Dentitions of Late Palaeozoic Orthacanthus species and new species of ?Xenacanthus (Chondrichthyes: Xenacanthiformes) from North America

Abstract

Orthacanthus lateral teeth have paired, variably divergent, smooth, usually carinated labio-lingually compressed principal cusps separated by a central foramen; one or more intermediate cusps; and an apical button on the base isolated from the cusps. Several thousand isolated teeth from Texas Artinskian bulk samples are used to define the heterodont dentitions of O. texensis and O. platypternus. The O. texensis tooth base has a labio-lingual width greater than the anteromedial-posterolateral length, the basal tubercle is restricted to the thick labial margin, the principal cusps are serrated to varying degrees, and the posterior cusp is larger. The O. platypternus tooth base is longer than wide, its basal tubercle extends to the center, the labial margin is thin, serrations are absent on the principal cusps, the anterior cusp is larger, and a single intermediate cusp is present. More than two hundred isolated teeth from Nebrasca (Gzhelian) and Pennsylvania (Asselian) provide apreliminary description of the heterodont dentition of O. compressus. The principal cusps are similar to O. texensis but usually(?) are not serrated, and the base is usually wider than long but has a thin or sometimes thick labial margin beneath a single intermediate cusp.

A few dozen very small isolated teeth define two ?Xenacanthus dentitions. ?X. ossiani sp. nov. (Gzhelian, Nebrasca) teeth have a thin, longer than wide base with a flange at one end, an isolated apical button, a centrally extended basal tubercle, and a central foramen; the principal and intermediate cusps are recumbent, divergent, highly compressed, smooth and lack serrations. ?X. slaughteri sp. nov. (Artinskian, Texas) teeth have nearly parallel, smooth, carinated, nonserrated, compressed principal cusps and intermediate cusp; the base is thin, longer than wide, with the apical button often in contact with the principal cusps, present or absent central foramen, and basal tubercle restricted to the labial margin.

The new species of ?Xenacanthus, as well as O. platypternus and other xenacanth species, appear to be endemic to North America. Other upper Palaeozoic species are endemic to Europe. However, O. compressus and possibly O. texensis are similar to some European species. Despite the Appalachian-Hercynian barrier, dispersal may have occurred in coastal marine waters during a migration phase of the reproductive cycle of some Orthacanthus species.