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.

软骨鱼类的牙列通常被解释为反映了祖先颚口动物的状况，但从这个角度对骨鱼牙齿进化的解释已被证明是不成功的，也许是因为软骨鱼类的牙列同样特化，或者是独立进化的。Ischnacanthid acanthodians 是干软骨鱼纲；作为骨鱼和冠软骨鱼的系统发育中间体，它们神秘的齿列的性质可能会揭示同源性和祖先颚口动物的状况。在这里，我们表明，坐棘鱼的边缘牙列是固定齿，由远端分叉的多尖牙组成，通过近端叠加替换，而它们的联合牙轮与软骨鱼和骨鱼的对应物相当。祖先状态估计表明颚口动物冠祖先的下颌上存在口腔结节；盾皮鱼、骨鱼、长棘鱼、其他棘皮动物和冠软骨鱼的齿轮或齿排是独立进化的。冠软骨鱼的齿列是相对于颌骨冠祖先而来的，后者具有简单的齿列并且缺乏永久的牙板，在软骨鱼和硬骨鱼中独立进化。