Sauropterygia

Sauropterygia is an extinct group of aquatic reptiles that includes plesiosaurs and several earlier marine lineages. Its members evolved from terrestrial ancestors and adapted to life in the sea.

Placodontia: An extinct group of marine reptiles, many with crushing teeth and protective body armor. Placodonts were an early sauropterygian branch adapted to feeding on hard-shelled prey.
Aquatic locomotion: Movement through water using limbs, body undulation, or other specialized structures. Sauropterygian swimming relied increasingly on powerful, paddle-shaped limbs.
Triassic: The geologic period from about 252 to 201 million years ago, following the Permian. Sauropterygians first diversified during the Triassic.
Placodus: An extinct Triassic placodont marine reptile with robust teeth suited to crushing hard prey. Placodus exemplifies the shell-crushing feeding specialization of some sauropterygians.
Ichthyosauria: An extinct group of marine reptiles with streamlined bodies and fish-like or dolphin-like outlines. Ichthyosaurs were a separate marine reptile lineage, not sauropterygians.
Nothosauroidea: A group of extinct marine reptiles that includes nothosaurs and pachypleurosaurs. These early-diverging sauropterygians illuminate the group's varied forms.
Hydrodynamic streamlining: Body-shape adaptations that reduce resistance as an organism moves through a fluid. Streamlined bodies helped many sauropterygians move efficiently through seawater.
Permian–Triassic extinction event: The largest known mass extinction, occurring about 252 million years ago at the end of the Permian. The ecological upheaval preceding sauropterygian origins reshaped marine ecosystems.
Cyamodus: An extinct Triassic placodont with broad bony armor on its back and skull. Cyamodus shows how strongly some sauropterygians were armored.
Mosasauridae: An extinct family of large marine lizards that flourished during the Cretaceous Period. Mosasaurs entered marine ecosystems independently and belong to a different reptile lineage.

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