We started off our Triassic series looking at the placodonts, a group of turtle-like marine reptiles and then the nothosaurs. Both of these animal groups existed solely during the Triassic era before becoming extinct at the Triassic-Jurassic boundary. The final group of Triassic-exclusive marine reptiles that we are going to look at are the thalattosaurs.
The thalattosaurs (temporal range: 240-210 Mya) were a group of marine reptiles that first appeared in the seas of the Middle Triassic. These animals bore a significant resemblance to modern day lizards and their exact classification has been the cause of much controversy. Currently they are widely accepted as members of the diapsid clade but they have been considered close relatives of both the ichthyosaurs and the archosaurs. At this point in time they are classified alone within the Thalattosauria order, which was created in 1904 by American paleontologist John C. Merriam.
It is believed that the thalattosaurs would have spent their lives predominantly in water but they displayed a number of terrestrial qualities that suggest they would have come ashore at regular intervals. The thalattosaurs were fairly small marine reptiles, with some of the larger species growing to lengths of around 3 meters. Unlike the nothosaurs, whose primary method of propulsion was from their powerful limbs, the thalattosaurs relied on a specially adapted tail to power themselves through the water. This laterally compressed tail was long and paddle-like. This, coupled with slender main bodies, meant that the thalattosaurs were efficiently streamlined and would have made effective underwater hunters. It is believed that the thalattosaurs would have breathed air, needing to come up to the surface for oxygen when in the water.
The one variant feature of the different thalattosaur species was their snout. One of the main sub-families of the Thalattosauria order, the Askeptosauroidea, had long, narrow, pointed skulls with an extended rostra at the end of the skull which pushed the nostrils backwards so that they were positioned close to the eyes. The other sub-family, the Thalattosauroidea, have shorter, downturned snouts with an upper jaw that was longer than the lower one.
Picture: Miodentosaurus, a member of the thalattosaur family.