Cranial morphology of the Stellate Sturgeon, Acipenser stellatus Pallas 1771 (Acipenseriformes, Acipenseridae), with notes on the skulls of other sturgeons

Extant members of Acipenseridae are generally classified in four genera: Scaphirhynchus, Pseudoscaphirhynchus, Huso and “Acipenser,” which is widely recognized to be paraphyletic. Advances have been made in understanding the systematic relationships among sturgeons based on both morphological and molecular data. Analysis of mitochondrial DNA data suggested that Pseudoscaphirhynchus should be regarded as nested within “Acipenser,” specifically as sister group to the Stellate Sturgeon, A. stellatus. Recent morphological analyses also recovered this relationship, supported by a number of osteological synapomorphies, although these results were based on few and relatively small individuals. Here we describe the anatomy of the skull of A. stellatus based on newly prepared specimens of adult individuals, as well as examination of a large number of preserved individuals representing a broad range of ontogenetic stages. We present new anatomical data from all regions of the skull (dermatocranium, neurocranium, viscerocranium) and offer interpretations of these and other characters. In particular, we describe the allometry in the snout of A. stellatus, which undergoes substantial elongation relative to other sturgeons. Aspects of the skull of A. stellatus are compared to other members of the family, specifically the course of the occipital sensory canal and the morphology and distribution of cranial spines.