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

Remains of the chondrichthyan Psephodus Morris and Roberts, 1862, consist mainly of isolated tooth plates. The genus has a range from Late Devonian (Famennian) to 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 the Lophodus-like and Helodus-like teeth of the East Kilbride specimen. This distinguishes these teeth from typical Lophodus-like and Helodus-like teeth. As in Helodus simplex, some specimens of Psephodus display varying degrees of fusion of teeth from the same labiolingually oriented file into tooth plates. However, unlike Helodus simplex, some specimens of Psephodus display 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 as Cochliodus or Chondrenchelys, 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 An unpublished watercolour by Agassiz's artist Dinkel depicts five tooth plates, which can be designated as syntypes of P. magnus. A lectotype for Psephodus crenulatus is designated.