Jawbone From 200 Million-Year-Old Giant Dinosaur Found

Reconstruction of the Shonisaurus, a giant ichthyosaur. (Nobumichi Tamura.jpg)

(CN) – A recently discovered 205 million-year-old jawbone belongs to one of the largest animals ever and resolves a long-standing mystery, according to a new study.

The report, published Monday in the journal PLOS ONE, finds that the bone belongs to a giant ichthyosaur, a type of prehistoric aquatic reptile that would have been more than 85 feet long – approaching the size of a blue whale, experts estimate.

The fossil also solves a 150-year-old mystery of apparent “dinosaur bones” found in the United Kingdom.

Paul de la Salle, a fossil collector and co-author of the study, found the bone on the beach at Lilstock, Somerset, in May 2016. Upon returning to the site, he discovered even more pieces that together measured more than three feet in length.

“Initially, the bone just looked like a piece of rock but, after recognizing a groove and bone structure, I thought it might be part of a jaw from an ichthyosaur and immediately contacted ichthyosaur experts Dean Lomax (University of Manchester) and Professor Judy Massare (the College at Brockport, State University of New York) who expressed interest in studying the specimen,” said de la Salle. “I also contacted Dr. Ramues Gallois, a geologist who visited the site and determined the age of the specimen stratigraphically.”

Lomax and Massare identified the specimen as a surangular, or incomplete bone, from the lower jaw of a massive ichthyosaur. They compared it with several ichthyosaurs and visited the Royal Tyrrell Museum of Paleontology in Alberta, Canada, to examine the largest known specimen, the shastasaurid Shonisaurus sikanniensis, which is more than 68 feet long.

“As the specimen is represented only by a large piece of jaw, it is difficult to provide a size estimate, but by using a simple scaling factor and comparing the same bone in S. sikanniensis, the Lilstock specimen is about 25 percent larger,” said Lomax. “Other comparisons suggest the Lilstock ichthyosaur was at least 65-82 feet.

“Of course, such estimates are not entirely realistic because of differences between species. Nonetheless, simple scaling is commonly used to estimate size, especially when comparative material is scarce.”

In 1850, a large incomplete bone was described from the late Triassic, 208 million years ago, found in Aust Cliff, Gloucestershire. Four other similarly incomplete bones were also discovered and described, two of which are now missing and presumed destroyed. They have been identified as the limb bones of several dinosaurs – sauropods and stegosaurs – indeterminate dinosaurs and other reptiles.

However, the findings from the new study refute previous identifications and also the most recent pronouncement that the Aust bones represent an early experiment of dinosaur-like gigantism in terrestrial reptiles. They are, in fact, jaw fragments from sizeable, previously unrecognized ichthyosaurs.

“One of the Aust bones might also be an ichthyosaur surangular. If it is, by comparison with the Lilstock specimen, it might represent a much larger animal,” said Lomax.

“To verify these findings, we need a complete giant Triassic ichthyosaur from the U.K. – a lot easier said than done.”

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