The origin of vertebrate teeth and evolution of sensory exoskeletons.

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 teeth(3), 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 tubules(1) 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 teeth(16). Middle-Ordovician fossils now represent the oldest known evidence for vertebrate dental tissues.