Pseudooneotodus: A histological study of an Ordovician to Devonian vertebrate lineage

Detailed histological investigations have shed new light on the nature of Pseudooneotodus Drygant, 1974 (?Arenig/Llanvirn - Emsian). The genus has generally been interpreted as a conodont and is represented by squat phosphatic cones. These conodont elements show a differentiation into a lamellar cap, indistinguishable from vertebrate enamel, which is underlain by a spherulitic basal tissue with several characters indicative of dentine. The presence of these two issues in the elements of a conodont argues persuasively for the vertebrate classification of the clade, and illustrates that at least some conodonts have a hard tissue complex which is histologically indistinguishable from those of other primitive vertebrates. These observations have potentially important implications for conodont classification and the stratigraphic first appearances of vertebrate hard tissues. (C) 1996 The Linnean Society of London