Dental ontogeny in extinct synapsids reveals a complex evolutionary history of the mammalian tooth attachment system

The mammalian dentition is uniquely characterized by a combination of precise occlusion, permanent adult teeth, and a unique tooth attachment system. Unlike the ankylosed teeth in most reptiles, mammal teeth are supported by a ligamentous tissue that suspends each tooth in its socket, providing flexible and compliant tooth attachment that prolongs the life of each tooth and maintains occlusal relationships. Here we investigate dental ontogeny through histological examination of a wide range of extinct synapsid lineages to assess whether the ligamentous tooth attachment system is unique to mammals and to determine how it evolved. This study shows for the first time that the ligamentous tooth attachment system is not unique to crown mammals within Synapsida, having arisen in several non-mammalian therapsid clades as a result of neoteny and progenesis in dental ontogeny. Mammalian tooth attachment is here re-interpreted as a paedomorphic condition relative to the ancestral synapsid form of...哺乳动物牙列的独特特征，在于精准咬合、恒成体齿，以及独特的牙齿附着系统。与多数爬行类的锚定固着齿（ankylosed teeth）不同，哺乳动物的牙齿由韧带结缔组织支撑——该组织将每颗牙齿悬吊于齿槽之中，可提供柔性且可顺应的牙齿附着方式，既延长了单颗牙齿的使用寿命，又维持了咬合关系。本研究通过对多种已灭绝合弓类（Synapsida）支系开展组织学检查，探究牙齿个体发育（dental ontogeny）过程，以评估韧带性牙齿附着系统是否仅为哺乳动物所独有，并阐明其演化路径。本研究首次证实，在合弓类中，韧带性牙齿附着系统并非冠群哺乳动物（crown mammals）所独有：该系统已在若干非哺乳类兽孔类（therapsid）支系中演化出现，其成因是牙齿个体发育过程中的幼态持续（neoteny）与发育早熟（progenesis）。本研究将哺乳动物的牙齿附着方式，重新诠释为相对于祖先合弓类形态的幼态成形（paedomorphic）状态……