AN OVERVIEW OF ACIPENSERIFORMES

Acipenseriformes occupy a special place in the history of ideas concer ning fish evolution, but in many respects, phylogenetic studies df the group remain in their infancy. Even such basic questions as the monop hyly of Acipenser (the largest genus) are unanswered. We define relati onships based on comparative osteology, which allows us to incorporate well-preserved fossils into analyses. Acipenseriformes has existed at least since the Lower Jurassic (approximately 200 MYBP), and all foss il and recent taxa are from the Holarctic. Phylogenetic relationships among Paleozoic and Early Mesozoic actinopterygians are problematic, b ut most workers agree that Acipenseriformes is monophyletic and derive d from some component of 'paleonisciform' fishes. ('Paleonisciformes' is a grade of primitive non-neopterygian actinopterygians, sensu Gardi ner 1993.) Taxa discussed in comparison here are: dagger Cleirolepis, Polypterus, dagger Mimia, dagger Moythomasia, dagger Birgeria, dagger Saurichthys, Lepisosteus and Amia. We review generic diversity within the four nominal families of fossil and recent Acipenseriformes (dagge r Chondrosteidae, dagger Peipiaosteidae, Polyodontidae, and Acipenseri dae), and provide a cladogram summarizing osteological characters for those four groups. Monophyly of the two extant families is well-suppor ted, but there are no comprehensive studies of all of the known specie s and specimens of dagger Chondrosteidae and dagger Peipiaosteidae. As a result, sister-group relationships among dagger Chondrosteidae, dag ger Peipiaosteidae, and Acipenseroidei (= Polyodontidae + Acipenserida e) are unresolved. We discuss five features fundamental to the biology of acipenseriforms that benefit from the availability of our new phyl ogenetic hypothesis: (1) specializations of jaws and operculum relevan t to jaw protrusion, feeding, and ram ventilation; (2) anadromy or pot amodromy and demersal spawning; (3) paedomorphosis and evolution of th e group; (4) the biogeography of Asian and North American polyodontids and scaphirhynchines; and (5) the great abundance of electroreceptive organs in the rostral and opercular regions. Finally, we summarize ou r nomenclatural recommendations.