Its skull had large orbits and strong lower incisors, similar to modern lagomorphs.

[1] However, a 2022 study by Armella, Ercolli, Bonini and Garcia-Lopez found sufficient proof to recognize T. incipiens as a valid species.

One of the exceptional characteristics of Tremacyllus, as well as its relatives within Pachyrukhinae, such as Paedotherium, is the presence of a real sciuromorph condition in its chewing apparatus, defined by an anterior portion of the masseter muscle coming from a large zygomatic plate reaching the rostrum ; a characteristic traceable within Hegetotheriidae since the Oligocene.

This morphology would have permitted them to explore ecological niches unavailable to the histrichomorph rodents that coexisted with them.

This innovative acquisition seems to have appeared at the same time in sciuromorph rodents and pachyrukhins, and could be linked with the consumption of hard food.