Abstract
Shark and ray (elasmobranch) dentitions are well known for their multiple generations of teeth, with isolated teeth being common in the fossil record. However, how the diverse dentitions characteristic of elasmobranchs form is still poorly understood. Data on the development and maintenance of the dental patterning in this major vertebrate group will allow comparisons to other morphologically diverse taxa, including the bony fishes, in order to identify shared pattern characters for the vertebrate dentition as a whole. Data is especially lacking from the Batoidea (skates and rays), hence our objective is to compile data on embryonic and adult batoid tooth development contributing to ordering of the dentition, from cleared and stained specimens and micro-CT scans, with 3D rendered models. We selected species (adult and embryonic) spanning phylogenetically significant batoid clades, such that our observations may raise questions about relationships within the batoids, particularly with respect to current molecular-based analyses. We include developmental data from embryos of recent model organisms Leucoraja erinacea and Raja clavata to evaluate the earliest establishment of the dentition. Characters of the batoid dentition investigated include alternate addition of teeth as offset successional tooth rows (versus single separate files), presence of a symphyseal initiator region (symphyseal tooth present, or absent, but with two parasymphyseal teeth) and a restriction to tooth addition along each jaw reducing the number of tooth families, relative to addition of successor teeth within each family. Our ultimate aim is to understand the shared characters of the batoids, and whether or not these dental characters are shared more broadly within elasmobranchs, by comparing these to dentitions in shark outgroups. These developmental morphological analyses will provide a solid basis to better understand dental evolution in these important vertebrate groups as well as the general plesiomorphic vertebrate dental condition.
Highlights
Living chondrichthyans include sharks, rays and the holocephalans, which between them fill a diverse suite of marine, and some freshwater, niches
Sharks and rays are well known for the presence of multiple generations of teeth forming a ‘conveyor belt’ along the jaw, brought to the functional surface and subsequently lost. The development of these dentitions and how teeth are organized into highly functional dentitions characteristic of the group is poorly understood [1]. This lack of understanding makes it difficult to determine dental characteristics of the major chondrichthyan groups and to extrapolate observations from Recent taxa that would allow us to acquire a better understanding of fossil chondrichthyan dentitions
In order to better understand the development of chondrichthyan dentitions it is important that the dentitions of sharks, batoids and holocephalans (e.g., [1]) are studied in detail, early in ontogeny
Summary
Rays and the holocephalans (chimaeras), which between them fill a diverse suite of marine, and some freshwater, niches. The development of these dentitions and how teeth are organized into highly functional dentitions characteristic of the group is poorly understood [1] This lack of understanding makes it difficult to determine dental characteristics of the major chondrichthyan groups and to extrapolate observations from Recent taxa that would allow us to acquire a better understanding of fossil chondrichthyan dentitions. This hinders comparisons to the development of structural pattern to other major jawed vertebrate (gnathostome) groups such as the Actinopterygii (Osteichthyes), and to the fossil groups ‘Placodermi’ and ‘Acanthodii’. Our focus below is on the Batoidea, comprising a monophyletic clade containing diverse rays, skates, guitarfish and sawfish and representing the most speciose group of extant chondrichthyans
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