Skeletal Anatomy of the Shortnose Sturgeon, Acipenser brevirostrum Lesueur, 1818, and the Systematics of Sturgeons (Acipenseriformes, Acipenseridae)

Sturgeons of the family Acipenseridae comprise 25 extant species, making it the most species-rich extant family of basal (i.e., nonteleostean) actinopterygians. Because of their basal position within Actinopterygii, the anatomical study of sturgeons has a long and rich history, although there remains much to be discovered. Here we describe and illustrate the skeletal anatomy of the shortnose sturgeon, Acipenser brevirostrum, as a representative of the family Acipenseridae. Acipenser brevirostrum, which is distributed along the east coast of North America, is a relatively small species of sturgeon, reaching a maximum of just over 1 m in total length. Our study is based on 105 skeletal and 147 alcohol-stored specimens representing a broad range of ontogenetic stages (ca. 18 to 1000+ mm TL). This study emphasizes the bony portions of the skeleton, their ontogeny, and parts of the skeleton that persist as cartilaginous elements into the adult stage; the earliest stages of development of the chondrocranium, however, will be the subject of a future project. In the present study, we intend to provide baseline data for future comprehensive ontogenetic and morphological studies of Acipenseridae. Although the Acipenseriformes are extremely morphologically derived compared to most basal actinopterygians, these data will be useful in broader systematics studies of basal Actinopterygii generally. Based on previous studies of acipenseriform phylogeny and using new anatomical data collected in this study, we studied the phylogenetic relationships of fossil and living acipenserids. The monophyly of Acipenseriformes is supported by two synapomorphies: palatoquadrates with symphysis between pars autopalatina and the absence of premaxillae and maxillae. Contrary to other recent studies, we recover †Chondrosteus as most basal among Acipenseriformes rather than †Peipiaosteus. †Peipiaosteus + Acipenseroidei is supported by the presence of fewer than seven but more than one branchiostegals, the posterior margin of the branchiostegals serrated, the absence of ossified basibranchials, and the absence of teeth on the gill rakers. Acipenseroidei ( = Polyodontidae + Acipenseridae) is supported by one synapomorphy (presence of ventral rostral bones). Monophyly of both Polyodontidae and Acipenseridae is well supported. The family Polyodontidae (represented in our analysis by †Protopsephurus and Polyodon) is supported by the presence of well-developed anterior and posterior divisions o