Wumengosaurus delicatomandibularis (Jiang et al 2008, Middle Triassic) was originally considered a pachypleurosaurid with an unusually elongated rostrum. Wu et al. (2011) were less certain. Unfortunately neither tested Wumengosaurus against Stereosternum, Hupehsuchus or Utatsusaurus. Here, derived from a sister toSMF R 4710, Wumengosaurus phylogenetically preceded Thaisaurus at the base of the Ichthyosauria/Ichthyopterygia. Of course, mesosaurs and ichthyosaurs were derived from pachypleurosaurs like Pachypleurosaurus.
Distinct from the smaller specimen of Stereosternum the skull of Wumengosaurus had smaller, more numerous teeth. The premaxilla was elongated at the expense of the maxilla resulting in a slightly reduced rostral length. The postorbital portion of the skull was shortened and raised. The quadratojugal process of the jugal and the quadratojugal itself were greatly reduced to absent.
The cervicals decreased in size and increased in number. The dorsal vertebrae did likewise with an increase in neural spine height reaching an acme at the sacral series. The dorsal ribs were even more pachyostotic than in Mesosaurus.
The pectoral girdle was reduced, the humerus was more robust and the phalanges were more asymmetrical in a pattern more similar to Claudiosaurus.
The pelvis was reduced, the femur was more robust and metatarsal V plus its digit were not elongated in a pattern more similar to Claudiosaurus.
Despite the apomorphic length of the neck, Wumengosaurus is a transitional taxon between mesosaurs and ichthyosaurs. Tthe neural spine extensions of the posterior dorsal vertebrae precede the same structure on Hupehsuchus.
Maisch (2010) found that ichthyosaurs nested either with mesosaurs or procolophonids. The second result is obviously an error caused by too small of an inclusion set.
Below is a subset of the large reptile tree highlighting relationships among mesosaurs, thalattosaurs and ichthyosaurs.
A tree of the Reptilia is here.