Mesenosaurus romeri (Efremov 1938, Reisz and Berman 2001; Late Carboniferous to Early Permian ~300 to ~260 mya) was originally considered a varanopseid, like Varanops, but it lies outside that clade. Here Mesenosaurus was derived from a sister to Archaeovenator and phylogenetically preceded Spinoaequalisand Milleropsis within the Protodiapsida.
Distinct from Archaeovenator, the skull width of Mesenosaurus was at least twice the height. In palatal view, the snout was slightly constricted. The nasals were widest at mid length. The premaxilla extended beyond the naris. The ascending portion of the maxilla separating the naris from the lacrimal became longer. The nasals and frontals were subequal. The orbit was half again as long as tall. The prefrontal contacted the maxilla. The jugal had not quadratojugal process. The last maxillary tooth was below the posterior orbit. Large canines were present.
The neck was elongated. The dorsal ribs were short. The sacrum was wide. The anterior caudal transverse processes were not bent posteriorly. The tail was very slender.
The scapula and coracoid were not fused. The ulna and tibia were straighter and closer together.
The tibia and fibula were straighter and closer together. The proximal metatarsals were subequal in width.
The Protodiapsida tree is here. The expanded family tree of the Reptilia is here. Below are several Mesenosaurus skulls as interpreted by several authors.