Turtle evolution mystery deepens
Newly discovered fossil suggests “evolutionary reversal”, reports Stephen Fleischfresser.
A fossil discovered in China has brought science one step closer to connecting the dots of turtle evolution, reveals a report published in Nature.
Scientists know that the precursors of modern turtles and tortoises that had a generally recognisable body plan first appeared in the late Triassic period, roughly 200 million years ago. However, exactly what group of reptiles they descended from remains one of evolution’s most enduring puzzles.
For more than a century the oldest known turtle was Proganochelys. It had a fully formed shell, but its origins remained unknown. However, its dating has been overturned by a series of recent findings.
A fossil form first identified in the nineteenth century, called Eunotosaurus, has lately been suggested as being the last common ancestor of turtles. Eunotosaurus was a land-dwelling lizard-like reptile with bone structure similar to that of turtles, hailing from Africa 260 million years ago.
Subsequently, transitional fossils between Eunotosaurus and Proganochelys have been discovered and are helping scientists understand how modern turtles connect to the reptiles of the Mesozoic Era, of which the Triassic was the first part.
In 2015, a pair of German researchers revealed Pappochelys, a semi-aquatic transitional turtle species dating back to 240 million years. This built on the earlier discovery in 2008 of the possibly sea-dwelling Odontochelys, another transitional fossil dating back to 220 million years.
The study of turtle evolution is dogged by an ongoing conflict between sources of evidence: morphological or anatomical studies suggest a connection to an early sister clade to reptiles known as Parareptilia, while molecular evidence points to a place amongst the true reptiles.
Odontochelys led many to place turtles in the vast and diverse clade known as the diapsids, which includes birds and crocodiles. This was an unusual case of morphological evidence supporting a reptile classification.
Now, three of the four discoverers of Odontochelys have revealed yet another transitional species. Chun Li of the Chinese Academy of Sciences, Beijing, Olivier Rieppel of the Field Museum of Natural History, Chicago, in the US, Xiao-Chun Wu of the Canadian Museum of Nature, Ottawa, are joined by Nicholas Fraser of the National Museums Scotland in the UK and have published their findings on Eorhynchochelys sinensis.
E. sinensis, discovered in south western China, is a two-and-a-half-metre complete fossil of a turtle ancestor that was without a fully developed shell. Dating back to 228 million years, it fits between Pappochelys and Odontochelys, but oddly shows much more pronounced turtle-like characteristics than the younger Odontochelys, having a head very similar to modern turtles and sporting the true turtle beak.
However, E. sinensis also has anatomical features that hark back to species predating Eunotosaurus, in what the authors call “an evolutionary reversal”.
While evidence is always welcome, the most recent find does little to clarify turtle evolution. If anything, it points to an increasingly complex story that will require a great deal more work to make sense of.