Abstract
The teeth of sharks famously form a series of parallel, continuously replacing files borne directly on the jaw cartilages. In contrast, bony fishes possess site-specific shedding dentition borne primarily on dermal plates. Understanding how these disparate systems evolved is challenging, not least because of poorly understood relationships amongst early chondrichthyans and the profusion of morphologically and terminologically diverse bones, cartilages, splints and whorls that they possess. Here we use tomographic methods to investigate mandibular structures in several early branching ‘acanthodian’-grade stem-chondrichthyans. We characterise the dentigerous jaw bones of disparate genera of ischnacanthids as growing bones with non-shedding dentition. Mandibular splints, which support the ventro-lateral edge of the Meckel’s cartilage in some acanthodians, are formed from dermal bone and may be an acanthodid synapomorphy. We strengthen the case for Acanthodopsis as an acanthodid deeply nested within an edentulous radiation and show that its teeth are borne directly on the mandibular cartilage, unexpectedly representing an independent origin of teeth. Poor resolution of relationships amongst ‘acanthodians’ represents a major barrier to understanding the evolution and homology of teeth and associated oral structures.
Competing Interest Statement
The authors have declared no competing interest.
Footnotes
We have made a number of changes following review, including: - reframing the introduction and discussion to focus more explicitly on oral structure variation and diversity in early chondrichthyans - moving anatomical details from the supplement to the main text - expanding the review of oral structures in early chondrichthyans and moving it into the main text - revising a phylogenetic analysis to generate our tree topology - adding more explicit consideration of the distribution of different oral structures across different tree topologies
