Department of Anthropology, University of Winnipeg, 515 Portage Avenue, Winnipeg, MB R3B 2E9, Canada.
J Hum Evol. 2010 Jun;58(6):505-21. doi: 10.1016/j.jhevol.2010.03.008. Epub 2010 May 4.
We describe a virtual endocast produced from ultra high resolution X-ray computed tomography (CT) data for the microsyopid, Microsyops annectens (middle Eocene, Wyoming). It is the most complete and least distorted endocast known for a plesiadapiform primate and because of the relatively basal position of Microsyopidae, has particular importance to reconstructing primitive characteristics for Primates. Cranial capacity is estimated at 5.9 cm(3), yielding encephalization quotients (EQ) of 0.26-0.39 (Jerison's equation) and 0.32-0.52 (Eisenberg's equation), depending on the body mass estimate. Even the lowest EQ estimate for M. annectens is higher than that for Plesiadapis cookei, while the range of estimates overlaps with that of Ignacius graybullianus and with the lower end of the range of estimates for fossil euprimates. As in other plesiadapiforms, the olfactory bulbs of M. annectens are large. The cerebrum does not extend onto the cerebellum or form a ventrally protruding temporal lobe with a clear temporal pole, suggesting less development of the visual sense and a greater emphasis on olfaction than in euprimates. Contrasts between the virtual endocast of M. annectens, and both a natural endocast of the same species and a partial endocast from the earlier-occurring Microsyops sp., cf. Microsyops elegans, suggest that the coverage of the caudal colliculi by the cerebrum evolved within the Microsyops lineage. This implies that microsyopids expanded their cerebra and perhaps evolved an improved visual sense independent of euprimates. With a growing body of data on the morphology of the brain in primitive primates, it is becoming clear that many of the characteristics of the brain common to euprimates evolved after the divergence of stem primates from other euarchontans and likely in parallel in different lineages. These new data suggest a different model for the ancestors of euprimates than has been assumed based on the anatomy of the brain in visually specialized diurnal tree shrews.
我们描述了一种从超高分辨率 X 射线计算机断层扫描 (CT) 数据中生成的 microsyopid（中始新世，怀俄明州）虚拟内颅。它是最完整和失真最小的原始灵长类动物内颅，并且由于 Microsyopidae 的相对基础位置，对于重建原始灵长类动物的特征具有特殊意义。脑容量估计为 5.9 立方厘米，产生的脑量指数 (EQ) 为 0.26-0.39（杰里森方程）和 0.32-0.52（艾森伯格方程），具体取决于体重估计。即使是 Microsyopidae 的最低 EQ 估计值也高于 Plesiadapis cookei 的 EQ 估计值，而估计值的范围与 Ignacius graybullianus 的范围重叠，并且与化石真灵长类动物的估计值的低端重叠。与其他原始灵长类动物一样，Microsyopidae 的嗅球较大。大脑没有延伸到小脑或形成一个有明显颞极的向腹外突出的颞叶，这表明视觉感的发育程度较低，嗅觉的强调程度高于真灵长类动物。虚拟内颅的对比 M. annectens，与同一种类的自然内颅以及较早出现的 Microsyops sp. 的部分内颅，cf. Microsyops elegans，表明大脑覆盖尾状丘的部分在 Microsyops 谱系中进化。这意味着 microsyopids 扩大了它们的大脑，并且可能独立于真灵长类动物进化出了更好的视觉感。随着越来越多的关于原始灵长类动物大脑形态的数据，很明显，许多与真灵长类动物共同的大脑特征是在原始灵长类动物从其他合弓类动物分支出来之后进化的，并且可能在不同的谱系中平行进化。这些新数据表明，与基于视觉特化的日间树鼩的大脑解剖学所假设的祖先模型相比，真灵长类动物的祖先有不同的模型。
