Orthacanthus platypternus (Cope, 1883) (Chondrichthyes: Xenacanthiformes) teeth and other isolated vertebrate remains from a single horizon in the early Permian (Artinskian) Craddock Bonebed, lower Clear Fork Group, Baylor County, Texas, USA

Abstract

An unusual 6–8 cm layer of prismatic cartilage and matrix containing some 8,800 teeth, coprolites, incomplete occipital spines, and denticles of Orthacanthus platypternus (Cope, 1883) occurs in the lower Permian (Artinskian) Craddock Bonebed in Texas, USA. It is the only species of shark present in the Clear Fork Group except for three worn Xenacanthus Beyrich, 1848 occipital spine fragments and two teeth of ?Lissodus (Polyacrodus) zideki (Johnson, 1981) (Hybodontoidei), both being the first occurrences in this unit. Analysis of measurements of teeth with complete bases randomly selected from 3,050 initially available teeth failed to reveal the presence of sexual dimorphism or the discrete presence of juveniles as expected, based on an independent study which identified the presence of Orthacanthus juvenile occipital spines. A few highly symmetrical small teeth are present, which had not been previously observed in the Texas lower Permian. They may be symphyseals and restricted only to juveniles. Other unusual teeth include germinal teeth and deformed teeth, both of which occur in the Clear Fork and underlying Wichita groups. One tooth displays an apparent example of the equivalent of an “enamel pearl” on one of its cusps. The most unusual teeth are those that appear to have undergone various stages of resorption. Only the lingual margin of the base is affected in which the apical button is resorbed to varying degrees until only the labial margin with the basal tubercle and the three cusps are all that remain. If the teeth were undergoing resorption, then the perplexing problem is why the apical button is resorbed and not the superjacent basal tubercle. Other vertebrate remains include palaeoniscoid scales and teeth and unidentified tetrapod bone fragments, jaw fragments, and teeth. Rare fragments of bones (scales?) bear a “comb edge” which have not been previously observed in the Texas lower Permian.