Early development of rostrum saw-teeth in a fossil ray tests classical theories of the evolution of vertebrate dentitions

Moya Smith, Alex Riley, Gareth J Fraser, Charlie Underwood, Monique Welten, Jürgen Kriwet, Cathrin Pfaff, Zerina Johanson

Research output: Contribution to journalArticle (Academic Journal)peer-review

8 Citations (Scopus)
288 Downloads (Pure)


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.

Original languageEnglish
Article number20151628
Number of pages8
JournalProceedings of the Royal Society B: Biological Sciences
Issue number1816
Publication statusPublished - 7 Oct 2015


  • Cretaceous ray
  • tooth regeneration
  • developmental model
  • convergent evolution
  • tooth development


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