Department of Anatomical Sciences, Renaissance School of Medicine, Stony Brook University, Stony Brook, New York, USA.
Am J Phys Anthropol. 2021 Apr;174(4):728-743. doi: 10.1002/ajpa.24177. Epub 2021 Jan 23.
The Willwood Formation of the southern Bighorn Basin, Wyoming is a fluvial rock sequence that spans approximately 3 million years of early Eocene time. It has yielded one the largest collections of fossil mammals in the world including thousands of dentitions of extinct lemur-like primates known as notharctines. In the southern Bighorn Basin, specimens of these primates have been collected on numerous paleontological expeditions and the stratigraphic levels yielding the dentitions have been carefully recorded. Notharctine dentitions represent a rare opportunity to study morphological variation in a single anatomical system through time among closely related individuals.
Prior studies of Bighorn Basin notharctines through time produced measurements of hundreds of specimens but I report here results from measurement and comparison of the dentitions and dentaries of more than 3,000 specimens, all stratigraphically mapped.
Variation in premolar and molar area and variation in dentary depth are apparent throughout the section. Specimens with relatively small teeth and dentaries are known from the older part of the section. In younger rocks, variation in tooth area among specimens increases. Variation in tooth area is continuous and overlaps extensively both within and between stratigraphic levels. Other dental variables examined by inspection change in a mosaic and continuous fashion through the section. These features include variation in the presence and number of paraconids on the lower fourth premolar (p4), the size and shape of the entoconid notch on the lower first and second molars, and the relative development of the pseudohypocone, mesostyle, and cingula on the upper molars.
These broad patterns can be identified despite notharctine alpha taxonomy being in need of extensive revision and, importantly, simplification. Such revision is beyond the scope of this article but is essential if we are to develop a taxonomy that is both free of stratigraphic influence and useful for rapid, repeatable species assignment. Boundaries among the patterns of tokogenesis, anagenesis, and cladogenesis are blurred in this dense sample of extinct primates. While pattern of evolution, a population-level phenomenon, may be difficult to falsify in the fossil record, this notharctine sample suggests that in the rare instance such as this, when the fossil record is densely sampled, change through time is continuous and more consistent with gradual evolution.
怀俄明州南部大角盆地的威利伍德组是一段河流岩石序列，跨越了大约 300 万年的始新世早期。它拥有世界上最大的化石哺乳动物收藏之一，包括数千种已灭绝的狐猴状灵长类动物——不知猴的牙齿。在南部大角盆地，这些灵长类动物的标本已在多次古生物学考察中被采集，并且产生牙齿的地层水平已被仔细记录。不知猴的牙齿为研究密切相关个体随时间推移在单一解剖系统中的形态变化提供了难得的机会。
先前对大角盆地不知猴进行的时间研究产生了数百个标本的测量结果，但我在此报告了对 3000 多个标本的牙齿和齿骨进行测量和比较的结果，所有标本均进行了地层映射。
在前牙区和臼齿区的面积以及齿骨深度方面都存在明显的变化。在较老的地层中，牙齿和齿骨相对较小的标本是已知的。在较年轻的岩石中，标本之间的牙齿面积变化增加。牙齿面积的变化是连续的，并且在同一地层水平内和地层水平之间广泛重叠。通过检查发现，其他牙齿变量以镶嵌和连续的方式在整个地层中发生变化。这些特征包括下第四前臼齿（p4）上的原尖的出现和数量的变化、下第一和第二臼齿的后尖窝的大小和形状、以及上臼齿的假次尖、中嵴和副嵴的相对发育。
尽管不知猴的阿尔法分类学需要广泛的修订，并且重要的是需要简化，但仍可以识别出这些广泛的模式。这种修订超出了本文的范围，但如果我们要建立一种既不受地层影响又对快速、可重复的物种分配有用的分类法，这是必不可少的。在这个灭绝灵长类动物的密集样本中，起源、进化和分支进化的模式之间的界限是模糊的。虽然进化模式是一种种群水平的现象，在化石记录中可能难以被证伪，但这个不知猴样本表明，在这种罕见的情况下，当化石记录被密集采样时，随时间的变化是连续的，更符合渐进式进化。
