Sturgeon and paddlefish (Order Acipenseriformes) make up the largest group of ‘living fossil’ fishes with 27 extant species. But despite this large number of living species and a fossil record that dates back 200 million years, defining the phylogenetic relationships of these fishes has proven challenging. The family-level split between Acipenseridae (sturgeons) and Polydontidae (paddlefishes) is relatively simple and non-controversial. Polydontidae is further divided into the genera of Polydon (North American paddlefish) and Psephurus (Chinese paddlefish) and is equally non-controversial. However, arriving at a consensus for genera within Acipenseridae remains elusive. Reasons for this are the large amount of ontogenetic and morphologic variation, hybridization, and extirpations that effectively blurred the evolutionary relationships. Early attempts were based on morphologic characteristics and a working hypothesis was derived that divided Acipenseridae into four genera: Huso, Acipenser, Scaphirhynchus, and Pseudoscaphirhynchus. Karyological and molecular data has since been used to propose that both Huso species actually belong within Acipenser, bringing the number of genera within Acipenseridae to three. But more recently, osteological evidence has been provided that refutes this re-grouping and supports the original four-genera schema by placing Huso outside Acipenser as a sister taxon to all other sturgeons. More research is needed before Acipensersid phylogenetic relationships can be definitively characterized.
For more in-depth reading, see:
Findeis, E. K. 1997. Osteology and phylogenetic interrelationships of sturgeons (Acipenseridae). Environmental Biology of Fishes 48: 73-126.
Bemis, W. E. and B. Kynard. 1997. Sturgeon rivers: an introduction to Acipenseriform biogeography and life history. Environmental Biology of Fishes 48: 167-183.
Birstein, V. J. and R. DeSalle. 1998. Molecular phylogeny of Acipenserinae. Molecular Phylogenetics and Evolution 9(1): 141-155.