A new species of extinct turtle has been named based on a fossil from the Yixian Formation, part of the famous ‘Jehol Biota.’
Reconstruction of Xiaochelys ningchengensis in its freshwater environment, preying on a small fish called Lycoptera. Image credit: W.S. Wang.
The Jehol Biota is a rich Cretaceous ecosystem preserved in a multi-layered rock formation cropping out in the Chinese provinces of Liaoning, Hebei and Inner Mongolia. A huge variety of ancient organisms became fossilized there 125 million years ago.
Over the course of the last two decades, a large number of turtle fossils were recovered from the Jehol Biota but very few of them were described.
Dr Chang-Fu Zhou of the Shenyang Normal University and Dr Márton Rabi of the University of Tübingen and the Hungarian Academy of Sciences have now identified a new species of Jehol turtle and named it Xiaochelys ningchengensis.
According to a new paper published in the journal Scientific Reports, this new species is so unique that it also represents a new genus.
Dr Rabi and Dr Zhou also investigated its possible relationship with today’s chelonians (turtles, tortoises, and terrapins).
Xiaochelys ningchengensis: positive and negative impressions preserved in the layers of stone, and an illustration of the skeleton (right). Image credit: Chang-Fu Zhou.
They wanted to test a hypothesis that the Jehol turtles belong to a lineage that eventually gave rise to today’s sea turtles.
“According to our findings, the Jehol turtles are instead found on the lineage leading to the cryptodiran turtles,” Dr Zhou said.
“The cryptodires – which also include sea turtles – are able to pull their heads and necks vertically into their shells using an S-shaped motion.”
“However, a placement of the Jehol turtles close to sea turtles on the family tree is only slightly less supported statistically. Therefore, the earliest known sea turtles are likely to have looked much like the species found in the Jehol Biota,” Dr Rabi added.
“These well-preserved fossils give us insights into the origin of cryptodires. About three-quarters of today’s turtles belong to that group,” Dr Zhou said.
It remains unclear, however, just how the main adaptations of sea turtles arose for a marine habitat.
These evolutionary changes included the reduction of their skeleton and the development of large and rigid paddles which enable the creatures to swim in a style which is best described as underwater flight.
“The origin of sea turtles was a major morphological transition in vertebrate evolution but we still don’t really understand how it happened,” Dr Rabi said.
“It was a highly successful adaptation and it is truly depressing to see that the last surviving marine reptiles are threatened with extinction after more than 130 million years.”
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Chang-Fu Zhou & Márton Rabi. A sinemydid turtle from the Jehol Biota provides insights into the basal divergence of crown turtles. Scientific Reports 5, article number: 16299; doi: 10.1038/srep16299
