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. 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.

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. An expanded cladogram of the Reptilia is here.

not Eretmorhipis (Cheng et al. 2019; YAGM V1401; Early Triassic, 250 mya) was considered conspecific with a headless hupehsuchid Eretmorhipis (WGSC V26020). Here YAGM V1401 nests prior to hupesuchids.

The eyes are actually large relative to the skull, in the new YAGM specimen (Fig. 2), but the skull is tiny relative to the body. The rostrum is narrow relative to the cranium. Typically that enables binocular vision. The authors did not provide a reconstruction of the skull.

The authors created a chimaera when they added the hands and feet of the holotype WGSC specimen to the new YAGM specimen in their Nature paper. Here faint impressions of the foot appear in the matrix, but they are not scored.

The YAGM specimen has a cleithrum and a ventrally broad clavicle along with an interclavicle and other traits found in Wumengosaurus, but lacking in hupehsuchids.

The Origin of Ichthyosaurs
The evolution of ichthyosaurs and Hupehsuchus begins with Stereosternum and continues through Wumengosaurus. At the bottom the same taxa are shown to scale with Utatsusaurus, a basal ichthyosaurs.

Chen X-H, Motani R, Cheng L, Jiang D-Y and Rieppel O 2015. A new specimen of Carroll’s mystery hupehsuchian from the Lower Triassic of China. PLoS One 10, e0126024, https://doi.org/10.1371/journal.pone.0126024 (2015).

Cheng L, Motani R, Jiang D-Y, Yan C-B, Tintori A and Rieppel O 2019. Early Triassic marine reptile representing the oldest record of unusually small eyes in reptiles indicating non-visual prey detection. Nature Scientific Reports Published online January 24, 2019.

Jiang D-Y, Rieppel O, Motani R, Hao W-C, Sun Y-I, Schmitz L and Sun Z-Y. 2008. A new middle Triassic eosauropterygian (Reptilia, Sauropterygia) from southwestern China. Journal of Vertebrate Paleontology 28:1055–1062.

Maisch MW 2010. Phylogeny, systematics, and origin of the Ichthyosauria - the state of the art. Palaeodiversity 3: 151-214.

Wu X-C, Cheng Y-N, Li C, Zhao L-J and Sato T 2011. New Information on Wumengosaurus delicatomandibularis Jiang et al., 2008, (Diapsida: Sauropterygia), with a Revision of the Osteology and Phylogeny of the Taxon. Journal of Vertebrate Paleontology 31(1):70–83.

wiki/Wumengosaurus