Thalattosauria

From Palaeos

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

The thalattosaur Miodentosaurus brevis

Thalattosauria (meaning "ocean lizards") were a group of marine reptiles that lived during the mid-late Triassic period. Some species of thalattosaur grew to over 4 meters (13 feet) in length, including a long, flattened tail used in underwater propulsion. While they bore a superficial resemblance to lizards, the exact relationships of thalattosaurs are unknown; most experts place them somewhere between the ichthyosaurs and archosaurs.

Systematics

The Thalattosauria and family Thalattosauridae were named by John C. Merriam in 1905 for a distinctive group of marine reptiles, at the time containing only the first known genus, Thalattosaurus. The classification of thalattosaurs has varied from study to study, especially since some paleontologists began including the askeptosaurids as true thalattosaurs. While some authors have placed the traditional thalattosaurs and the askeptosaurids within an expanded order Thalattosauria, some cladistic definitions (see Phylogeny below) reserve the name Thalattosauria for a clade comprising the traditional thalattosaurs, and name the larger thalattosaur-askeptosaurid group Thalattosauriformes. In studies that use a more inclusive Thalattosauria, the name Thalattosauroidea has been used for the more exclusive thalattosaur group (Rieppel et al., 2000).

Taxonomy

The taxonomy presented here includes genera listed in an online survey by Mikko Haaramo, with additional taxa added based on their referral in subsequent papers. Thalattosauria is used in its most inclusive sense after Cheng et al. (2007). Note that the family Thalattosauridae is paraphyletic, as it includes the direct ancestors of the family Claraziidae.

• Order Thalattosauria
  • Superfamily Askeptosauroidea
    • Genus Miodentosaurus
    • Family Askeptosauridae
      • Genus Anshunsaurus
      • Genus Askeptosaurus
    • Family Endennasauridae
      • Genus Endennasaurus
  • Superfamily Thalattosauroidea
    • Genus Nectosaurus
    • Genus Xinpusaurus
    • Family Claraziidae
      • Genus Clarazia
      • Genus Hescheleria
    • Family Thalattosauridae*
      • Genus Agkistrognathus
      • Genus Paralonectes
      • Genus Thalattosaurus

Phylogeny

While Thalattosauria has traditionally been used for the largest group containing all families of "thalattosaurs", several authors have recently used this name for what would otherwise be known as Thalattosauroidea, and used the clade Thalattosauriformes for the larger group (Müller et al., 2005). The phylogeny presented here again follows Mikko Haaramo.

Thalattosauriformes
  |--Askeptosauroidea
  |    |--Endennasaurus
  |    `--Askeptosauridae
  |         |--Askeptosaurus
  |         `--Anshunsaurus
  `--Thalattosauria
       |--+--Nectosaurus
       |  `--Xinpusaurus
       `--Thalattosauridae
            |--Agkistrognathus
            `--+--Paralonectes
               `--+--Thalattosaurus
                  `--Claraziidae
                       |--Clarazia
                       `--Hescheleria

References

Cheng, Y., X. Wu, C. Li & S. Tamaki. 2007. A new thalattosaurian (Reptilia: Diapsida) from the Upper Triassic of Guizhou, China.

Müller, J., S. Renesto & S. E. Evans. 2005. The marine diapsid reptile Endennasaurus (Reptilia: Thalattosauriformes) from the Late Triassic of Italy. Palaeontology 48: 15-30.

Rieppel, O., J. Liu & B. Bucher. 2000. The first record of a thalattosaur reptile from the Late Triassic of Southern China (Guizhou Province, PR China). Journal of Vertebrate Paleontology 20 (3): 507-514.

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