Sciuromorphy outside rodents reveals an ecomorphological convergence between squirrels and extinct South American ungulates

Marcos D Ercoli,Alicia Álvarez,Adriana M Candela

doi:10.1038/s42003-019-0423-5

Abstract

Notoungulates were a diverse group of South American ungulates that included the rodent-like typotherians. However, they are typically compared with other ungulates and interpreted as grazers. Here we present the first detailed reconstruction of the masticatory muscles of the pachyrukhine typotherians Paedotherium and Tremacyllus. An outstanding feature is the presence of a true sciuromorph condition, defined by an anterior portion of the deep masseter muscle originating from a wide zygomatic plate that reaches the rostrum, a trait traceable since the Oligocene pachyrukhines. Consequently, pachyrukhines are the first case of sciuromorph non-rodent mammals. This morphology would have allowed them to explore ecological niches unavailable for the exclusively hystricomorph coexisting rodents. This innovative acquisition seems to be synchronous in Pachyrukhinae and sciuromorph rodents and related to hard-food consumption. We postulate the expansion of nut and cone trees during the major environmental changes at Eocene−Oligocene transition as a potential trigger for this convergence.

Highlights

Notoungulates were a diverse group of South American ungulates that included the rodentlike typotherians
Sciurmorphy is defined as the extension of the deep masseter muscle onto the rostrum, which is attached to a widened and anteriorly tilted zygoma, i.e., zygomatic plate[3,4,5]
The group which more closely resemble rodents, by morphology and size, are the Pachyrukhinae, a Hegetotheriidae subfamily probably recorded from the early Oligocene[30] and whose biochron extends to Late Pliocene[26,31]

Summary

Introduction

Notoungulates were a diverse group of South American ungulates that included the rodentlike typotherians. An outstanding feature is the presence of a true sciuromorph condition, defined by an anterior portion of the deep masseter muscle originating from a wide zygomatic plate that reaches the rostrum, a trait traceable since the Oligocene pachyrukhines. The extinct species Douglassciurus jeffersoni (Protosciurus jeffersoni, late Eocene, 36 Ma13) is typically considered as the first sciurid, but contrary to later representatives of the family it has a protrogomorph zygomasseteric configuration, in which the deep masseter muscle originates from the anterior root of the zygomatic arch which is not expanded and does not reach the rostrum[4,5,13]. There exist only some comments on the paleobiology of this extinct group and there are few ecomorphological studies regarding the cranial morphology of typotherioids[19,21,24,25,26,28,33,34,35]

Methods

Results

Conclusion