| Summary: | The Acipenseridae comprises the largest group of non-teleostean actinopterygians and has served a central role in evolutionary studies of the Actinopterygii, and yet no comprehensive study of morphology and systematics of the family has ever been made. The family includes twenty-four species within four traditionally recognized genera, but the traditional generic designations lack rigorous definition within a cladistic scheme and are confounded by plesiomorphic characters. As background, revised diagnoses, synonymy lists, and overviews of life history are provided for every acipenserid species and genus. The systematics of the Acipenseridae are addressed with emphasis on generic-level relationships based on characters from examinations of skeletal morphology. A complete description of the skeleton of the shovelnose sturgeon Scaphirhynchus platorynchus provides a template of the skeleton of acipenserids. Skeletal comparisons with representatives of the remaining three acipenserid genera (Huso, Acipenser, and Pseudoscaphirhynchus), as well as Polyodontidae, $\dagger$Chondrosteidae, $\dagger$Peipiaosteidae, and other actinopterygians were used to identify, define, and test putative synapomorphies. Ninety-six synapomorphies are identified defining all major nodes within Acipenseriformes. A novel phylogeny of Acipenseridae is proposed, with Huso recognized as the sister group to a newly redefined subfamily Acipenserinae comprising all other acipenserids. The shovelnose sturgeons (genera Scaphirhynchus and Pseudoscaphirhynchus) compose a monophyletic Scaphirhynchini. No synapomorphies are recognized defining Acipenser, leaving the genus as a potentially untenable assemblage within Acipenseridae. The proposed phylogeny suggests that evolution within Acipenseridae followed a markedly different course than typically assumed, with progressive increase in morphological specializations for benthic life. Huso and polyodontids (as immediate outgroups) define an outgroup morphology and life history founded on mid-water habitats and piscivory. In contrast, most acipenserines, and more markedly in scaphirhynchines, are more benthic as exemplified by their ventral jaws and substrate-oriented diets. This phylogeny further suggests that peramorphosis, not paedomorphosis, played a central role in acipenserid evolution, exemplified by progressive acquisition of novel bones, increased scalation, and stronger dermal ornament. While putative paedomorphic characters are coincident with these peramorphic characters, addition of characters through peramorphosis belies the historical idea that paedomorphic or degenerate features dominate acipenserid evolution.
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