THE ACTINOPTERYGIAN FISHES OF THE NEWARK SUPERGROUP AND CHINLE FORMATION, NORTH AMERICA: NEW DISCOVERIES AND COMMENTS ON THE DIVERSITY OF FISHES FROM THE EARLY MESOZOIC

Understanding the evolutionary relationships of lower actinopterygian fishes is crucial to understanding how modern fish lineages have evolved. A large percentage of the actinopterygian diversity (including the advanced palaeonisciform †Redfieldiiformes and the ginglymodian †Semionotiformes) occurs in two major Triassic-age deposits in North America: the Chinle Formation of southwestern United States and the Newark Supergroup of eastern North America. Fish fossils found in the Newark Supergroup have been the subject of several extensive studies, whereas many fish fossils from the Chinle Formation have remain undescribed, and our understanding of the extent of diversity of fishes from the Upper Triassic Chinle Formation remains incomplete. Recent work in the southwestern United States has provided insight into the diversity of fishes in the Chinle Formation, including the discovery of a new genus within the family Semionotidae. The new genus, Lophionotus, is directly compared to specimens of Semionotus from the Newark Supergroup, and displays distinct morphological characteristics that diagnose it as a new genus with two new species. Some important characteristics include a dorsoventrally expanded infraorbital series and broad parietal (frontal) bones on the skull roof, whereas Semionotus from the Newark Supergroup have narrow infraorbital bones and slender parietal (frontal) bones. The assemblage of fishes from the Newark Supergroup is key in identifying and distinguishing the fish fauna found in the Upper Triassic Chinle Formation, and a comparison of these early Mesozoic assemblages is presented, including ginglymodian, palaeonisciform, and redfieldiiform fishes.