Phattalung, Thailand. This specimen consists of a skull, partial vertebral series, and an incomplete fore- and hindlimb and was preserved in resistant dolomite, alongside two other poorly preserved natural molds of other specimens. The type specimen was subsequently extracted in 1988 by a joint paleontological expedention from Thailand and France. The specimen is housed in the Department of Mineral Resources of Bangkok, where it received the number TF2454.[3] The specimen has yet to be fully prepared.[4]: 63 Mazin and colleagues described the specimen in 1991, naming it Thaisaurus chonglakmanii, in honor of the country it came from and its discoverer.[3]
Description
Thaisaurus is a small ichthyopterygian, a group of marine reptiles.
tail fins and their limbs were encased in flippers.[6] While rather similar to its fellow ichthyopterygian Chaohusaurus,[5]Thaisaurus also shows some differences.[7] Some features of Thaisaurus resemble those of the later, better adapted ichthyosaurs while others represent the group’s ancestral characteristics.[8] However, the anatomy of Thaisaurus is poorly understood, with further preparation and study needed.[4]: 63 [7]
The skull of Thaisaurus bears enlarged, rounded
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.[8] An elongated ridge is present along the rear edges of the postorbitals.[9] The postfrontals (a pair of skull roof bones) do not form any part of the borders of the supratemporal fenestrae.[7]
The narrow,
sockets. The outer surfaces of their crowns are smooth and the roots lack infolding. The teeth in its lower jaw are smaller than those in its upper.[3] Unlike Grippia and Chaohusaurus, Mazin and colleagues reported that Thaisaurus did not have blunted teeth at the back of its jaws.[3][5] However, Motani noted in 2003 that the back teeth of Thaisaurus are not exposed, so the presence or absence of blunt teeth cannot be confirmed.[4]
: 63
The
metacarpals are identical in size.[7] There are a total of five digits in each hindlimb.[8]
Classification
Validity
Mazin and colleagues in 1991 considered the features exhibited by Thaisaurus to be unique among ichthyopterygians.
monotypic family, Thaisauridae, for the former. Nevertheless, he also agreed that restudy was necessary.[7]
Phylogeny
While Thaisaurus is widely regarded as a basal (early-diverging) ichthyopterygian,
omphalosaurids, while the latter group, to which they assigned Thaisaurus, were considered part of the "true" ichthyosaur line that persisted beyond the Triassic.[3]
However, later studies of ichthyopterygians did not support this dichotomy.
grade leading to the "intermediate" ichthyosaurs of the later Triassic, which in turn gave rise to the post-Triassic ichthyosaurs. Due to its poor preservation, Motani did not include Thaisaurus in the analysis, instead listing it as Ichthyopterygia incertae sedis.[5][6] Another cladistic analysis of the group was conducted by Maisch and Matzke in 2000. While not without differences, their topology was broadly similar to that of Motani's. They included Thaisaurus in their analysis, finding it to be the first-diverging member of the group.[9] However, Motani voiced doubts about the stability of the topology at the base of the cladogram recovered by Maisch and Matzke.[4]
^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.
