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

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.

Stasis of functionally versatile specialists

“…Our primary objective is to determine whether functionally versatile specialists with broad dietary niches have a static trophic morphology. Tree squirrels provide a useful model system in that they are morphologically specialized, being extreme among rodents in their adaptations for forceful biting/gnawing (Druzinsky 2010; Cox et al 2012;Ercoli et al 2019), but despite that specialization, they are not restricted to hard foods-their specialization enables them to eat hard foods without compromising their ability to consume soft foods. Most tree squirrels eat not only hard nuts and hard-shelled fruits but also small seeds, pulpy fruits, nectar, insects, and other soft foods (Moller 1983;Emmons and Feer 1997;Roth and Mercer 2008;Thorington et al 2012).…”

“…2012; Ercoli et al. 2019), but despite that specialization, they are not restricted to hard foods—their specialization enables them to eat hard foods without compromising their ability to consume soft foods. Most tree squirrels eat not only hard nuts and hard‐shelled fruits but also small seeds, pulpy fruits, nectar, insects, and other soft foods (Moller 1983; Emmons and Feer 1997; Roth and Mercer 2008; Thorington et al.…”

Snout shape and masticatory apparatus of the rodent‐like mesotheriid ungulates (Notoungulata, Typotheria): exploring evolutionary trends in dietary strategies through ancestral reconstructions

“…2018; Ercoli et al . 2019, 2020) it can be inferred that the masticatory apparatus of the analysed mesotheriids and hegetotheriids would enable them to perform stronger masticatory efforts than other notoungulates and extant ungulates. These functional inferences are unexpected for species whose diets are composed exclusively of soft leaves, but do not preclude their consumption (Zelditch et al .…”

“…Patterson (1934) recognized that trachytheriines ( Trachytherus Morphotype) possess an incipient development of the zygomatic plate, whereas this structure is well developed in mesotheriines (‘Intermediate’ and Mesotherium morphotypes), reaching a proto‐sciuromorph masseteric condition convergent with some rodents and other typotherians (Patterson 1934; Ercoli et al . 2019, 2020). Our analyses also reveal differences in the development of the zygomatic plate even within mesotheriines.…”

“…2020) but exploiting dietary niches more similar to other coexisting typotherians and rodents than to those of other native ungulates (MacFadden 2005; Ercoli et al . 2019, 2020). These adaptations do not preclude access to soft food items; rather, they would expand rather than constrain the ecological niche.…”