PHYLOGENY AND PALEOBIOGEOGRAPHY OF PLATYSIAGIFORM STEM ACTINOPTERYGIANS

Revision of the type series of the platysiagiform Helmolepis gracilis Nybelin, 1977 and several newly discovered specimens of the same species reveal that a phylogenetically significant cheek bone in actinopterygians, the purported interopercle in Helmolepis, is a broken ventral portion of the subopercle in the holotype specimen mistaken for an interopercle. As a corollary, the systematic position of the order Platysiagiformes was misinterpreted in subsequent phylogenetic analyses. Discovery of two new species allows also partial reconstructions of the head and body skeleton of early platysiagiforms, recognition of derived characters in the snout and primitive characters in the caudal fin and reconstruction of their phylogenetically isolated history. In their revised phylogenetic position, platysiagiform actinopterygians represent either "basal perleidiforms" or even more primitive stem actinopterygians of pivotal yet unknown Early Triassic or Late Permian origin. Paleobiogeographic mapping of the history of all platysiagiform species as an isolated phylogenetic lineage throughout the Mesozoic highlights that the earliest Triassic species are extraordinarily small if compared to Middle Triassic to Jurassic species: this evidence is interpreted as the first record of dwarfism among basal actinopterygians following the great end-Permian extinction event.