Fish biodiversity in the marine Norian (Late Triassic) of northern Italy: the first Neopterygian radiation

The Middle-Late Norian (220-215 Mya) vertebrate fauna from the Zorzino Limestone had numerous neopterygian genera. Though known since the Permian, neopterygians were not particularly diverse until the Norian, when major orders (Semionotiformes, Pycnodontiformes, Macrosemiiformes, Pholidophoriformes) radiated. Currently, 12 genera belonging to these orders are known from this fauna, a few more awaiting description. Palaeopterygians are also well represented (e.g., Saurichthys, Birgeria Thopracopterus and Gabanellia). Genera showing durophagous dentitions, adapted for crushing or nibbling different prey, belong to the Neopterygii, while paleopterygians mainly have predatory adaptations. Thus it is likely that the radiating neopterygians were not able to compete with more primitive forms on the highest trophic levels of active predation. Compared to earlier actinopterygians they developed an 'underexploited' trophic adaptation, durophagy. A new, more favourable environment was unlikely to have been the reason for this change. The Norian shallow, marine environment was rather poor in benthic lift, especially if compared with the Late Ladinian-Carnian reef complexes, from where no true durophagous neopterygian is known. Rather, it is suggested that the blooming of durophagous fishes was related to the improvement in jaw mechanics that characterizes the origin of neopterygians.