Sauropterygia ("lizardflippers") is anextinct taxon of diverse, aquaticdiapsidreptiles that developed from terrestrial ancestors soon after theend-Permian extinction and flourished during theTriassic before all except for thePlesiosauria becameextinct at the end of that period. The plesiosaurs would continue to diversify until the end of theMesozoic, when they became extinct as part of theend-Cretaceous mass extinction. Sauropterygians are united by a radical adaptation of theirpectoral girdle, adapted to support powerful flipper strokes. Some later sauropterygians, such as thepliosaurs, developed a similar mechanism in theirpelvis. Other than being diapsids, their affinities to other reptiles have long been contentious. Sometimes suggested to be closely related toturtles, other proposals have considered them most closely related toLepidosauromorpha orArchosauromorpha, and/or the marine reptile groupsThalattosauria andIchthyosauromorpha.
The earliest sauropterygians appeared about 247million years ago (Ma), at the start of theMiddle Triassic: the first definite sauropterygian with exact stratigraphic datum lies within the Spathian division of theOlenekian 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). TheTriassic-Jurassic extinction event wiped them all out except for theplesiosaurs. During theEarly Jurassic, these diversified quickly into both long-necked small-headed plesiosaurs proper, and short-necked large-headedpliosaurs. 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 of sauropterygians has been difficult. The demands of an aquatic environment caused the same features to evolve multiple times among reptiles, an example ofconvergent evolution. Sauropterygians arediapsids, and since the late 1990s, scientists have suggested that they may be closely related toturtles. The bulky-bodied, mollusc-eatingplacodonts 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 toarchosaurs (birds and crocodilians) than tolepidosaurs (lizards and snakes).[2] Some authors have suggested that Sauropterygians form a clade with two other groups of marine reptiles,Ichthyosauromorpha andThalattosauria, with this clade either being placed as non-saurian diapsids or as basalarchosauromorphs.[3][4]
The cladogram shown hereafter is the result of an analysis of sauropterygian relationships (using just fossil evidence) conducted by Neenan and colleagues, in 2013.[3]
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 aparaphyletic assemblage ofstem turtles.[2]
Placodonts are thought to have beendurophagous, using rounded teeth to crush hard shelled organisms.[6] Members of Eosauropterygia are thought to have been piscivores and carnivores.[7]
^Ji Cheng,et al. 2013. "Highly diversified Chaohu fauna (Olenekian, Early Triassic) and sequence of Triassic marine reptile faunas from South China", in Reitner, Joachimet al., eds.Palaeobiology and Geobiology of Fossil Lagerstätten through Earth History p. 80
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