Silcox Mary T, Dalmyn Claire K, Bloch Jonathan I
Department of Anthropology, University of Winnipeg, 515 Portage Avenue, Winnipeg, MB R3B 2E9, Canada.
Proc Natl Acad Sci U S A. 2009 Jul 7;106(27):10987-92. doi: 10.1073/pnas.0812140106. Epub 2009 Jun 22.
Extant primates are distinctive among mammals in having relatively large brains. As stem primates, Paleogene plesiadapiforms provide direct information relevant to the earliest stages in the evolution of this characteristic. Here we describe a virtual endocast reconstructed from ultra high resolution X-ray computed tomography data for the paromomyid plesiadapiform Ignacius graybullianus (USNM 421608) from the early Eocene of Wyoming. This represents the most complete endocast known for a stem primate, allowing for an unprecedented study of both size and fine details of anatomy. Relative to fossil and extant euprimates, I. graybullianus had large olfactory lobes, but less caudal development of the cerebrum and a poorly demarcated temporal lobe, suggesting more emphasis on olfaction and a less well developed visual system. Although its brain was small compared to those of extant primates, the encephalization quotient of I. graybullianus is higher than that calculated for Paleocene Plesiadapis cookei and overlaps the lower portion of the range documented for fossil euprimates. Comparison to the basal gliroid Rhombomylus suggests that early primates exhibited some expansion of the cerebrum compared to their ancestors. The relatively small brain size of I. graybullianus, an arboreal frugivore, implies that neither arboreality nor frugivory was primarily responsible for the expanded brains of modern primates. However, the contrasts in features related to the visual system between I. graybullianus and fossil and extant euprimates suggest that improvements to these portions of the brain contributed to increases in brain size within Euprimates.
现存的灵长类动物在哺乳动物中具有相对较大的大脑，这一点颇为独特。作为灵长类动物的祖先，古近纪的近猴类为这一特征演化的最早阶段提供了直接相关信息。在此，我们描述了一个根据超高分辨率X射线计算机断层扫描数据重建的虚拟脑腔模型，该模型来自怀俄明州始新世早期的近猴科近猴类伊氏 Ignacius graybullianus（美国国家自然历史博物馆编号421608）。这是已知的最完整的灵长类动物祖先脑腔模型，使得对其大小和解剖细节进行前所未有的研究成为可能。相对于化石和现存的真灵长类动物，伊氏 Ignacius graybullianus 的嗅叶较大，但大脑的尾部发育较差，颞叶界限不清晰，这表明其更强调嗅觉，视觉系统发育程度较低。尽管与现存灵长类动物相比，它的大脑较小，但伊氏 Ignacius graybullianus 的脑化商高于为古新世的库克氏近猴（Plesiadapis cookei）计算的脑化商，并且与化石真灵长类动物记录范围的下限重叠。与基干类啮齿动物菱形兽（Rhombomylus）的比较表明，与它们的祖先相比，早期灵长类动物的大脑出现了一些扩张。伊氏 Ignacius graybullianus 是一种树栖食果动物，其相对较小的大脑尺寸意味着树栖生活和食果习性都不是现代灵长类动物大脑扩张的主要原因。然而，伊氏 Ignacius graybullianus 与化石和现存真灵长类动物在与视觉系统相关特征上的差异表明，大脑这些部分的改善促进了真灵长类动物大脑尺寸的增加。
