Thaisaurus

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Thaisaurus
Temporal range: Early Triassic
Thaisaurus chonglakmanii 5.JPG
Fossil
Scientific classification e
Kingdom: Animalia
Phylum: Chordata
Class: Reptilia
Superorder: †Ichthyopterygia
Family: †Thaisauridae
Maisch, 2010
Genus: †Thaisaurus
Mazin et al., 1991
Type species
†Thaisaurus chonglakmanii
Mazin et al., 1991

Thaisaurus is an extinct genus of ichthyopterygian marine reptile that lived during the Spathian[1] (late Olenekian, Early Triassic). Fossils have been found in Thailand.[2]

Description

Thaisaurus is a small ichthyopterygian, a group of marine reptiles.[3] While later members of this group were not unlike fish in body shape, Early Triassic ichthyopterygians had more lizard-like bodies. However, they still possessed low tail fins and their limbs were encased in flippers.[4] While rather similar to its fellow ichthyopterygian Chaohusaurus,[3] Thaisaurus also shows some differences.[5] Some features of Thaisaurus resemble those of the later, better adapted ichthyosaurs while others represent the group’s ancestral characteristics.[6]

The elongated snout of Thaisaurus is thin. Thaisaurus has enlarged, rounded orbits (openings that housed the eyes). The massive frontals and parietals, two pairs of skull roof bones, both contact the pineal foramen (a small hole along the skull’s midline). The small, broad supratemporal fenestrae (openings behind the orbits) have quadrangular boundaries. The outer edges of these openings are formed by the well-developed postorbitals (paired skull bones behind the orbits). To the rear edges of the postorbitals are the small squamosals.[6] An elongated ridge is present along the rear edges of the postorbitals.[7] The postfrontals (a pair of skull roof bones) do not form any part of the borders of the supratemporal fenestrae.[5]

The narrow[5] teeth of Thaisaurus are all identical in shape and firmly implanted in extensive,[3] partially enclosed sockets. The outer surfaces of their crowns and roots are smooth while their tips are pointed. The teeth in its lower jaw are smaller than those in its upper.[6] Unlike Grippia and Chaohusaurus, Thaisaurus does not have blunted teeth at the back of its jaws.[3]

The presacral vertebrae, those in front of the hips, are short measured from front to back, differing from the much more elongate vertebrae in the tail.[6] The forelimbs of Thaisaurus are longer than its hindlimbs, with both sets of limbs composed of elongated bones. Each humerus (upper arm bone) lacks a projecting flange on its front edge. Both the humeri and femora (upper leg bones) are not curved.[6] Similar to Chaohusaurus, Thaisaurus has a large, forwards projection on the upper end of each radius (front lower arm bone).[3] The first and fifth metacarpals are identical in size.[5] There are a total of five digits in each hindlimb.[6]

See also

References

  1. ^ Tongtherm, Kittichai; Shigeta, Yasunari; Sardsud, Apsorn; Sashida, Katsuo; Agematsu, Sachiko (October 2020). "Age of the Early Triassic Ichthyopterygian Thaisaurus Inferred from Ammonoid Biostratigraphy". Paleontological Research. 24 (4): 276–284. doi:10.2517/2019PR022. S2CID 222136795.
  2. ^ New Material of Qianichtyosaurus Li, 1999 (Reptilia, Ichthyosauria) from the late Triassic of southern China, and Implications for the Distribution of Triassic Ichthyosaurs. Elizabeth L. Nicholls, Chen Wei, Makoto Manabe.
  3. ^ a b c d e Motani, Ryosuke (1999). "Phylogeny of the Ichthyopterygia" (PDF). Journal of Vertebrate Paleontology. 19 (3): 473–496. doi:10.1080/02724634.1999.10011160. Archived from the original (PDF) on 2012-04-15.
  4. ^ Motani, R. (2005). "Evolution of fish-shaped reptiles (Reptilia: Ichthyopterygia) in their physical environments and constraints" (PDF). Annual Review of Earth and Planetary Sciences. 33: 395–420. Bibcode:2005AREPS..33..395M. doi:10.1146/annurev.earth.33.092203.122707. Archived from the original (PDF) on 17 May 2017.
  5. ^ a b c d Maisch, M. W. (2010). "Phylogeny, systematics, and origin of the Ichthyosauria – the state of the art" (PDF). Palaeodiversity. 3: 151–214.
  6. ^ a b c d e f Mazin, J. M.; Sander, P. M. (1993). "Palaeobiogeography of the Early and Late Triassic Ichthyopterygia". Paleontologia Lombarda. New Series. 2: 93–107.
  7. ^ Maisch, M. W.; Matzke, A. T. (2000). "The Ichthyosauria". Stuttgarter Beiträge zur Naturkunde, Serie B. 298: 1–159.

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