, has a fully formed shell and offers no clue as to its origin. Here we describe a new 220-million-year-old turtle from China, somewhat older than Proganochelys, that documents an intermediate step in the evolution of the shell and associated structures. A ventral plastron is fully developed, but the dorsal carapace consists of neural plates only. The dorsal ribs are expanded, and osteoderms are absent. The new species shows that the plastron evolved before the carapace and that the first step of carapace formation is the ossification of the neural plates coupled with a broadening of the ribs. This corresponds to early embryonic stages of carapace formation in extant turtles, and shows that the turtle shell is not derived from a fusion of osteoderms. Phylogenetic analysis places the new species basal to all known turtles, fossil and extant. The marine deposits that yielded the fossils indicate that this primitive turtle inhabited marginal areas of the sea or river deltas.

Specialization for underwater hearing by the tympanic middle ear of the turtle,Trachemys scripta elegans

“…Specializations for underwater hearing may reflect the primitive condition in turtles if they originated in aquatic habitats. A marine origin is inferred for turtles considered closely related to extinct marine sauropterygian diapsids [44].…”

Jaw musculature during the dawn of turtle evolution

“…As mentioned above, a cavum tympani is absent in O. semitestacea (Li et al 2008). Although slightly concave in all early Testudinata, Sterli and de la Fuente (2010) determined that a particularly enlarged otic region first occurs just below the appearance of the Testudines clade.…”

“…This could, on the one hand, speak in favour of the hypothesis that related muscle characters have been conserved since the first occurrence of Testudinata. However, first, O. semitestacea still had teeth on the dentary and bones related to the palate region (Li et al 2008) and, as mentioned above, a basicranial kinesis of the turtle skull is still assumed to have been present in O. semitestacea and P. quenstedti. Both features may require different muscle arrangements to those reconstructed in my analysis.…”

“…The quadrate is weakly concave posteriorly; a cavum tympani is absent." To what degree O. semitestacea shows a derived or juvenile condition (Li et al 2008;Scheyer et al 2012) cannot be discussed adequately herein. The presumably second most "basal" turtle is Proganochelys quenstedti (Gaffney 1990), which, together with (and as a sister taxon to) the remaining Testudinata, is sister to O. semitestacea (Li et al 2008;Lyson et al 2010).…”

“…To what degree O. semitestacea shows a derived or juvenile condition (Li et al 2008;Scheyer et al 2012) cannot be discussed adequately herein. The presumably second most "basal" turtle is Proganochelys quenstedti (Gaffney 1990), which, together with (and as a sister taxon to) the remaining Testudinata, is sister to O. semitestacea (Li et al 2008;Lyson et al 2010). Following the study of Sterli and de la Fuente (2010), the following evolutionary changes have to be considered for the adductor chamber in turtles:…”