Abstract
A well-known characteristic of chondrichthyans (e.g. sharks, rays) is their covering of external skin denticles (placoid scales), but less well understood is the wide morphological diversity that these skin denticles can show. Some of the more unusual of these are the tooth-like structures associated with the elongate cartilaginous rostrum ‘saw’ in three chondrichthyan groups: Pristiophoridae (sawsharks; Selachii), Pristidae (sawfish; Batoidea) and the fossil Sclerorhynchoidea (Batoidea). Comparative topographic and developmental studies of the ‘saw-teeth’ were undertaken in adults and embryos of these groups, by means of three-dimensional-rendered volumes from X-ray computed tomography. This provided data on development and relative arrangement in embryos, with regenerative replacement in adults. Saw-teeth are morphologically similar on the rostra of the Pristiophoridae and the Sclerorhynchoidea, with the same replacement modes, despite the lack of a close phylogenetic relationship. In both, tooth-like structures develop under the skin of the embryos, aligned with the rostrum surface, before rotating into lateral position and then attaching through a pedicel to the rostrum cartilage. As well, saw-teeth are replaced and added to as space becomes available. By contrast, saw-teeth in Pristidae insert into sockets in the rostrum cartilage, growing continuously and are not replaced. Despite superficial similarity to oral tooth developmental organization, saw-tooth spatial initiation arrangement is associated with rostrum growth. Replacement is space-dependent and more comparable to that of dermal skin denticles. We suggest these saw-teeth represent modified dermal denticles and lack the ‘many-for-one’ replacement characteristic of elasmobranch oral dentitions.
Highlights
Sharks and rays have been studied extensively to address the origin and evolution of teeth (e.g. [1])
Our observations and comparisons of the rostrum saw-teeth to the oral dentitions demonstrate that along the rostrum, developmental order and arrangement of denticles are distinct with clear boundaries in the embryo delineating rostrum saw-teeth from that of the oral dentition
We suggest that the tooth-like denticles on the rostrum-saw were more similar, in their initiation pattern and replacement order, to that seen in generalized dermal denticles, and originated from the denticle odontogenic modules that became enlarged and arranged along the rostrum and chondrocranium, forming distinct sets in 18 Pristiophoridae and Sclerorhychoidea
Summary
Teeth and tooth-like structures are readily observed in sharks and rays; in addition to true teeth present along the jaws, dermal denticles are present in most species, being present both on the skin and in the oro-pharyngeal cavity. The classic ‘outside–in’ hypothesis proposed that oral teeth were derived from the external skin denticles through a heterotopic evolutionary shift into the oro-pharyngeal cavity, from which the functional dentition on the jaw was derived. This implies that denticles and oral teeth share a common developmental ‘toolkit’, as morphogenetic units (odontodes), and including the spatio-temporal patterning and ordered replacement that characterizes the dentition. Denticles on the skin surface would retain the potential to be patterned (e.g. [5,6,7,8,9]) and to be replaced on a regular basis and in advance of function, as in an ordered set of teeth
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