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Scientists have reclassified a prehistoric marine reptile discovered in 1935 as one of the last thalatosaurs, rather than an early choristoder, following new discoveries and detailed images.

The true identity of a local prehistoric marine reptile has been revealed after experts determined that some of its remains actually belonged to a fish.

Researchers from the University of Bristol and the University of Southampton have found that bones found in Triassic the rocks in 1935 were from one of the last thalatosaurs, a large marine lizard that behaved like an otter.

For years, the ancient animal was thought to be one of the first choristoderas, another group of crocodile-like marine reptiles. However, in the study published in Journal of Vertebrate Paleontologythe team examined the original named specimen from 1935. They compared this with a remarkable new specimen from Pachystropheusknown as “Annie,” which contains hundreds of bones from several individuals, as well as evidence of sharks, bony fish, and even land-based dinosaurs.

Advanced techniques and features

Jacob Quinn, who is studying for an MSc in palaeobiology at Bristol’s School of Earth Science, traveled with the two specimens to Southampton where they were scanned with CT scans, producing stacks of X-rays through the blocks that allowed him to reconstruct a full 3D model of all buried in the blocks.

“Thalatosaurs were around throughout the Triassic,” explained Jacob. “Some of them reached four meters (13 feet) in length and would have been the terrors of the seas. But ours Pachystropheus it was only a meter long and half of that was its long tail. It also had a long neck, a small head about the size of a matchbox that we hadn’t found, and four oars. If it were like its relatives, it would have very small sharp teeth, perfect for catching fish and other small, wriggling prey.

Rhaetian (205 million years ago) food web of the Bristol Archipelago containing Pachystropheus rhaeticus. Arrows indicate who eats whom – red and black indicate inference, and blue arrows are based on ecological and fossil associations observed during this study. Credit: Jacob Quinn

“Pachystropheus was previously identified as the first of the choristoderes, another group of crocodile-like marine reptiles, and was treated as very important because it was the oldest,” said Professor Mike Benton, one of Jacob’s supervisors. “Jacob was able to show that some of the bones actually came from fish, and the others, which really belonged to Pachystropheus, showed that it was actually a small Thalatosaurus.” So, from what was thought to be the first of the choristoderas, it has now been identified as the last of the thalatosaurs.

Discovery and reconstruction efforts

Evangelos R. Mato-Raven of Peterborough discovered Annie while on holiday in Somerset in 2018 and then painstakingly reassembled and cleaned her to display the bones in his spare time. He said: “I noticed pieces of fallen rock on the beach about 10 meters from the base of the cliff. I was excited as their exposed surfaces showed some fossil bones. It wasn’t until a few days later that I saw that the pieces, collected over two days, fit together. After a few weeks of preparation, we could see that something special was brewing. It took me about 350 hours and about a year to complete the pattern.

Evangelos R. Matheau-Raven during the production of “Annie”. Credit: Evangelos R. Matheau-Raven/Andrea Matheau-Raven

“Pachystropheus probably lived the life of a modern otter, feeding on small fish or invertebrates like shrimp,” says Dr. David Whiteside, another supervisor. “These slender reptiles had long necks, a tail flattened for swimming, and remarkably strong forelimbs for a marine animal, suggesting Pachystropheus they may have come onto land to feed or avoid predators. At the time, the Bristol area, and indeed much of Europe, were shallow seas, and these animals may have lived in a large colony in the warm, shallow waters around the island archipelago.

Annie will now be housed at Bristol Museum and Art Gallery for further study.

“We are delighted that this incredible fossil is now part of the collection at Bristol Museum and Art Gallery, courtesy of the Friends of Bristol Museums, Galleries and Archives. We are thrilled to be able to share the story of this new fossil and all the work the team has achieved with museum visitors,” said Bristol Museum and Art Gallery’s Curator of Geology, Deborah Hutchinson.

Reference: “The Relationships and Paleoecology of Pachystropheus rhaeticus, an Enigmatic Late Triassic Marine Reptile (Diapsida: Thalattosauria)” by Jacob G. Quinn, Evangelos R. Mato-Raven, David I. Whiteside, John EA Marshall, Deborah J. . Hutchinson and Michael J. Benton, June 4, 2024, Journal of Vertebrate Paleontology.
DOI: 10.1080/02724634.2024.2350408