Abstract
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.
Funder
University of Toronto Scarborough
University of Toronto
Natural Sciences and Engineering Research Council of Canada
Subject
General Agricultural and Biological Sciences,General Environmental Science,General Immunology and Microbiology,General Biochemistry, Genetics and Molecular Biology,General Medicine
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