The Palaeozoic genusPsephodus(Chondrichthyes, Cochliodontiformes) and the transition from teeth to tooth plates in holocephalians

Abstract

Remains of the chondrichthyanPsephodusMorris and Roberts, 1862consist mainly of isolated tooth plates. The genus has a range from the Late Devonian (Famennian) to the Late Mississippian (Serpukhovian). The most complete specimen is a partially articulated set of teeth and tooth plates of the type species,P. magnus, from the early Serpukhovian of East Kilbride, Lanarkshire, Scotland. A row of minute nodes is present along the basal margins of theLophodus-like andHelodus-like teeth of the East Kilbride specimen. This distinguishes these teeth from typicalLophodus-like andHelodus-like teeth. As inHelodus simplex, some specimens ofPsephodusdisplay varying degrees of fusion of teeth from the same labiolingually oriented file into tooth plates. However, unlikeHelodus simplex, some specimens ofPsephodusdisplay fusion of mesiodistally separated tooth plates, each representing a fused tooth file, into a larger tooth plate.Psephodus, which crosses the Devonian–Carboniferous boundary, may be ancestral to other Mississippian holocephalians, such asCochliodusorChondrenchelys, in which dentitions consist of a few tooth plates with only few, or no, separate teeth. An unpublished watercolour by Agassiz's artist Dinkel depicts five tooth plates, which can be designated as syntypes ofP. magnus.A lectotype forPsephodus crenulatusis designated.