Sauropterygia

From Palaeos.org

o Sauria
|?-Thalattosauriformes
`--o LEPIDOSAUROMORPHA
   |--Acerosodontosaurus [Acerosodontosauridae]
   |--+--Sauropterygia
   |  `?-Testudinata (Rieppel & deBraga)
   `--Lepidosauriformes

Plesiosaurus drawing CC-by-sa Adam Stuart Smith

Introduction and Description

The Sauropterygia or "lizard flippers" are an important group which includes a wide range of aquatic lizard-like, crocodile-like, long-necked "Loch Ness monster"-like, and large-headed killer whale-like aquatic reptiles. Among the great prehistoric success stories, they first appeared at (or just before) the very start of the Mesozoic and flourished right until the end of the era. Yet as with the Ichthyosauria, they are so specialized their evolutionary relationships are still somewhat obscure.

Earlier forms (pachypleurosaurs) were mostly small amphibious lizard-like animals with slender, elongated limbs and small heads, that lived in estuarine and near-shore environments. They quickly increased to several meters in size and ventured further out to sea, although still retaining amphibious traits and sinuous lizard-like form (nothosaurs). By the Middle Triassic there were a number of divergent lineages. In addition to the line leading to the successful Jurassic plesiosaurs, there were several divergent branches of early sauropterygians, including pachypleurosaurs, several lines of nothosaurs, and the transitional pistosaurs. These early divergent branches show successive stages in adaptation to oceanic life.

The Carnian extinction took out most of these early Sauropterygia, but the one surviving group, the plesiosaurs, underwent a tremendous evolutionary radiation around the very end of the Triassic period and the very start of the Jurassic, diverging rapidly into variously lineages in a typical example of a macroevolutionary adaptive radiation. It is not yet clear whether this event coincided with and was caused by the full development of the plesiosaur locomotor plan, or whether it was simply opportunistic factors, such as the mass extinction of potential competitors in the Rhaetian turnover, that allowed the plesiosaurs to exploit the potential of a previously evolved locomotor plan (see below). In any case by the early Jurassic there were a diverse range of long- medium- and short-necked forms, some with small heads others with large. These were all open ocean types, completely adapted for marine life, and usually 3 metres or more in length. The two principal morphs were the plesiosaurs (long necked "Loch Ness monster" types) and pliosaurs (reptilian killer whales). However, the evolutionary relationships of the Plesiosauria are complex, and it appears that long-necked and short-necked types occurred in a number of lineages.

A remarkably similar arrangement was evolved in the pelvic girdle of derived sauropterygians, e.g. plesiosaurs, and both girdles were connected by a net of gastralia (stomach ribs) probably reinforced by stout ligaments. This arrangement and the problems of plesiosaur locomotion, will be covered on a subsequent page. MAK991206, ATW061220.

o Lepidosauromorpha
`--o Sauropterygia
   `--+?-Claudiosaurus 
      `--+?-Thalattosauriformes 
         |?-Placodontia
         `--o Eusauropterygia
            |--Pachypleurosauridae
            `--+--Nothosauridae
               `--+--Corosaurus 
                  `--+--Cymatosaurus 
                     `--+--Pistosauridae  
                        `--o Plesiosauria
                           |==Pliosauroidea
                           `--o Plesiosauroidea
                              |--Plesiosaurus
                              `--+--Elasmosauridae
                                 `--Cryptocleidoidea

Information

Range: Late Permian to Late Cretaceous.

Phylogeny: Lepidosauromorpha: Lepidosauriformes + Sauropterygia : Placodontia + Nothosauria

Characters

Characters: Postorbital skull elongated; upper fenestra enlarged; "euryapsid" (lower temporal arch lost) [CG85]; quadrate rigid (primitive) [CG85]; elongate neck (>7 cervicals); limbs relatively large [CG85]; curved humerus; radius equal length to ulna.

References:

• Carroll, RL & Gaskill, P (1985), The nothosaur Pachypleurosaurus and the origin of plesiosaurs. Phil. Trans. Roy. Soc. Lond. B 309: 343-393.

Taxonomy

This section from Wikipedia:

• Class Sauropsida (reptiles)
  • Infraclass Lepidosauromorpha (lizards and relatives)
    • SUPERORDER SAUROPTERYGIA
      • Order Thalattosauriformes
        • Suborder Thalattosauria
      • Order Placodontia
      • (unranked) Eusauropterygia
        • Order Nothosauroidea
          • Suborder Pachypleurosauria
          • Suborder Nothosauria
        • (unranked) Pistosauroidea
          • Corosaurus
          • Chinchenia
          • Kwangsisaurus
          • Cymatosaurus
          • Family Pistosauridae
          • Order Plesiosauria
            • Suborder Plesiosauroidea (long-necked plesiosaurs)
            • Suborder Pliosauroidea (short-necked plesiosaurs)

Links

• Sauropterygia
• Turtle Origins
• ???????? (Japanese)
• Fossil Room

Credits

MAK991206, ATW061216, ATW061220 Palaeos com