Bertrand Ornella C, Amador-Mughal Farrah, Silcox Mary T
Department of Anthropology, University of Toronto Scarborough, Toronto, Ontario, Canada M1C 1A4
Department of Anthropology, University of Toronto Scarborough, Toronto, Ontario, Canada M1C 1A4.
Proc Biol Sci. 2016 Jan 27;283(1823). doi: 10.1098/rspb.2015.2316.
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.
了解早期啮齿动物的大脑进化模式对于重建啮齿类动物以及真盲缺总目其他成员（包括灵长类动物）的原始状态至关重要。我们描述了已知最古老的化石啮齿动物虚拟脑腔模型，它们属于科普副鼠（始新世早期）和精致副鼠（始新世中期）。相对于后来出现的啮齿动物，副鼠的两个标本的嗅球相对于总脑颅体积更大，小脑旁叶更小，这可能是啮齿目的原始特征。科普副鼠和精致副鼠的脑化商（EQ）高于后来出现的（渐新世）典型强棱鼠，这与所有哺乳动物目EQ随时间增加的假设相矛盾。然而，副鼠的两个物种的相对新皮质表面积都比后来的啮齿动物低，这表明该目新皮质化随时间发生，尽管程度低于灵长类动物。副鼠的EQ比任何灵长类干群都高，但新皮质比例更低。这一结果与灵长类动物在整体脑化程度上总是特殊的观点形成对比，并表明相对脑大小和新皮质表面积不一定随时间共同变化。因此，这些数据与关于真盲缺总目大脑进化模式的假设相矛盾。
