Sauropterygia

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Sauropterygians
Thalassomedon.jpg
Reconstructed skeleton of Thalassomedon hanningtoni, American Museum of Natural History
Scientific classification edit
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 Triassic before all except for the Plesiosauria became extinct at the end of that era. The plesiosaurs would continue to diversify till the end of the Mesozoic. 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.

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.

Classification[edit]

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]

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]

Pantestudines

 Lepidosauromorpha 

Kuehneosauridae

Lepidosauria

 Archosauromorpha 

Prolacertiformes

Choristodera

Rhynchosauria

Trilophosaurus

Archosauriformes

Ichthyopterygia

Thalattosauria

Eusaurosphargis

Hanosaurus

Helveticosaurus

Sinosaurosphargis

 Sauropterygia 

Placodontiformes

 Eosauropterygia 
 Pistosauria 

Yunguisaurus

Plesiosauria

Pistosaurus

Augustasaurus

Corosaurus

Cymatosaurus

 Nothosauria 

Simosaurus

Germanosaurus

Nothosaurus

Lariosaurus

Diandongosaurus

 Pachypleurosauria 

Dianopachysaurus

Keichousaurus

Wumengosaurus

Anarosaurus-Dactylosaurus

Neusticosaurus-Serpianosaurus

Size and ecology[edit]

Each morphotype filled a specific ecological role. The large pliosaurs, such as 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, such as 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.

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
  2. ^ Lee, M. S. Y. (2013). "Turtle origins: Insights from phylogenetic retrofitting and molecular scaffolds". Journal of Evolutionary Biology. 26 (12): 2729–2738. doi:10.1111/jeb.12268. PMID 24256520.
  3. ^ Neenan, J. M.; Klein, N.; Scheyer, T. M. (2013). "European origin of placodont marine reptiles and the evolution of crushing dentition in Placodontia". Nature Communications. 4: 1621. doi:10.1038/ncomms2633. PMID 23535642.

External links[edit]