Re-assessment of the genus Helmolepis Stensiö 1932 (Actinopterygii: Platysiagidae) and the evolution of Platysiagids in the Early-Middle Triassic

Abstract

The type material of the rare, small actinopterygian Helmolepis gracilis Stensiö 1932 from the lower Griesbachian of East Greenland has been re-discovered in the Geologisk Museum of the University of Copenhagen. Two additional specimens, one presumably juvenile, have been identified in the same collection. More precise meristic features are reported and the reconstruction of the skull skeleton is considerably improved. Furthermore, another species, Helmolepis manis sp. nov. has been discovered in the Lower Triassic (Dienerian) of northwest Madagascar. The evidence of similar small-sized actinopterygian species in the Early Triassic suggests that platysiagids - recently also described from the Lower Smithian of western Canada - showed worldwide distribution at least by the Dienerian. Discovery of this new species further supports the assumption, that the Platysiagidae were not the offspring of derived Middle Triassic "perleidiform" actinopterygian fishes, but probably shared a common (unknown) ancestor older than the early Smithian, possibly of early Griesbachian age. The occurrence of closely related, small-sized actinopterygians in high latitudes by the early Dienerian and in the eastern Panthalassic Ocean by the early Smithian may be explained by an early or rapid dispersal event in combination with dwarfism in the recovery phase following the end-Permian mass extinction.