The authors report, “Since its first brief description, however, T. chonglakmanii has never been restudied in detail, and its exact stratigraphic and phylogenetic position remained elusive. Here we revisit the well prepared holotype specimen of T. chonglakmanii. This is the earliest record of Mesozoic marine reptiles, two million years earlier
than the earliest previous record.” The authors do not record an outgroup for the Ichthyosauria. The LRT provides dozens in a lineage going back to Devonian tetrapods. Late surviving Wumengosaurus nests as the basalmost ichthyosaur in the LRT (Fig. 2) and mesosaurs are the sister clade appearing as early as the Early Permian. So that gives plenty of time for ichthyosaurs to diverge from primitive mesosaur/sauropterygians. And we should be finding basal ichthyosaurs throughout the Permian.
Thaisaurus chonglakmanii (Mazin et al. 1991; Early Triassic; Fig. 1.) was considered the most basal ichthyosaur by Maisch (2010). That is largely confirmed in the large reptile tree where Thaisaurus nests between Wumengosaurus and the remainder of the Ichthyosauria (sensu Maisch 2010, Fig. 2).
Nice to see that everyone is in agreement
on the taxonomic nesting of Thaisaurus.
Thaisaurus was a late-survivor in the Early Triassic,
a time in which ichthyosaurs were diversifying rapidly. Or did ichthyosaurs just appear in the fossil record then, having diversified throughout the Permian?
Liu J, Samathi A and Chanthasit P 2018. The earliest ichthyosaur from the middle Lower Triassic of Thailand.
Maisch MW 2010. Phylogeny, systematics, and the origin of the Ichthyosauria – the state of the art. Palaeodiversity 3:151-214.
Mazin J-M et al. 1991. Preliminary description of Thaisaurus chonglakmanii n. g. n. sp. a new ichthyopterygian (Reptilia) from the Early Triassic of Thailand. – Comptes- Rendus des Séances de l’Académie de Sciences Paris, Série II, 313: 1207-1212.