New teeth belonging to Karksiodus mirus Ivanov & Märss (Chondrichthyes) were found together with putative chondrichthyan scales in five new localities of the Leningrad Region, northwestern Russia, within the Aruküla and Burtnieki regional stages, Givetian, Middle Devonian. The teeth exhibit variability in the number of cusps, angles between the lateral cusps, base curvature, length of lateral parts and the prominence of the wall of the transversal basal canal. Karksiodus tooth material collected from these sites suggests that this taxon possesses an heterodont dentition and a specific, complex vascularization system affecting the dental base and the crown. Enameloid tissue seems to be absent, thus the surface striations on the cusps are presumably made up by orthodentine. The fish fauna from these localities is listed.
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