Retention of fish-like odontode overgrowth in Permian tetrapod dentition supports outside-in theory of tooth origins
Retention of fish-like odontode overgrowth in Permian tetrapod dentition supports outside-in theory of tooth origins
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DOI：
10.1098/rsbl.2019.0514
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发表时间：
2019-09
期刊：
影响因子：
3.3
通讯作者：
Y. Haridy;B. Gee;F. Witzmann;J. Bevitt;R. Reisz
中科院分区：
文献类型：
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作者：
Y. Haridy;B. Gee;F. Witzmann;J. Bevitt;R. Reisz
牙齿通常被认为是排列在口腔边缘的结构;然而，称为齿突的牙齿状结构通常在许多古生代脊椎动物的真皮骨骼上发现，包括早期的无颌鱼类。“牙本质”是所有具有同源组织和发育的牙齿样牙本质结构的广义术语。这个定义包括真牙和早期“鱼类”的齿突，最近对这些牙齿进行了研究，以获得对牙齿起源的新见解。在这场争论中，两个主要的假说经常被引用：“由外向内”假说，该假说认为真皮齿突进化到口腔中，以及“由内而外”假说，该假说认为牙齿起源于口咽腔，然后向外移动到口腔中。在这里，我们表明，不像众所周知的一对一的边缘齿系的更换模式，腭齿系的早二叠世四足动物，包括dissorophoid两栖动物Cacops和早期爬行动物Captorhinus，是过度生长的新骨层，最新的牙齿，然后连接。在早期鱼类的皮肤和口腔齿突（即牙齿）中也有很好的记录。我们认为，这种模式代表了脊椎动物的原始状态，甚至可能早于颌骨的起源。因此，这种模式跨越鱼类四足动物的过渡，并保留这种祖先的模式，在腭齿系的早期陆生四足动物提供了强有力的支持牙齿起源的“外-内”假说。
Teeth are often thought of as structures that line the margins of the mouth; however, tooth-like structures called odontodes are commonly found on the dermal bones of many Palaeozoic vertebrates including early jawless fishes. ‘Odontode’ is a generalized term for all tooth-like dentine structures that have homologous tissues and development. This definition includes true teeth and the odontodes of early ‘fishes’, which have been recently examined to gain new insights into the still unresolved origin of teeth. Two leading hypotheses are frequently referenced in this debate: the ‘outside-in’ hypothesis, which posits that dermal odontodes evolutionarily migrate into the oral cavity, and the ‘inside-out’ hypothesis, which posits that teeth originated in the oropharyngeal cavity and then moved outwards into the oral cavity. Here, we show that, unlike the well-known one-to-one replacement patterns of marginal dentition, the palatal dentition of the early Permian tetrapods, including the dissorophoid amphibian Cacops and the early reptile Captorhinus, is overgrown by a new layer of bone to which the newest teeth are then attached. This same overgrowth pattern has been well documented in dermal and oral odontodes (i.e. teeth) of early fishes. We propose that this pattern represents the primitive condition for vertebrates and may even predate the origin of jaws. Therefore, this pattern crosses the fish–tetrapod transition, and the retention of this ancestral pattern in the palatal dentition of early terrestrial tetrapods provides strong support for the ‘outside-in’ hypothesis of tooth origins.