05/29/2026
Now THAT’S What I Call : Pachystropheus Edition!
Hey there, Cameron (our Paleontology Collections Manager), give me the deets, boy (and free my soul):
“Pachystropheus (meaning ‘thick vertebrae’) is a genus of prehistoric marine reptile found in Late Triassic rocks of southwestern England. It is the youngest known thalattosaur—an extinct group of marine reptiles superficially resembling, but only distantly related to, lizards.
Pachystropheus was named by Erika von Huene in 1935, who described the animal as a choristodere (another extinct group of marine reptiles). Decades went by before Dr. David Gower and our very own Dr. Glenn Storrs redescribed the specimen and determined it may represent the youngest occurrence of choristoderes in the fossil record, extending their record 45 million years. However, this conclusion has brought skepticism; notably, there is no confirmed skull material for Pachystropheus, and most of the diagnostic features of choristoderes are skull features.
Pachystropheus reached a maximum length of ~2.5 meters, though most known individuals don't exceed 1 meter in length. It was an active swimmer, likely using its limbs as paddles for steering and thrust, along with additional propulsion from the tail. The ribs and vertebrae of the body display pachyostosis—a non-pathological thickening of the bone seen in modern manatees and dugongs—likely as an adaptation to living in a marine environment. No teeth are known for Pachystropheus and it may have been toothless, feeding on small fish and cephalopods.
The Westbury Formation is a mudstone representing a shallow shelf marine environment. The yellow-gold crystals visible on the surface of the slab pictured are the iron sulfide mineral, pyrite (FeS2), commonly known as ‘Fool's Gold’. In marine settings, pyrite commonly forms in organic-rich sediment where bacterial reduction of seawater sulfate occurs, particularly in anoxic basins; such environments also happen to be suitable for accumulating and preserving fossils.”
CMC VP6616
Pachystropheus rhaeticus
Marine reptile
Westbury Formation
Late Triassic
Westbury-on-Severn, Avon, England
