An unusual tricuspid chondrichthyan tooth from the Lower Permian of Texas, U.S.A.

Abstract

A single small, well-preserved complete tooth from the upper Petrolia Formation (early Artinskian), Wichita Group, in north-central Texas, possesses a crown composed of three nearly parallel labially-positioned principal cusps of about equal length and stature. Secondary cusps are absent. The lateral cusps are nearly straight and cylindrical, but proximally compressed; they each bear about nine mostly straight cristae on all margins, mostly restricted to the distal half. The central cusp is similar, although less compressed, and bears about seven cristae with none on the lingual side. The base is longer than wide (labial-lingual) and possesses a similarly elongated basal tubercle and apical button. The basal tubercle is slightly concave, restricted to the labial half of the aboral surface, and has a prominent labial rim which is indented. The prominent apical button covers the oral surface of the lingually extended base, lies partly between the lateral cusps, is in contact with all three cusps, and its lingual margin bears prominent centrally positioned foramina, resulting in a slightly bifurcated appearance. These characters are similar in some aspects to the Late Devonian phoebodontiforms Phoebodus politus and Jalodus. Among xenacanthiform sharks, the tooth is most similar to Bransonella from the Carboniferous and Early Permian (Asselian). The Texas Permian tooth shows no evidence of wear from transport. It probably represents a new taxon, and its morphology supports the concept that phoebodontiforms and xenacanthiforms are closely related. The shapes of the apical button and basal tubercle, and especially the three equal cusps, suggest this tooth may be allied more closely to the phoebodontiforms, but its affinity cannot presently be determined.