title: Early development of rostrum saw-teeth in a fossil ray tests classical theories of the evolution of vertebrate dentitions.
abstract: In classical theory, teeth of vertebrate dentitions evolved from co-option of external skin denticles into the oral cavity. This hypothesis predicts that ordered tooth arrangement and regulated replacement in the oral dentition were also derived from skin denticles. The fossil batoid ray Schizorhiza stromeri (Chondrichthyes; Cretaceous) provides a test of this theory. Schizorhiza preserves an extended cartilaginous rostrum with closely spaced, alternating saw-teeth, different from sawfish and sawsharks today. Multiple replacement teeth reveal unique new data from micro-CT scanning, showing how the 'cone-in-cone' series of ordered saw-teeth sets arrange themselves developmentally, to become enclosed by the roots of pre-existing saw-teeth. At the rostrum tip, newly developing saw-teeth are present, as mineralized crown tips within a vascular, cartilaginous furrow; these reorient via two 90° rotations then relocate laterally between previously formed roots. Saw-tooth replacement slows mid-rostrum where fewer saw-teeth are regenerated. These exceptional developmental data reveal regulated order for serial self-renewal, maintaining the saw edge with ever-increasing saw-tooth size. This mimics tooth replacement in chondrichthyans, but differs in the crown reorientation and their enclosure directly between roots of predecessor saw-teeth. Schizorhiza saw-tooth development is decoupled from the jaw teeth and their replacement, dependent on a dental lamina. This highly specialized rostral saw, derived from diversification of skin denticles, is distinct from the dentition and demonstrates the potential developmental plasticity of skin denticles.
authors.affiliation: Department of Earth Sciences, Natural History Museum, London SW75BD, UK Dental Institute, Craniofacial Development, King's College London, London SE1 9RT, UK.
authors.forename: Moya Meredith
authors.initials: MM
authors.lastname: Smith
authors.affiliation: Department of Earth Sciences, Natural History Museum, London SW75BD, UK.
authors.forename: Alex
authors.initials: A
authors.lastname: Riley
authors.affiliation: Department of Animal and Plant Sciences, University of Sheffield, Sheffield S10 2TN, UK.
authors.forename: Gareth J
authors.initials: GJ
authors.lastname: Fraser
authors.affiliation: Department of Earth and Planetary Sciences, Birkbeck, University of London, London WC1E 7HX, UK.
authors.forename: Charlie
authors.initials: C
authors.lastname: Underwood
authors.affiliation: Department of Earth Sciences, Natural History Museum, London SW75BD, UK.
authors.forename: Monique
authors.initials: M
authors.lastname: Welten
authors.affiliation: Department of Palaeontology, University of Vienna, Vienna 1090, Austria.
authors.forename: Jürgen
authors.initials: J
authors.lastname: Kriwet
authors.affiliation: Department of Palaeontology, University of Vienna, Vienna 1090, Austria.
authors.forename: Cathrin
authors.initials: C
authors.lastname: Pfaff
authors.affiliation: Department of Earth Sciences, Natural History Museum, London SW75BD, UK z.johanson@nhm.ac.uk.
authors.forename: Zerina
authors.initials: Z
authors.lastname: Johanson
mesh_terms: D000818:Animals; D005075:Biological Evolution; D018940:Elasmobranchii / Q000033:anatomy & histology* / Q000145:classification / Q000254:growth & development; D005580:Fossils / Q000033:anatomy & histology*; D010802:Phylogeny; D014070:Tooth / Q000033:anatomy & histology* / Q000254:growth & development
keywords: Cretaceous ray; convergent evolution; developmental model; tooth development; tooth regeneration