Long Adam, Bloch Jonathan I, Silcox Mary T
Department of Anthropology, University of Toronto, Toronto, ON, Canada, M5S 2S2.
Florida Museum of Natural History, University of Florida, Gainesville, FL, 32611.
Am J Phys Anthropol. 2015 Jul;157(3):363-73. doi: 10.1002/ajpa.22724. Epub 2015 Feb 18.
Extant euprimates (=crown primates) have a characteristically expanded neocortical region of the brain relative to that of other mammals, but the timing of that expansion in their evolutionary history is poorly resolved. Examination of anatomical landmarks on fossil endocasts of Eocene euprimates suggests that significant neocortical expansion relative to contemporaneous mammals was already underway. Here, we provide quantitative estimates of neocorticalization in stem primates (plesiadapiforms) relevant to the question of whether relative neocortical expansion was uniquely characteristic of the crown primate radiation. Ratios of neocortex to endocast surface areas were calculated for plesiadapiforms using measurements from virtual endocasts of the paromomyid Ignacius graybullianus (early Eocene, Wyoming) and the microsyopid Microsyops annectens (middle Eocene, Wyoming). These data are similar to a published estimate for the plesiadapid, Plesiadapis tricuspidens, but contrast with those calculated for early Tertiary euprimates in being within the 95% confidence intervals for archaic mammals generally. Interpretation of these values is complicated by the paucity of sampled endocasts for older stem primates and euarchontogliran outgroups, as well as by a combination of effects related to temporal trends, allometry, and taxon-unique specializations. Regardless, these results are consistent with the hypothesis that a shift in brain organization occurred in the first euprimates, likely in association with elaborations to the visual system.
现存的真灵长类动物（即冠类灵长动物）相对于其他哺乳动物而言，其大脑的新皮质区域有典型的扩张，但在它们进化历史中这种扩张发生的时间却鲜有定论。对始新世真灵长类动物化石脑模上解剖学标志的研究表明，相对于同时期的哺乳动物，新皮质的显著扩张已经开始。在此，我们针对相对新皮质扩张是否是冠类灵长动物辐射的独特特征这一问题，对原猴类（近猴形类）的新皮质化进行了定量估计。利用来自怀俄明州早始新世的副鼠猴（Ignacius graybullianus）和怀俄明州中始新世的微鼠猴（Microsyops annectens）虚拟脑模的测量数据，计算了近猴形类动物新皮质与脑模表面积的比率。这些数据与已发表的对三尖近猴（Plesiadapis tricuspidens）的估计值相似，但与为早第三纪真灵长类动物计算的数据形成对比，因为它们总体上处于古代哺乳动物95%的置信区间内。由于早期原猴类和真灵长总目外类群的脑模样本稀少，以及与时间趋势、异速生长和类群独特特化相关的多种综合影响，对这些数值的解释变得复杂。尽管如此，这些结果与以下假设一致：在最早的真灵长类动物中发生了大脑组织的转变，这可能与视觉系统的精细发展有关。
