Virtual endocasts of fossil Sciuroidea: brain size reduction in the evolution of fossoriality

Abstract

Aplodontia rufa (mountain beaver) is the only extant member of the Aplodontidae. The fossil record indicates that this family displayed greater taxonomic and ecological diversity in the past, and that the burrowing adaptations of Aplodontia might be derived. We describe the first virtual endocasts of A. rufa and of three fossil aplodontids: Prosciurus relictus and Pros. aff. saskatchewaensis (early Oligocene), and Mesogaulus paniensis (early Miocene). Our results show that the endocasts of early aplodontid rodents are more similar to those of early arboreal squirrels than to those of the later occurring aplodontids in terms of both relative size and morphology. The endocranial features observed in sciurids and early aplodontids, missing in later aplodontids, have been associated with better vision and the development of arboreality in squirrels. Basal Aplodontidae known from postcrania have been described as generalists with some features for arboreality, which may provide a basis for these similarities. In contrast, the relatively small endocasts of the later occurring aplodontids, which lack traits related to visual specialization, may reflect their burrowing adaptations, as they would be less reliant on visual cues. When integrated with data from the most primitive fossil rodents, the Ischyromyidae, these new data suggest that early squirrels and aplodontids diverged from more terrestrial ischyromyids to become more arboreal, with relatively larger brains showing traits for improved vision. Recent Aplodontidae with fossorial adaptations returned to a more ischyromyid‐like condition in their endocranial features. These results are consistent with previous observations that changes in locomotion are reflected in the endocranial anatomy of rodents.

Virtual endocranial and inner ear endocasts of the Paleocene ‘condylarth’ Chriacus: new insight into the neurosensory system and evolution of early placental mammals

“…These cerebellar structures play a role in the control of eye movements, also known as vestibulo‐ocular reflex (VOR; Rambold et al., ). Large petrosal lobules are present in more active and visually oriented animals, such as arboreal squirrels, compared with more slow‐moving and fossorial taxa like the mountain beaver ( Aplodontia rufa ; Bertrand et al., , 2018b). The average petrosal volume percentage is 0.96%, with a range of 0.01–2.34%, for 49 species of mammals (Ferreira‐Cardoso et al., ) and 1.78%, with a range of 0.39–3.35%, for 33 Ischyromyidae and Sciuroidea rodent species (Bertrand et al., 2018b).…”

“…() and references therein for cranial and petrosal anatomy, Bertrand et al. (2018b) for endocranial anatomy and Orliac & O'Leary () for bony labyrinth anatomy.…”

“…This method was used by Bertrand et al. (,b) because, in their study, some specimens lacked a completely preserved neocortex; importantly, they found that neocortical height was strongly correlated with neocortical surface area ratio in rodents (Table ).…”

“…There is a relationship between the rhinal fissure and the orbitotemporal canal (sulcus) in rodents, primates and other taxa (e.g. Novacek, ; Martin, ; Silcox et al., ; Bertrand et al., ,b). However, this relationship is not consistent across Mammalia and in some cases the rhinal fissure may occupy a more dorsal position compared with the orbitotemporal canal.…”

The cranium of Proviverra typica (Mammalia, Hyaenodonta) and its impact on hyaenodont phylogeny and endocranial evolution

“…; Bertrand et al . , , b); for instance, terrestrial squirrel species have a lower EQ than arboreal squirrel species. In Chiroptera, diet preferences may be involved in the value of the EQ with frugivorous species having a larger EQ than insectivorous species (Pirlot & Stephan ).…”