Pachystropheus

The earliest known remains attributable to Pachystropheus rhaeticus were collected between 1834 and 1841 and were given the name Rysosteus oweni by Richard Owen in his 1842 publication "Report on British Fossil Reptiles, Part II". The specimen consisted of a vertebra and limb bones in the collection of James Johnson, and they were sold alongside much of his collection in 1845. They have since become lost.

The holotype is NMHUK R747, consisting of around 35 backbones/centra and neural spines including dorsal vertebrae, ribs and gastralia, a femur, an ilium, and aproblematic fibula; a hybodontiform cephalic spine was preserved alongside the holotype. It was presented to the Natural History Museum, London in 1866.

Pachystropheus was named by Erika von Huene in 1935 based on the holotype from the Westbury Formation in Somerset. She also synonymised Rysosteus oweni with Pachystropheus rhaeticus.

A younger specimen was reported from the Hettangian of Somerset in 1996, and BRSMG Cg3102 was discovered in summer 2018 as only the second specimen from the Wesbury Formation to show many identifiable bones.

In 2026, remains of Pachystropheus were reported from the Rhineland-Palatinate region of Germany in the Exter Formation.

Pachystropheus reached an estimated maximum total body length of around 2.5 metres (8.2 ft), though most known individuals are much smaller, less than 1 metre (3.3 ft) in length. The ribs of the body display pachyostosis and the vertebrae exhibit thickening, likely as an adaptation to living in a marine environment. As no teeth are known for Pachystropheus, it may have been toothless like its close relative Endennasaurus.

When describing the animal, von Huene (1935) described Pachystropheus as a choristodere, but this was overlooked for decades until its redescription by Storrs and Gower in 1993. This reevaluation would extend the fossil record of choristoderes back 45 million years. However, other authors consider attribution of Pachystropheus to Choristodera problematic, stating that it depends on vertebral and girdle characters that are also found in the skeletons of aquatic reptiles other than choristoderes; most of the diagnostic features of choristoderes are skull features, but the presence of these cannot be confirmed in Pachystropheus, as there is no confirmed skull material for this taxon.

Silvio Renesto (2005) found similarities in the postcranial skeleton of Pachystropheus and the thalattosaur genus Endennasaurus; according to Renesto, these similarities may indicate that Pachystropheus and Endennasaurus are close relatives, but they might as well simply be a case of a convergent evolution triggered by the aquatic lifestyle of both taxa.

The placement as a thalattosaur has been supported by other researchers, including a thorough analysis published in 2024, which found it to be a thalattosaur based on postcranial characters, including the presence of a "finely striated blade of the ilium", and a member of the subclade Askeptosauroidea based on its dumbbell-shaped radius. The phylogenetic analysis recovered it as closely related to Endennasaurus.

The Westbury Mudstone where Pachystropheus was found represents a shallow shelf marine environment. Pachystropheus likely used its limbs as paddles along with propulsion from the tail. It probably fed on small fish and cephalopods. Finds in coprolites suggests that Pachystropheus was fed upon by larger marine reptiles.