Sauropterygia

Sauropterygia is an extinct clade of marine reptiles that lived from the Early Triassic to the Late Cretaceous. Emerging after the Permian–Triassic extinction, sauropterygians adapted to life in marine environments and became a major component of Mesozoic seas. They gave rise to a diversity of forms that occupied coastal to open-ocean habitats.

The group splits into two principal lineages: Nothosauroidea (nothosaurs) and Plesiosauria. Nothosaurs were typically semi-aquatic with elongated bodies and limbs suited for swimming, often found in coastal settings. Plesiosauria evolved into two well-known body plans: the long-necked, small-headed plesiosauroids and the short-necked, large-headed pliosauroids. In both groups, the limbs were transformed into broad paddles, and many species achieved efficient, whale-like locomotion in open water. Some sauropterygians also developed tail fins and highly streamlined bodies.

Ecology and biology varied among lineages. Most sauropterygians were carnivorous, feeding on fish, cephalopods, and other marine animals. Teeth and jaw shapes reflect resource specialization across genera. Reproductive strategies are inferred from fossils that suggest viviparity in several lineages, consistent with a fully aquatic lifestyle. Growth and maturity data from fossils indicate rapid development in some species.

Fossil evidence is especially abundant in Triassic and Jurassic deposits, with plesiosaurs persisting into the Cretaceous. The sauropterygians ultimately declined and disappeared by the end of the Mesozoic, along with other large marine reptiles, as modern marine ecosystems took shape.

Sauropterygia remains a key group for understanding the evolution of aquatic locomotion and the diversification of marine reptiles in the Mesozoic.