Department of Biology, University of Toronto Mississauga, Mississauga, Ontario, Canada.
PLoS One. 2013 Sep 4;8(9):e74697. doi: 10.1371/journal.pone.0074697. eCollection 2013.
Tooth implantation provides important phylogenetic and functional information about the dentitions of amniotes. Traditionally, only mammals and crocodilians have been considered truly thecodont, because their tooth roots are coated in layers of cementum for anchorage of the periodontal ligament, which is in turn attached to the bone lining the alveolus, the alveolar bone. The histological properties and developmental origins of these three periodontal tissues have been studied extensively in mammals and crocodilians, but the identities of the periodontal tissues in other amniotes remain poorly studied. Early work on dental histology of basal amniotes concluded that most possess a simplified tooth attachment in which the tooth root is ankylosed to a pedestal composed of "bone of attachment", which is in turn fused to the jaw. More recent studies have concluded that stereotypically thecodont tissues are also present in non-mammalian, non-crocodilian amniotes, but these studies were limited to crown groups or secondarily aquatic reptiles. As the sister group to Amniota, and the first tetrapods to exhibit dental occlusion, diadectids are the ideal candidates for studies of dental evolution among terrestrial vertebrates because they can be used to test hypotheses of development and homology in deep time. Our study of Permo-Carboniferous diadectid tetrapod teeth and dental tissues reveal the presence of two types of cementum, periodontal ligament, and alveolar bone, and therefore the earliest record of true thecodonty in a tetrapod. These discoveries in a stem amniote allow us to hypothesize that the ability to produce the tissues that characterize thecodonty in mammals and crocodilians is very ancient and plesiomorphic for Amniota. Consequently, all other forms of tooth implantation in crown amniotes are derived arrangements of one or more of these periodontal tissues and not simply ankylosis of teeth to the jaw by plesiomorphically retaining "bone of attachment", as previously suggested.
牙齿植入为羊膜动物的牙齿提供了重要的系统发生和功能信息。传统上，只有哺乳动物和鳄形目动物被认为是真正的槽齿类，因为它们的牙根被覆有一层牙骨质，用于固定牙周韧带，牙周韧带又附着在牙槽骨的内层，即牙槽骨上。这三种牙周组织的组织学特性和发育起源在哺乳动物和鳄形目动物中得到了广泛研究，但其他羊膜动物的牙周组织的特征仍研究不足。早期对基础羊膜动物牙齿组织学的研究得出结论，大多数动物具有简化的牙齿附着方式，其中牙根与由“附着骨”组成的基座融合，而附着骨又与颌骨融合。最近的研究得出结论，非哺乳类、非鳄形目动物也存在典型的槽齿类组织，但这些研究仅限于冠群或次生水生爬行动物。作为羊膜动物的姐妹群，也是第一个表现出牙齿咬合的四足动物，双孔类是研究陆地脊椎动物牙齿进化的理想候选者，因为它们可以用于测试在深时中发育和同源性的假说。我们对石炭二叠纪合弓类四足动物牙齿和牙齿组织的研究揭示了存在两种类型的牙骨质、牙周韧带和牙槽骨，因此这是在四足动物中最早记录的真正的槽齿类。这些在基干羊膜动物中的发现使我们能够假设，产生哺乳动物和鳄形目动物特征性的槽齿类组织的能力在羊膜动物中非常古老且是原始的。因此，冠群羊膜动物中所有其他形式的牙齿植入都是这些牙周组织的一种或多种的衍生排列，而不是像以前认为的那样，通过保留原始的“附着骨”使牙齿简单地与颌骨融合。
