Eusauropterygia
| Sauropterygians Temporal range: Early Triassic - Late Cretaceous |
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| Reconstructed skeleton of Thalassomedon hanningtoni, American Museum of Natural History | |
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Clade: | Pantestudines |
| Superorder: |
†Sauropterygia Owen, 1860 |
| Subgroups | |
Sauropterygia ("lizard flippers") is an extinct, diverse taxon of aquatic reptiles that developed from terrestrial ancestors soon after the end-Permian extinction and flourished during the Mesozoic before they became extinct at the end of that era. When the End-Cretaceous extinction event occurred, only a single lineage, the Xenopsaria, represented by the two groups Elasmosauridae and Polycotylidae, was still present. Sauropterygians are united by a radical adaptation of their pectoral girdle, designed to support powerful flipper strokes. Some later sauropterygians, such as the pliosaurs, developed a similar mechanism in their pelvis.
The earliest sauropterygians appeared about 245 million years ago (Ma), at the start of the Triassic period: the first definite sauropterygian with exact stratigraphic datum lies within the Spathian division of the Olenekian era in South China. Early examples were small (around 60 cm), semi-aquatic lizard-like animals with long limbs (pachypleurosaurs), but they quickly grew to be several metres long and spread into shallow waters (nothosaurs). The Triassic-Jurassic extinction event wiped them all out except for the plesiosaurs. During the Early Jurassic, these diversified quickly into both long-necked small-headed plesiosaurs proper, and short-necked large-headed pliosaurs. Originally, it was thought that plesiosaurs and pliosaurs were two distinct superfamilies that followed separate evolutionary paths. It now seems that these were simply morphotypes in that both types evolved a number of times, with some pliosaurs evolving from plesiosaur ancestors, and vice versa.
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