Are there two distinct types of hypocone in Eocene primates? The 'pseudohypocone' of notharctines revisited
Are there two distinct types of hypocone in Eocene primates? The 'pseudohypocone' of notharctines revisited
复制标题
始新世灵长类动物是否有两种不同类型的下锥体？
DOI：
10.26879/329
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发表时间：
2012
影响因子：
2
通讯作者：
W. Dirks
中科院分区：
文献类型：
--
作者：
R. Anemone;M. Skinner;W. Dirks
现代人类和大多数现存灵长类动物的上臼齿有四个牙尖，它们是从兽目哺乳动物的原始楔齿进化而来的。这些包括原始三角形的三个尖点（例如，副锥、后锥和原锥），以及增加远舌尖或下锥。在始新世的灵长类动物中，与舌扣带相关的“真”次锥（adapine形式）和与原锥远端边缘相关的“假次锥”（notharctine形式）之间的区别由来已久。这两种类型的远舌尖的发展过程是未知的，和有效性的区别是基于系统发育的效用和同源性，而不是尖的位置，在其他哺乳动物群体。为了解决这个问题，我们使用微型计算机断层扫描，以揭示形态的次锥和相关的尖和牙釉质牙本质交界处（EDJ）的波峰。EDJ保留了牙冠发育的初始步骤，可用于阐明磨损或损坏的化石牙齿中牙冠形态的详细方面。我们的研究样本包括来自欧洲的adapine物种和来自北美的notharctines。我们证实，pseudohocone notharctines之间发现的是一个真正的尖，因为它的形式作为一个牙本质角在冠的发展。我们的研究结果也证实，这两种形式的hypocone是发育不同的，并在这两个灵长类进化分支收敛。对古生物学文献的回顾表明，尽管在哺乳动物的分支中，关于次锥的发育的同质性很普遍，但只有在notharctines中，我们才能找到这个尖点的替代名称。
Upper molars of modern humans and most extant primates have four cusps that have evolved from the original tribosphenic tooth of therian mammals. These include the three cusps of the original trigon (e.g., paracone, metacone, and protocone), and the addition of the distolingual cusp or hypocone. Among Eocene primates of the family Adapidae, a distinction has long been made between a “true” hypocone associated with the lingual cingulum (adapine form) and a “pseudohypocone” associated with the distal margin of the protocone (notharctine form). The developmental processes underlying these two types of distolingual cusp are unknown, and the validity of the distinction is based on phylogenetic utility and homology rather than cusp position, as in other mammalian groups. To address this issue we use micro-computed tomography to reveal the morphology of the hypocone and associated cusps and crests on the enamel-dentine junction (EDJ). The EDJ preserves the initial steps of tooth crown development and can be used to clarify detailed aspects of crown morphology in variably worn or damaged fossil teeth. Our study sample includes both adapine species from Europe and notharctines from North America. We confirm that the pseudohypocone found among notharctines is a true cusp since it forms as a dentine horn during crown development. Our results also confirm that these two forms of hypocone are developmentally distinct and have evolved convergently in these two primate clades. A review of the paleontological literature suggests that, in spite of the fact that homoplasy is rampant among mammalian clades with respect to the development of the hypocone, only among the notharctines do we find an alternative name for this cusp.