The
American paddlefish (
Polyodon spathula) is a species of
basal ray-finned fish closely related to
sturgeons in the
order Acipenseriformes. Fossil records of
paddlefish date back over 300million years, nearly 50million years before dinosaurs first appeared. American paddlefish are smooth-skinned freshwater fish commonly called paddlefish, but are also referred to as Mississippi paddlefish, spoon-billed cats, or spoonbills. They are one of only two
extant species in the paddlefish family,
Polyodontidae. The other is the critically endangered
Chinese paddlefish (
Psephurus gladius)
endemic to the
Yangtze River basin in
China. American paddlefish are often referred to as
primitive fish, or
relict species because they retain some morphological characteristics of their early ancestors, including a skeleton that is almost entirely
cartilaginous, a paddle-shaped
rostrum (snout) that extends nearly one-third their body length, and a
heterocercal tail or
caudal fin, much like that of
sharks. American paddlefish are a highly
derived fish because they have evolved with adaptations such as filter feeding. Their rostrum and cranium are covered with tens of thousands of sensory receptors for locating swarms of
zooplankton, which is their primary food source.