Jiang et al. (2008) considered Wumengosaurus delicatomandibularis an odd pachypleurosaurid, but Wu et al. (2011) found 102 shortest trees nesting Wumengosaurus between [pachypleurosaurs + nothosaurs] and [plesiosaurs + their kin]. Mesosaurs and ichthyosaurs were not tested. Here, using a larger gamut of taxa, Wumengosaurus nested between Stereosternum and Hupehsuchus + ichthyosaurs + thalattosaurs, which is not far from the base of the sauropterygia.
Here (Fig. 1) the palate of Wumengosaurus (with reidentified palatal elements) fits in, like the rest of its morphology, as a transitional taxon linking the mesosaur, Stereosternum, to Hupehsuchus, ichthyosaurs and basal thalattosaurs like Askeptosaurus. The palatine in enaliosaurs varies substantially (Fig. 2) and it is difficult to interpret due to fusion of certain elements in certain taxa, along with taphonomic crushing and shattering.
Loss of the Suborbital Fenestra
Petrolacosaurus, Araeoscelis and Claudiosaurus all have a narrow suborbital fenestra. The rest of the Enaliosauria, including Wumengosaurus did not. Whether the choana and suborbital fenestra became confluent or the suborbital fenestra simply disappeared is not apparent with present data.
Anterior Extension of the Ectopterygoid
The ectopterygoid in mesosaurs (Fig. 2) extended anteriorly with ventral reduction of the palatine. The expansion of the palatal portion of the maxilla may be key to this extension because predecessor taxa did not have either. Expanding the maxillae medially created a rostral palate plate that displaced the internal nares (choanae) posteriorly, far from the premaxillary teeth. This derived trait appeared first in mesosaurs and was retained by all descendant taxa, including Wumengosaurus. A similar pattern of bones appears in all descendant taxa, including thalattosaurs and ichthyosaurs.
As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.
Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.
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.
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.