ACIPENSERIFORMES

The order Acipenseriformes is formed by four families of fishes, including the ?Chondrosteidae (three
species in three genera from the Jurassic of Europe), the ?Peipiaosteidae (four species in two genera from
the Jurassic and Cretaceous of Asia), the sturgeons in the family Acipenseridae, and the paddlefishes of family Polyodontidae.

Acipenser ruthenus (cleared and stained), juvenile (Courtesy P. Bartsch)

These fishes are believed to have split off from other ray-finned fishes around 150 million years ago, giving the living members their reputation as "living fossils." Acipenseriform fishes are widely considered to be the closest living relatives of gars, bowfins, and teleosts * a group that forms over one-half of all living vertebrate species.

Acipenser brevirostrum (cleared and stained) (Courtesy E. Hilton)

Compared to the 25 living species of sturgeons, which are classified in four genera (Scaphirhynchus, Pseudoscaphirhynchus, Acipenser, and Huso), the fossil record of sturgeons is quite poor. Most sturgeon fossils are fragmentary pieces of bony scutes or pectoral fin spines, and cannot be diagnosed as a particular species. The monotypic genus Protoscaphirhynchus from the Late Cretaceous of Montana was based on a very poorly preserved specimen that included parts of the skull, pectoral girdle, and body. In 2006, Grande and Hilton described the first well preserved fossil sturgeon, Psammorhynchus longipinnis, from the Late Cretaceous of Montana, recovered from inside a hadrosaur dinosaur. This specimen was nearly complete and the head and pectoral girdles were preserved in three dimensions.

Acipenser ruthenus (cleared and stained), juvenile (Courtesy P. Bartsch)

The fossil record of paddlefishes is much more extensive than that of sturgeons, with three genera (?Protopsephurus, Jurassic of Asia; ?Paleopsephurus, Cretaceous of Montana; and *?Crossopholis, Eocene of Wyoming), and a fossil
species of the genus Polyodon (?P. tuberculata, Paleocene of Montana).