Abstract
A tooth cusp of a Palaeozoic xenacanth shark, Orthacanthus sp. (Xenacanthiformes, Elasmobranchii) was discovered from a black massive mudstone of the upper part of the Senmatsu Formation (Changhsingian, Upper Permian) in Kanoko, Motoyoshi Town, Miyagi Prefecture, South Kitakami Belt, northeast Japan.The specimen is one of two lateral cusps of the tooth with a small median cusp. The cusp is slender and curves to inner or outer side at the one fourth cervical side of the total length. There are two serrated cutting edges in the cusp. The number of serrae is 28 in one side and 16 in the middle part of the other side. The surface of the cusp is ornamented with shiny black enameloid layer. The total length of the cusp is 12.4+mm and the maximum width of the cusp is 2.1 mm.This report is the first record of genus Orthacanthus from the Upper Permian in the world and terrestrial freshwater vertebrate fossil found from the Palaeozoic of Japan. It is possible, therefore, that many kinds of freshwater fishes, amphibians and reptiles may also be found from the Palaeozoic era of the Japanese Islands.
