Researchers identified a prehistoric marine reptile found in 1935 as a thalattosaur and not a choristodere, using CT scans and a new specimen. Reconstruction of Pachystrofeus rhaeticuspictured next to a hybodont shark feeding on a Birgeria fish. Credit: James Ormiston
Scientists have reclassified a prehistoric marine reptile discovered in 1935 as one of the last thalattosaurs, and not an early choristodere, after new findings and detailed imaging.
The true identity of a local prehistoric marine reptile has been discovered after experts determined some of its remains were actually fish.
Researchers from the University of Bristol and the University of Southampton have determined that bones have been found in Trias rocks from 1935 are from one of the last thalattosaurs, a large sea lizard that behaved like an otter.
For years, the ancient animal was believed to be one of the first choristoderes, another group of crocodilian marine reptiles. However, in the study, published in the Journal of vertebrate paleontologythe team examined the original name-bearing copy from 1935. They compared it with a remarkably new example of Pachystropheusknown as ‘Annie’, which contains hundreds of bones from different individuals, as well as evidence of sharks, bony fish and even land dinosaurs.
Advanced techniques and features
Jacob Quinn, who is studying for his Masters in Palaeobiology at Bristol School of Earth Sciences, traveled with the two specimens to Southampton where they were scanned with a CT scan, producing stacks of X-rays through the blocks to create a complete 3D model all kinds of things buried in the blocks.
“Thalattosaurs existed throughout the Triassic period,” Jacob explained. “Some of them reached four meters (13 feet) in length and are said to have been the terrors of the sea. But our Pachystropheus was only three feet long, and half of that was its long tail. He also had a long neck, a small head the size of a matchbox, which we hadn’t found yet, and four paddles. If it looked like its relatives, it would have had many sharp teeth, ideal for grabbing fish and other small, wriggly prey.”
Rhaetian (205 million years ago) food web from the Bristol archipelago with Pachystrofeus rhaeticus. The arrows indicate who eats who – red and black means inferred, and blue arrows are based on ecology and fossil associations observed during this study. Credit: Jacob Quinn
“Pachystrofeus was previously identified as the first of the choristoderes, another group of crocodilian marine reptiles, and was considered 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 that were really from Pachystrofeus showed that it was actually a small thalattosaur. So after being considered the first of the choristoderes, it is now identified as the last of the thalattosaurs.”
Discovery and reconstruction efforts
Evangelos R. Matheau-Raven from Peterborough discovered Annie while on holiday in Somerset in 2018, and he then painstakingly reassembled and cleaned it to uncover the bones in his spare time. He said: “I saw parts of a fallen rock on the beach about 10 meters from the foot of the cliff. I was excited when their exposed surfaces showed some fossil bones. It wasn’t until a few days later that I could see that the pieces collected two days apart fit together. After a few weeks of preparation we could see that something special was emerging. The one took me about 350 hours and about a year to complete.”
Evangelos R. Matheau-Raven during the preparation of ‘Annie’. Credit: Evangelos R. Matheau-Raven/Andrea Matheau-Raven
“Pachystropheus probably lived the life of a modern otter, eating small fish invertebrates like shrimp,” said Dr. David Whiteside, another supervisor. “These slender reptiles had a long neck, a tail that was flat for swimming and remarkably robust forelimbs for a marine animal, suggesting Pachystropheus may have come ashore to feed or avoid predators. At that time, the area around Bristol, and much of Europe, consisted of 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 the Bristol Museum & Art Gallery for further study.
“We are very pleased that this incredible fossil is now part of the Bristol Museum & Art Gallery collection, thanks to the kind help of the Friends of Bristol Museums, Galleries & Archives. We are delighted to share the story of this new fossil and all the work the team has done with visitors to the museum,” says Bristol Museum & Art Gallery geology curator Deborah Hutchinson.
Reference: “The Relationships and Paleoecology of Pachystrofeus rhaeticus, an Enigmatic Latest Triassic Marine Reptile (Diapsida: Thalattosauria)” by Jacob G. Quinn, Evangelos R. Matheau-Raven, David I. Whiteside, John E. A. Marshall, Deborah J. Hutchinson and Michael J. Benton, June 4, 2024, Journal of vertebrate paleontology.
DOI: 10.1080/02724634.2024.2350408