Sauropterygia ("lizard flippers") is an extinct clade of diverse, generally marine 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. 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.

thumb|upright=.8|[[Mary Anning's plesiosaur: specimen NHMUK PV OR 22656 of Plesiosaurus dolichodeirus]]

Description

Sauropterygians are characterised by a number of synapomorphies, including having only a single upper temporal fenestra rather than two temporal fenestrae as is ancestral for diapsids, the supratemporal, postparietal, tabular, and lacrimal bones of the skull have been lost, the nares (external nasal opening) is retracted, there is a large retroarticular process on the mandible, the trunk vertebrae lack intercentra, the sacrum is composed of three or more vertebrae, a sternum is absent, the scapula is positioned superficially relative to the clavicle, the scapulocoracoid is divided, the pectoral and thyroid bear fenestration. Other characters suggested to be ancestral for Sauropterygia include the squamosal bone descends to the lower (ventral) margin of the skull, the quadrate bone of the skull is covered by the squamosal and quadratojugal when the skull is viewed side-on (lateral view), the bones exhibit pachyostosis (high density), a reduced or absent posterior process on the interclavicle, a curved humerus, and intertrochanteric fossa is small but distinct. taken to the extreme in elasmosaurid plesiosaurs, which have up to 76 neck vertebrae, with the long neck ancestral for plesiosaurs becoming secondarily shortened in some plesiosaur lineages like pliosaurs. By contrast, placodonts have very short necks with only 6 vertebrae.

The vast majority of sauropterygians are generally thought to have relied on using their limbs to propel themselves through the water (paraxial propulsion) as opposed to undulating their bodies as is typical of living reptiles like crocodilians, Members of Eosauropterygia are also characterised by slender and curved femurs. They also lack osteoderms which are present in some other sauropterygians. Members of Eosauropterygia are thought to have been piscivores and carnivores. Sauropterygians were generally marine, While pleisosaurs were fully aquatic, some non-plesiosaur sauropterygians may have been capable of limited terrestrial locomotion.

Origins and evolution

thumb|[[Kronosaurus and Woolungasaurus, the plesiosaurs]]

The earliest sauropterygians appeared about 248-247 million years ago (Ma) during the Spathian towards the end of the Early Triassic, by which time they already showed considerable diversification. By the Middle Triassic, some sauropterygians had already reached large body sizes, with some nothosaurs belonging to the genus Nothosaurus from this time period reaching a body length of . The Triassic-Jurassic extinction event wiped out all sauropterygians aside from the plesiosaurs, which would continue to diversify until the end of the Mesozoic, when they would be wiped out by the K-Pg extinction event.

Classification and relationships to other reptiles

It has long been uncertain how Sauropterygia is related to other reptile groups. The demands of an aquatic environment caused the same features to evolve multiple times among reptiles, an example of convergent evolution. Sauropterygians are widely agreed to be diapsids, and since the late 1990s, scientists have suggested that they may be closely related to turtles, 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.

Some other Triassic marine reptiles have been suggested to be more closely related to Sauropterygia than to Ichthyosauromorpha and Thalattosauria, including Hanosaurus, Helveticosauridae, Palatodonta, Eusaurosphargis, and Saurosphargidae (some studies find varying members of this group to be within Sauropterygia proper). Wang et al. 2022 coined Sauropterygiformes to include Sauropterygia (which was found to include Palatodonta) proper, as well as Hanosaurus, Helveticosaurus, and Saurosphargidae (including Eusaurosphargis as a member).

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.

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:

References

  • Unit 220: 100: Lepidosauromorpha. Palaeos. July 15, 2003. Retrieved January 19, 2004.
  • A review of the Sauropterygia. Adam Stuart Smith. The Plesiosaur Directory. Retrieved April 17, 2006.
  • Paleofile taxalist - lists every species and synonyms. Retrieved February 26, 2006