Squamation of the Permian actinopterygian Toyemia Minich, 1990: evenkiid (Scanilepiformes) affinities and implications for the origin of polypteroid scales

Abstract

occur in the continental upper Permian successions of European Russia. Due to their abundance and character­ istic morphology, they have been used as index fossils for biostratigraphic zonation schemes in order to define substage­level assemblage zones and even the so­called Toyemia superassemblage zone, slightly exceeding the duration of a stage (Esin & Mashin 1996, Minikh & Minikh 2009). The onset of this superassemblage, cha r ­ acterized by a high degree of endemism, coincides with the disappearance of several generalist fish forms as well as shifts in tetrapod and aquatic invertebrate communities, and has been interpreted in the context of major palaeoenvironmental changes on a regional scale (Bakaev et al. 2017). Toyemia was originally described as a member of Gonatodidae Gardiner, 1967, although similarities with the scanilepiform Evenkia Berg, 1941 have equally been pointed out (Minikh & Minikh 1990). Toyemia is reconstructed as a medium­sized predator of freshwater habitats (Minikh & Minikh 2009, Romano et al. 2016). For a better understanding of the palaeobiology and interrelationships of this both ecologically and strati ­ graphically significant taxon, we are conducting a com­ prehensive morphological study based on new and rediscovered, previously undescribed material. In this contribution, we present new morphological and histo ­ l ogical data on the squamation of Toyemia and the evenkiids Evenkia and Oshia Sytchevskaya, 1999, and discuss their possible phylogenetic implications.