JOHNSON, G.D.; Smithsonian Institution: Diversity of the Acanthomorpha

The spiny-rayed fishes, Acanthomorpha, are the crown group of the Teleostei. With over 300 families and 16,000 species, they comprise more than 60% of extant teleosts and about one-third of living vertebrates. They exhibit staggering diversity in size (1cm-17m), body form, skeletal and soft anatomy, habitat, behavior, etc., much of which may differ strikingly through ontogeny. This presentation will attempt to illustrate some sense of the breadth of morphological diversity, including an ontogenetic perspective. Fossil acanthomorphs first appear sparsely at the base of the Upper Cretaceous. By the late Paleocene, the fauna is a bit more diverse, but at the Middle-Eocene, an explosive radiation essentially laid out the majority of acanthomorph diversity. Three unique innovations probably contributed to the extensive radiation of acanthomorphs: reductive restructuring of the dorsal gill arches, true fin spines, and a single median rostral cartilage facilitating upper jaw protrusion. Acanthomorph monophyly is well-supported morphologically, but hypotheses of intrarelationships are controversial or non-existent. Johnson and Patterson (1993) proposed an admittedly imperfect hypothesis of relationships among basal acanthomorph groups and redefined the crown group, Percomorpha, comprising about 245 families and 14,000 species, that best represent Nelson’s (1989)unresolved “bush at the top.” Despite considerable progress in establishing monophyly and composition of many percomorph orders, suborders and families, relationships among the majority remain obscure. Even the largest and most diverse order of vertebrates, the Perciformes (9800 species in over 200 families), remain uncharacterized. Clearly, THE outstanding problem in systematic ichthyology is sorting out the percomorph/perciform morass, which undoubtedly will also alter our current concept of basal acanthomorph relationships.