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Triassic Nothosaur

Paleontologists have found two nearly complete skeletons from a new genus and species of nothosauroid marine reptile that lived during the Middle Triassic in what is now China. Dubbed Brevicaudosaurus jiyangshanensis, the ancient creature used its short and flat tail for balance and floated near the bottom of the edges in shallow water, picking off prey with fang-like teeth.

An artist’s reconstruction of Brevicaudosaurus jiyangshanensis. Image credit: Tyler Stone, tylerstoneart.wordpress.com.

An artist’s reconstruction of Brevicaudosaurus jiyangshanensis. Image credit: Tyler Stone, tylerstoneart.wordpress.com.

Brevicaudosaurus jiyangshanensis lived during the Triassic period, approximately 240 million years ago.

The extinct reptile had a small head, fangs, flipper-like limbs, a long neck, and a short and flat tail.

The two nearly complete skeletons of Brevicaudosaurus jiyangshanensis were collected from a thin layer of limestone in two quarries in Fuyuan County, southwest China.

The specimens were examined by Dr. Qing-Hua Shang from the Institute of Vertebrate Paleontology and Paleoanthropology at the Chinese Academy of Sciences and colleagues.

“Our analysis of two well-preserved skeletons reveals a reptile with a broad, pachyostotic body (denser boned) and a very short, flattened tail,” they said.

“A long tail can be used to flick through the water, generating thrust, but the new species was probably better suited to hanging out near the bottom in shallow sea, using its short, flattened tail for balance, like an underwater float, allowing it to preserve energy while searching for prey.”

The forelimbs of Brevicaudosaurus jiyangshanensis are more strongly developed than its hind limbs, suggesting they played a role in helping the reptile to swim.

However, the bones in the front feet are short compared to other species, limiting the power with which it could pull through the water.

Most of the reptile’s bones, including the vertebrae and ribs, are thick and dense, further contributing to the stocky, stout appearance and limiting its ability to swim quickly but increasing stability underwater.

However, thick, high-mass bones act as ballast. What Brevicaudosaurus jiyangshanensis lost in speed, it gained in stability.

Dense bones, known as pachyostosis, may have made it neutrally buoyant in shallow water.

Together with the flat tail, this would have helped the predator to float motionless underwater, requiring little energy to stay horizontal.

Neutral buoyancy should also have enabled it to walk on the seabed searching for slow-moving prey.

Brevicaudosaurus jiyangshanensis skeletons in dorsal view. Image credit: Shang et al., doi: 10.1080/02724634.2020.1789651.

Brevicaudosaurus jiyangshanensis skeletons in dorsal view. Image credit: Shang et al., doi: 10.1080/02724634.2020.1789651.

Highly dense ribs of Brevicaudosaurus jiyangshanensis may also suggest the reptile had large lungs.

As suggested by the lack of firm support of the body weight, nothosaurs were oceanic nut they needed to come to the water surface for oxygen.

They have nostrils on the snout through which they breathed. Large lungs would have increased the time the species could spend under water.

Brevicaudosaurus jiyangshanensis features a bar-shaped bone in the middle ear called the stapes, used for sound transmission.

The stapes was generally lost in other nothosaurs or marine reptiles during preservation.

Paleontologists had predicted that if a stapes was found in a nothosaur, it would be thin and slender like in other species of this branch of the reptilian family tree.

However, in Brevicaudosaurus jiyangshanensis it is thick and elongate, suggesting it had good hearing underwater.

“Perhaps this small, slow-swimming marine reptile had to be vigilante for large predators as it floated in the shallows, as well as being a predator itself,” said Dr. Xiao-Chun Wu, a researcher at the Canadian Museum of Nature.

The discovery is reported in a paper published today in the Journal of Vertebrate Paleontology.

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Qing-Hua Shang et al. A New Ladinian Nothosauroid (Sauropterygia) from Fuyuan, Yunnan Province, China. Journal of Vertebrate Paleontology, published online October 29, 2020; doi: 10.1080/02724634.2020.1789651

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