Abstract

Two new acanthodian taxa are described. The ischnacanthid Xylacanthus kenstewarti is based on large, dentigerous jaws, and Granulacanthus joenelsoni is based on isolated spines. The isolated remains of these species are similar in that they both possess pustulose denticles or tubercles, either on the mesial ridge (X. kenstewarti) or on the fin spines (G. joenelsoni). Jaws of X. kenstewarti are similar in size to those of Xylacanthus minutus, Ischnacanthus kingi, and I. wickhami and smaller than those of X. grandis. The jaws of X. kenstewarti are most similar to those of X. minutus, but are distinguished from this and other ischnacanthid species by a tapering patch of pustulose denticles that is widest midway along the jaw, mesial denticles that are simple blisterlike structures, the monocuspid, striated primary teeth that are subcircular in cross section, and a posterodorsal process that is enlarged. The spines of G. joenelsoni have distinctive tubercular ornamentation. Tubercles, or nodular ornaments on fin spines, are characteristic of primitive acanthodians, but the slender shape of the spines, the low number of spine ribs, and the fine striations posterior to the main ribs of each spine suggest that G. joenelsoni is a relatively advanced acanthodian. Xylacanthus kenstewarti and G. joenelsoni are from the Silurian (Wenlock or Ludlow) of the southern Mackenzie Mountains. Xylacanthus kenstewarti represents the earliest representative of the genus, the earliest unequivocal remains of a gnathostome from the Mackenzie Mountains, and extends the known geographical range of the genus from the Mackenzie Mountains east to Spitsbergen.

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