Small within the largest: brain size and anatomy of the extinct Neoepiblema acreensis , a giant rodent from the Neotropics

Abstract

The ecomorphological diversity of caviomorph rodents in South America included giant forms, such as the chinchilloid Neoepiblema acreensis from the Upper Miocene of Brazil. The evolution of the brain anatomy and size of these animals can be now studied with non-invasive imaging techniques and exceptional fossils. Caviomorphs show diversity in the traits of the olfactory bulbs, cerebrum, cerebellum, cranial nerves, and blood vessels. Neoepiblema acreensis had a gyrencephalic brain, with an expansion of the frontal lobe, lacking an evident paraflocculus. Compared to the predictions based on extant taxa, even when considering taphonomical effects, N. acreensis , a rodent that weighted almost 80 kg, had a very low encephalization quotient compared to other rodents. The adaptive value of a low energetic cost and other ecological factors could explain the presence of a small brain in this giant rodent––a pattern we also hypothesize for other Neogene giant rodents.

Virtual brain endocast of Antifer (Mammalia: Cervidae), an extinct large cervid from South America

“…Interchange, the predatory pressures were distinctly different (low) when compared with the moment after the arrival of the placental carnivores, which are more active predators (Fernández-Monescillo et al, 2017;Ferreira et al, 2020). Cervids, other ungulates, and large carnivorous mammals arrived in South America at a similar time and evolved in parallel over the last 2 million years.…”

“…Changes in encephalization through time, such as an increase in the endocranial volume under the selective pressure of predators and competition, or stabilization/reduction (negative or neutral selection) in the cases of the absence of predators, domestication, and/or fossorial lifestyles (among others), have been documented in several mammalian groups (Bertrand et al, 2018; Dunbar, 2009; Jerison, 1973; Köhler & Moyà‐Solà, 2004; Kruska, 2005; Palombo et al, 2008; Weston & Lister, 2009). However, few studies have analyzed changes in mammalian encephalization during the Cenozoic of South America (Dozo & Martínez, 2015; Fernández‐Monescillo et al, 2017; Ferreira et al, 2020). It is hypothesized that, before the Great American Biotic Interchange, the predatory pressures were distinctly different (low) when compared with the moment after the arrival of the placental carnivores, which are more active predators (Fernández‐Monescillo et al, 2017; Ferreira et al, 2020).…”

“…However, few studies have analyzed changes in mammalian encephalization during the Cenozoic of South America (Dozo & Martínez, 2015; Fernández‐Monescillo et al, 2017; Ferreira et al, 2020). It is hypothesized that, before the Great American Biotic Interchange, the predatory pressures were distinctly different (low) when compared with the moment after the arrival of the placental carnivores, which are more active predators (Fernández‐Monescillo et al, 2017; Ferreira et al, 2020). Cervids, other ungulates, and large carnivorous mammals arrived in South America at a similar time and evolved in parallel over the last 2 million years.…”

“…The neurosensorial evolution of extinct South American Cenozoic mammals (e.g., notoungulates, caviomorph rodents, xenarthrans, and primates) has been studied using natural or artificial brain endocasts (usually produced with latex; Dechaseaux, 1958Dechaseaux, , 1962Dozo, 1987Dozo, , 1994Dozo, , 1997Dozo, Vucetich, & Candela, 2004;Patterson, 1937; F I G U R E 1 1 Bivariate plots (log) of brain mass versus body mass (a); neocortical surface versus total endocranial surface (b). The identification of the dots is the same as the Figure 9 Radinsky, 1981) and more recently using virtual endocasts (Aristide et al, 2019;Boscaini et al, 2018;Fernández-Monescillo et al, 2017;Ferreira, Negri, Sánchez-Villagra, & Kerber, 2020;Tambusso & Fariña, 2015a, 2015b. Although terrestrial cetartiodactyls are a conspicuous component of the mammalian fauna of this continent (Menegáz & Ortiz-Jaureguizar, 1995), no paleoneurological data have been provided to date.…”

“…Art by Márcio L. Castro in the cases of the absence of predators, domestication, and/or fossorial lifestyles (among others), have been documented in several mammalian groups (Bertrand et al, 2018;Dunbar, 2009;Jerison, 1973;Köhler & Moyà-Solà, 2004;Kruska, 2005;Palombo et al, 2008;Weston & Lister, 2009). However, few studies have analyzed changes in mammalian encephalization during the Cenozoic of South America (Dozo & Martínez, 2015;Fernández-Monescillo et al, 2017;Ferreira et al, 2020). It is hypothesized that, before the Great American Biotic…”