An ancestral turtle from the Late Triassic of southwestern China

TLDR: A new 220-million-year-old turtle from China is described that documents an intermediate step in the evolution of the shell and associated structures and shows that the turtle shell is not derived from a fusion of osteoderms.

Abstract: A well preserved 220-million-year-old fossil from marine deposits of the Late Triassic of Guizhou in southwest China sheds light on the intermediate steps in the acquisition of the unique turtle body-plan. Transitional forms are scarce in this lineage, making this transition one of the mysteries of reptile evolution. The find is the most primitive turtle known. It has a fully developed plastron, the ventral dermal armour, evolved before the carapace, the dorsal (upper) part of the shell structure. In this fossil the carapace consists of neural plates only. This suggest that the carapace developed via ossification of the neural plates and broadening of the ribs — a sequence that echoes the developmental pattern in young turtles today. 220-million-year-old fossils from southwestern China represent the most primitive turtle known, and shed light on intermediate steps in the acquisition of the unique and highly specialized turtle body-plan. 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 Germany1, 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.