The origin of vertebrate teeth and evolution of sensory exoskeletons.

Abstract

The earliest record of tooth antecedents and the tissue dentine^1,2, an early-vertebrate novelty, has been controversially represented by fragmentary Cambrian fossils identified as Anatolepis heintzi^3-5. Anatolepis exoskeletons have the characteristic tubules of dentine that prompted their interpretation as the first precursors of teeth³, known as odontodes. Debates over whether Anatolepis is a legitimate vertebrate^6-8 have arisen because of limitations in imaging and the lack of comparative exoskeletal tissues. Here, to resolve this controversy and understand the origin of dental tissues, we synchrotron-scanned diverse extinct and extant vertebrate and invertebrate exoskeletons. We find that the tubules of Anatolepis have been misidentified as dentine tubules and instead represent aglaspidid arthropod sensory sensilla structures^9,10. Synchrotron scanning reveals that deep ultrastructural similarities between odontodes and sensory structures also extend to definitive vertebrate tissues. External odontodes of the Ordovician vertebrate Eriptychius^11-13 feature large dentine tubules¹ that are morphologically convergent with invertebrate sensilla. Immunofluorescence analysis shows that the external odontodes of extant chondrichthyans and teleosts retain extensive innervation suggestive of a sensory function akin to teeth^14-16. These patterns of convergence and innervation reveal that dentine evolved as a sensory tissue in the exoskeleton of early vertebrates, a function retained in modern vertebrate teeth¹⁶. Middle-Ordovician fossils now represent the oldest known evidence for vertebrate dental tissues.