A New Morphotype of Fish Teeth of the Order Eurynotoidiformes (Actinopterygii) from the Upper Permian Deposits of European Russia

A new morphotype of multicuspid teeth belonging to the actinopterygian order Eurynotoidiformes and supposedly associated with Isadia opokiensis A. Minich et Andrushkevich, 2017 was found in the terminal deposits of the East European Severodvinian Stage (Maryushkina Sluda-C locality, Vologda Region). The original description of this species was based upon isolated scales obtained from the stratigraphically and geographically adjacent Mutovino locality. Teeth of the I. opokiensis morphotype are robust and are characterized by a powerful buttress and a wide crown bearing numerous (up to eight) spade-shaped cusps with high acrodine caps. Cusp tips form a slightly curved occlusal surface. Labially, these tips bear well defined facets formed by lifetime wear that suggest algal scraping as the predominant feeding mode. Tooth structure in Isadia opokiensis is morphologically transitional between that of I. suchonensis and I. arefievi, suggesting a morphoevolutionary succession. The similar (flat) crown shape in another Isadia species, I. aristoviensis, may have originated convergently by modification of a more primitive morphotype found in Urzhumian eurynotoidids.