Propachyrucos is an extinct genus of late Oligocene hegetotheriid notoungulate. It is known from a few mandibular fragments from the Sarmiento Formation, Chubut Province, Argentina.[1]
Possibly similar to Prosotherium, but almost 20% smaller, more similar in size to Pachyrukhos and Paedotherium, and almost 30% larger than Tremacyllus.
The molars had two internal folds separated by a further deep fold, like the Interatheriidae ; dental cementum was present, but to a lesser extent than in Interatherium. The lower teeth were flat on the outside and bilobed on the inside.
Propachyrucos was first described in 1897 by Florentino Ameghino, based on a right mandible fragment found in Oligocene terrains of the Sarmiento Formation in Argentina, initially believed by Ameghino to have been Cretaceous. The type species is Propachyrucos smithwoodwardi, but other species have been attributed to the genus, such as P. aequilatus, P. robustus, and P. ameghinorum, the latter species, described in 1945 by George Gaylord Simpson from fossils found in the Chubut Province, is known from a remarkably complete skeleton, permitting to reconstruct the animal appearance with high precision. However, recent researches tend to indicate that all the other species of Propachyrucos are in fact synonymous with the type species.[2]
Propachyrucos was a hegetothere, a group of notoungulates that evolved in isolation on the South American continent, giving rise to forms resembling hares and rabbits. Propachyrucos was a member of Pachyrukhinae, a subfamily of hegetotheres particularly similar to modern lagomorphs.
In a 1999 unpublished Ph.D. dissertation, the Propachyrucos species P. ameghinorum and P. simpsoni were synonymized with Prosotherium garzoni. This was supported by a 2019 reassessment as well as a number of other studies. As the most well preserved specimen of Propachyrucos was previously the nearly complete holotype of P. ameghinorum, Propachyrucos smithwoodwardi is now only known from a few mandibular fragments. The 2019 study recovers the features of P. ameghinorum as consistent with those of juvenile Prosotherium garzoni (although the holotype of P. ameghinorum was not directly examined). Despite this, until 2019, phylogenetic analyses regularly scored Propachyrucos for cranial and post-cranial characters, which are known only from the synonymized P. ameghinorum and P. simpsoni.
Propachyrucos and its relatives were animals well suited to running and jumping. Body proportions similar to those of Propachyrucos