Virtual endocasts of Eocene Paramys (Paramyinae): oldest endocranial record for Rodentia and early brain evolution in Euarchontoglires

Understanding the pattern of brain evolution in early rodents is central to reconstructing the ancestral condition for Glires, and for other members of Euarchontoglires including Primates. We describe the oldest virtual endocasts known for fossil rodents, which pertain to Paramys copei (Early Eocene) and Paramys delicatus (Middle Eocene). Both specimens of Paramys have larger olfactory bulbs and smaller paraflocculi relative to total endocranial volume than later occurring rodents, which may be primitive traits for Rodentia. The encephalization quotients (EQs) of Pa. copei and Pa. delicatus are higher than that of later occurring (Oligocene) Ischyromys typus, which contradicts the hypothesis that EQ increases through time in all mammalian orders. However, both species of Paramys have a lower relative neocortical surface area than later rodents, suggesting neocorticalization occurred through time in this Order, although to a lesser degree than in Primates. Paramys has a higher EQ but a lower neocortical ratio than any stem primate. This result contrasts with the idea that primates were always exceptional in their degree of overall encephalization and shows that relative brain size and neocortical surface area do not necessarily covary through time. As such, these data contradict assumptions made about the pattern of brain evolution in Euarchontoglires.

The first major primate extinction: An evaluation of paleoecological dynamics of North American stem primates using a homology free measure of tooth shape

OBJECTIVES

The disappearance of the North American plesiadapoids (stem primates, or plesiadapiforms) in the latest Paleocene has been attributed to competition with rodents over dietary resources. This study compares molar morphology of plesiadapoids and early rodents to assess whether all taxa were adapted to consuming foods of the same structural properties with similar mechanical efficacy.

MATERIALS AND METHODS

Micro-CT scans of second mandibular molars (M2 s) of plesiadapoids (n = 181) and ischyromyid (early fossil) rodents (n = 13) were evaluated using Dirichlet normal energy (DNE), a dental topographic metric that quantifies the curvature of a tooth's occlusal surface, independent of the orientation of the occlusal plane; this metric can be used to infer diet.

RESULTS

Comparisons of DNE values for plesiadapoids and rodents show that rodents shared functionally similar dental morphology with at least some plesiadapid plesiadapoids and thus were likely adapted to processing foods with similar physical properties. However, the DNE values for rodents contrast markedly with those for the other two plesiadapoid families, the Carpolestidae and Saxonellidae.

CONCLUSIONS

It is unlikely that direct competition over food resources with rodents played a major role in the extinction of carpolestids and saxonellids, as members of these families were capable of consuming a range of foods that were not accessible to rodents. Although several plesiadapid species overlap with rodents in their range of DNE values, only three overlap in time. One of these (Plesiadapis cookei) may have been too large to be in direct competition with rodents, another (Plesiadapis dubius) has DNE values substantially different (higher) than those of rodents, whereas the third, Chiromyoides, has teeth of both a similar size and DNE value to those of Clarkforkian rodents. If dietary niche overlap with rodents played a direct role in the decline of plesiadapiforms, it can only have potentially done so for Chiromyoides.