Sauropterygia
Group of Mesozoic aquatic reptiles
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Sauropterygia ("lizard flippers") is an extinct taxon of diverse, aquatic diapsid reptiles that developed from terrestrial ancestors soon after the end-Permian extinction and flourished during the Triassic before all except for the Plesiosauria became extinct at the end of that period. The plesiosaurs would continue to diversify until the end of the Mesozoic, when they became extinct as part of the end-Cretaceous mass extinction. Sauropterygians are united by a radical adaptation of their pectoral girdle, adapted to support powerful flipper strokes. Some later sauropterygians, such as the pliosaurs, developed a similar mechanism in their pelvis. Other than being diapsids, their affinities to other reptiles have long been contentious. Sometimes suggested to be closely related to turtles, other proposals have considered them most closely related to Lepidosauromorpha or Archosauromorpha, and/or the marine reptile groups Thalattosauria and Ichthyosauromorpha.
| Sauropterygians Temporal range: Early Triassic - Late Cretaceous, | |
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| Sauropterygia diversity. Clockwise from top left: Ceresiosaurus calcagnii (Nothosauroidea), Henodus chelyops (Placodontia), Brachauchenius lucasi, Aristonectes parvidens (Plesiosauria). | |
Scientific classification  | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Clade: | Neodiapsida |
| Superorder: | †Sauropterygia Owen, 1860 |
| Subgroups | |
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Origins and evolution
The earliest sauropterygians appeared about 247 million years ago (Ma), at the start of the Middle Triassic: the first definite sauropterygian with exact stratigraphic datum lies within the Spathian division of the Olenekian age in South China.[1] 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.
Classification
Classification of sauropterygians has been difficult. The demands of an aquatic environment caused the same features to evolve multiple times among reptiles, an example of convergent evolution. Sauropterygians are diapsids, and since the late 1990s, scientists have suggested that they may be closely related to turtles. The bulky-bodied, mollusc-eating placodonts may also be sauropterygians, or intermediate between the classic eosauropterygians and turtles. Several analyses of sauropterygian relationships since the beginning of the 2010s have suggested that they are more closely related to archosaurs (birds and crocodilians) than to lepidosaurs (lizards and snakes).[2] Some authors have suggested that sauropterygians form a clade with two other groups of marine reptiles, Ichthyosauromorpha and Thalattosauria, with this clade either being placed as non-saurian diapsids or as basal archosauromorphs.[3][4]
The cladogram shown hereafter is the result of an analysis of sauropterygian relationships (using just fossil evidence) conducted by Neenan and colleagues (2013), which placed the clade as the sister group to Sauria (Archosauromorpha + Lepidosauromorpha).[3]
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The cladogram shown below follows the most likely result found by an analysis of turtle relationships using both fossil and genetic evidence by M.S. Lee, in 2013. This analysis resolved Sauropterygia as a paraphyletic assemblage of stem turtles within Archosauromorpha.[2]
| Crown Reptilia/ | |
| Sauria |
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In the cladistic analysis of Shcoch and Sues 2015, Sauropterygia was placed within Pantestudines, the latter recovered within Lepidosauromorpha (the clade including modern lizards and snakes):[5]
| Sauria | |
| (=Archelosauria) |
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The following cladogram was found by Simões et al. (2022), where Sauropterygia was recovered within a larger clade of marine reptiles, including ichthyosauromorphs and thalattosaurs, diverging after pantestudines within Archosauromorpha:[4]
| Neoreptilia |
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In the cladistic analyses of Jenkins et al. 2026, two alternate placements for Sauropterygia were recovered; their parsimony analysis placed the clade as the earliest-diverging lineage of archosauromorphs, with turtles recovered more closely related to modern archosaurs. In contrast, the Bayesian analysis placed sauropterygians within Lepidosauromorpha. The results of the former analysis are displayed in the cladogram below:[6]
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Ecology
Placodonts are thought to have been durophagous, using rounded teeth to crush hard shelled organisms.[7] Members of Eosauropterygia are thought to have been piscivores and carnivores.[8]