Several new species of the ischnacanthiform acanthodian Ischnacanthus are described from a Lower Devonian (Lochkovian) fossil locality in the Mackenzie Mountains, Northwest Territories, Canada. These new species are distinguished based on features of the jaws, teeth, and tooth bearing bones. Ischnacanthus gracilis , a similar species from the Old Red Sandstone of Scotland, is not found at the Canadian fossil site. The presence of a range of sizes of specimens of Ischnacanthus suggests a growth series (juvenile to adult). The genus is found to develop in a largely isometric manner, with a few body proportions exhibiting slight allometry. Inability to diagnose species based on body fossils of Ischnacanthus implies...
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