Turtles are charismatic and recognizable creatures that sport a unique characteristic: a shell comprised of two parts. Their shell features a carapace that covers the dorsal or back side of the animal, and a plastron, the flat belly. This design has long served a line of creatures that witnessed the flowering and fading of the mightiest of the giant dinosaurs’ rule on Earth. However, until now, all previously known prehistoric turtles possessed the same body plan, creating a puzzling origin to the lineage.
According to U of T Mississauga biology professor Robert Reisz, the discovery of 220-million-year-old fossils in marine deposits in southwest China has “opened up a new chapter in the study of the origins and early history of these fascinating reptiles.” Belonging to a newly identified species named Odontochelys semitestacea, meaning “toothed turtle with a half-shell,” they are now the oldest known species of turtle. The 35-centimetre long Odontochelys has expanded dorsal ribs and possesses the expected flat belly plastron. However, on its back it has no bony upper carapace. These traits enabled a research team led by Chinese biologist Chun Li, who unearthed the creature, to infer that shell evolution was a two-step process for an ancestral aquatic turtle, whereby the plastron developed before the carapace. This fits with the observation that the plastron develops first in turtle embryos.
In an article by Reisz, co-authored with UTM biology professor Jason Head, an alternate interpretation was offered. The two scientists argued that the shell morphology of Odontochelys is not the primitive state for the turtle lineage. Under this view, Odontochelys is a more advanced animal whose carapace was much reduced and softened, as a secondary or derived adaptation from a land-dwelling ancestor that lived even earlier. The study noted that the sea-going Odontochelys’ ecology closely resembles modern creatures such as freshwater soft-shell turtles with heavily reduced shells, as well as many sea turtles and snapping turtles whose shells have become somewhat unhardened. “Up to now, all the evidence suggested that the oldest turtles are terrestrial,” explained Head. “With this interpretation, that the turtle could indeed be aquatic, we may be pushing the frontiers of turtle origins further back.”
Despite their differing views, both groups of scientists feel that the fossil is the most primitive turtle. The discovery demonstrates how important new fossils are, as well as their ability to transform how we look at the course of vertebrate evolution. “This fossil discovery has brought up a new perspective and has given us new ideas and challenges,” said Reisz.
Prior to this discovery, the oldest known fossil turtle was the larger land-dwelling Proganochelys found in Germany. A metre-long beast with a spiky clubbed tail, it lived about ten million years after Odontochelys.
