DENTINE CANALS IN CAMBRO-ORDOVICIAN OSTRACODERMS AND CRETACEOUS-EOCENE PYCNODONT FISH

Dentine canals in the dermal plates of the “ostracoderm” fish Astraspis disiderata from the Middle Ordovician Harding Sandstone, Cannon City, Colorado show a striking similarity to canals in teeth of the pycnodont fish Anomoeodus latidens from the Upper Cretaceous Demopolis Formation of Frankstown, Mississippi, and to those of an unidentified Early Eocene pycnodont from Khouribga, Morocco. We propose here a direct homology between the dentine canals in Cambro‑Ordovician fishes such as Anatolepis and Astraspis and similar canals observed in pycnodont teeth. Pycnodont teeth consist largely of dentine invested with fine canals, capped by (in Anomoeodus latidens) a thin enameloid layer. In support of our inference of homology, the teeth of pycnodonts (and their probable ancestors the semionotids) are arranged in geometrical dental arrays that represent a variation on the primordial patterning of the sclerites (teeth) in an ancestral ostracoderm scleritome. The pycnodont-semionotid lineage thus manifests crushing dentition that is less derived than the dentition of comparable teleost fish. Early tetrapods developed stacked rows of teeth (zahnreihen) that evidently reflected control of sclerite/tooth position by an ectodermal morphogenetic field. Zahnreihen considered as partial anterior expression of the morphogenetic field help to explain the presence of tooth arrays that run at an angle to the edge of the lower jaw, as seen in Paleozoic tetrapods such as the Lower Permian reptile Captorhinus aguti.