Naturalis Biodiversity Center, Leiden, The Netherlands.
School of Earth Sciences, University of Bristol, Life Sciences Building, Bristol, UK.
Nat Ecol Evol. 2021 Jul;5(7):919-926. doi: 10.1038/s41559-021-01458-4. Epub 2021 May 6.
Chondrichthyan dentitions are conventionally interpreted to reflect the ancestral gnathostome condition but interpretations of osteichthyan dental evolution in this light have proved unsuccessful, perhaps because chondrichthyan dentitions are equally specialized, or else evolved independently. Ischnacanthid acanthodians are stem-Chondrichthyes; as phylogenetic intermediates of osteichthyans and crown-chondrichthyans, the nature of their enigmatic dentition may inform homology and the ancestral gnathostome condition. Here we show that ischnacanthid marginal dentitions were statodont, composed of multicuspidate teeth added in distally diverging rows and through proximal superpositional replacement, while their symphyseal tooth whorls are comparable to chondrichthyan and osteichthyan counterparts. Ancestral state estimation indicates the presence of oral tubercles on the jaws of the gnathostome crown-ancestor; tooth whorls or tooth rows evolved independently in placoderms, osteichthyans, ischnacanthids, other acanthodians and crown-chondrichthyans. Crown-chondrichthyan dentitions are derived relative to the gnathostome crown-ancestor, which possessed a simple dentition and lacked a permanent dental lamina, which evolved independently in Chondrichthyes and Osteichthyes.
软骨鱼类的牙齿结构通常被解释为反映了祖先天生颌类的状态，但这种对硬骨鱼类牙齿进化的解释并不成功，这也许是因为软骨鱼类的牙齿同样具有专门性，或者它们是独立进化的。异棘鱼形类的软骨鱼为软骨鱼的基干类群；作为硬骨鱼类和真骨鱼类的进化中间环节，它们神秘的牙齿结构的本质可能有助于同源性和祖先天生颌类状态的研究。在这里，我们发现异棘鱼形类的边缘牙齿结构为固着齿，由多尖齿组成，呈向远侧发散的行排列，并通过近侧的重叠替换而增加；而它们的联合齿环则与软骨鱼类和硬骨鱼类的牙齿结构相当。祖先状态估计表明，在硬骨鱼类的冠群祖先的颌骨上存在口上嵴；齿环或齿列在盾皮鱼类、硬骨鱼类、异棘鱼形类、其他的棘鱼类和真骨鱼类中是独立进化的。真骨鱼类的牙齿结构是相对于硬骨鱼类的冠群祖先的衍生结构，冠群祖先具有简单的牙齿结构，并且缺乏永久性的齿质鞘，后者在软骨鱼类和硬骨鱼类中是独立进化的。
