Tremacyllus

The genus Tremacyllus was described in 1891 by Florentino Ameghino for the species Pachyrukhos impressus, described by Ameghino himself a few years earlier. He named the type species Tremacyllus impressus, from the Pliocene and the Early Pleistocene, and several other species were later described by Ameghino himself, such as T. chapalmalensis, T. diminutus and T. novus, and by Gaetano Rovereto, such as T. intermedius and T. incipiens, the latter of which lived during the Late Miocene. A 2017 study led by Renatta Sostillo, Esperanza Cerdeño and Claudia I. Montalvo however determined that the presumed differences between all the species of Tremacyllus could be explained by intraspecific variations, and therefore that the only valid species was the type, T. impressus.^[1] However, a 2022 study by Armella, Ercolli, Bonini and Garcia-Lopez found sufficient proof to recognize T. incipiens as a valid species.^[2] Tremacyllus was a specialized member of the Hegetotheriidae, a group of small-sized lagomorph-like notoungulates. Particularly, Tremacyllus was close from the genera Pachyrukhos and Paedotherium, and was one of the last hegetotheres known, as well as one of the last notoungulates.

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. Hence, those animals are the first case of a non-rodent mammal developing a sciuromorph condition. 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. It is therefore supposed that the expansion of nut trees and cone trees caused by major environmental changes during the Eocene-Oligocene transition may have been the potential trigger for this convergent evolution.^[3] A recent study on tooth microwear of Tremacyllus suggest it had a primarily fruit and seed-based diet.^[4]

• F. Ameghino. 1891. Mamiferos y aves fosiles argentinas. --Especies nuevas, adiciones y correcciones. Revista Argentina Historia Natural 1(4):240-259
• E. Cerdeno and M. Bond. 1998. Taxonomic Revision and Phylogeny of Paedotherium and Tremacyllus (Pachyrukhinae, Hegetotheriidae, Notoungulata) from the Late Miocene to the Pleistocene of Argentina. Journal of Vertebrate Paleontology 18(4):799-811
• F. D. Seoane, S. R. Juñent, and E. Cerdeño. 2017. Phylogeny and paleobiogeography of Hegetotheriidae (Mammalia, Notoungulata). Journal of Vertebrate Paleontology 37(1):e1278547:1-13
• Marcos D. Ercoli; Alicia Álvarez; S. Rocío Moyano; Dionisios Youlatos; Adriana M. Candela (2020). "Tracing the Paleobiology of Paedotherium and Tremacyllus (Pachyrukhinae, Notoungulata), the Latest Sciuromorph South American Native Ungulates – Part I: Snout and Masticatory Apparatus". Journal of Mammalian Evolution. in press. doi:10.1007/s10914-020-09516-7.
• Marcos D. Ercoli; Alicia Álvarez; Dionisios Youlatos; S. Rocío Moyano; Adriana M. Candela (2020). "Tracing the Paleobiology of Paedotherium and Tremacyllus (Pachyrukhinae, Notoungulata), the Latest Sciuromorph South American Native Ungulates – Part II: Orbital, Auditory, and Occipito-Cervical Regions". Journal of Mammalian Evolution. in press. doi:10.1007/s10914-020-09518-5