Sauropterygia

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Sauropterygians
Temporal range: Early Triassic - Late Cretaceous
Thalassiodracon BW.jpg
Reconstruction of the plesiosaurian Thalassiodracon
Scientific classification e
Kingdom: Animalia
Phylum: Chordata
Class: Reptilia
Clade: Neodiapsida
Infraclass: Lepidosauromorpha
Superorder: Sauropterygia
Owen, 1860
Orders

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. Sauropterygians are united by a radical adaptation of their pectoral girdle, designed to support powerful flipper strokes. Some later sauropterygians, like the pliosaurs, developed a similar mechanism in their pelvis.

Origins and evolution[edit]

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.[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.

Size and ecology[edit]

Each morphotype filled a specific ecological role. The large pliosaurs, like the Jurassic Rhomaleosaurus, Liopleurodon and Pliosaurus, and the Cretaceous Kronosaurus and Brachauchenius, were the superpredators of the Mesozoic seas, around 7 to 12 meters in length, and filled a similar ecological role to that of killer whales today. The long-necked plesiosaurs, meanwhile, included both those with medium-long necks, like the 3 to 5 metre-long Plesiosauridae and Cryptoclididae, and the Jurassic and Cretaceous Elasmosauridae, which evolved progressively longer and more flexible necks, so that by the middle and late Cretaceous the entire animal was over 13 metres in length (e.g. Elasmosaurus) - although as most of this was the neck, the actual body size was much smaller than that of the larger pliosaurs. These long-necked forms undoubtedly fed on fish, which they probably snared in their tooth-lined jaws with rapid lunges of the neck and head.

Extinction[edit]

The sauropterygians thrived throughout the Mesozoic. However, despite their success, they became extinct along with the non-avian dinosaurs, pterosaurs and mosasaurs during the end Cretaceous mass extinction.

Taxonomy[edit]

Classification is difficult because the demands of the aquatic environment caused the same characteristics to evolve multiple times, illustrating convergent evolution. While sauropterygians are considered diapsids, they are also sometimes classified with turtles. The bulky-bodied, mollusc-eating placodonts may also be sauropterygians. In addition to the modifications of the shoulder, the group is also united by several modifications in their skulls.

References[edit]

  1. ^ Ji Cheng, et al. 2013. "Highly diversified Chaohu fauna (Olenekian, Early Triassic) and sequence of Triassic marine reptile faunas from South China", in Reitner, Joachim et al., eds. Palaeobiology and Geobiology of Fossil Lagerstätten through Earth History p. 80

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