Updated April 13, 2015 with a revised subset of the large reptile tree (Fig. 2).
Earlier we looked at Mikdadocephalus as the basalmost ichthyosaur. Today, a more primitive taxon is presented.
Thaisaurus chonglakmanii (Mazin et al. 1991; Early Triassic; Fig. 1.) was considered the most basal ichthyosaur by Maisch (2010). That is confirmed in the large reptile tree where Thaisaurus nests between Wumengosaurus and the remainder of the Ichthyosauria (sensu Maisch 2010, Fig. 2).
Figure 1. Thaisaurus in situ, traced using DGS, elements of tracing shifted using DGS and restored. Click to enlarge. Confirming Maisch 2010, this is a basal ichthyosaur, transitional between Wumengosaurus and the remainder of the Ichthyosauria. Many of the bones are missing but their impressions remain.
Diagnosis (according to Maisch 2010) – “Autapomorphies are the macroscopically smooth, conical and slender tooth crowns (convergent to the Leptonectidae), and a postfrontal that remains separated from the fenestra supratemporalis. Plesiomorphies aiding in identification are: humerus without lamina anterior, humerus, femur and zeugopodials very elongate and slender, metatarsal five long and slender, as big as metatarsal one.”
Figure 2. Subset of the large reptile tree focusing on the Ichthyosauria. Note the basal position of Thaisaurus between Wumengosaurus and the remainder of the Ichthyosauria. Low bootstrap score around the base of the hupesuchids represent two skull-only taxa nested with a skull less taxon, Parahupehsuchus. Note the shift in position of the hupehsuchids as well as the various nodes at which the various specimens attributed to Shastaaurus nest.
The small size of Thaisaurus (Fig. 3) brings up the subject, once again, of phylogenetic miniaturization at the genesis of major clades. We’ve seen this before with mammals, birds, reptiles, pterosaurs, dinosaurs and others.
Figure 3. Basal ichthyosauria to scale. Here Wumengosaurus, Thaisaurus, Mikadocephalus and a specimen attributed to Shastasaurus are illustrated. Note the phylogenetic miniaturization shown by Thaisaurus, a trait often seen at the origin of major clades.
Apparently, and this should come as no surprise, the fore limbs of basal ichthyosaurs transformed into flippers prior to the hind limbs.
Apparently the high neural spines of Wumengosaurus were shorter in Thaisaurus, but these are poorly preserved.
Apparently the extreme reduction and multiplication of the cervicals of Wumengosaurus was an autapomorphy because outgroup taxa, like Stereosternum, do not have this trait.The elongation of metatarsal V is also a trait shared between Thaisaurus and Stereosternum.
Note the putative basal ichthyosaur, Cartorhynchus, nests instead with basal pachypleurosaurs and explained here.
More on Thaisaurus and other basal ichthyosaurs later.
References
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.
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