''Crossopholis'' is an extinct fish known from the early

Eocene The Eocene ( ) is a geological epoch (geology), epoch that lasted from about 56 to 33.9 million years ago (Ma). It is the second epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), Era. The name ''Eocene'' comes ...

(

Ypresian In the geologic timescale the Ypresian is the oldest age (geology), age or lowest stage (stratigraphy), stratigraphic stage of the Eocene. It spans the time between , is preceded by the Thanetian Age (part of the Paleocene) and is followed by th ...

) of North America, approximately 52 million years ago. It is a close relative of the contemporary

American paddlefish The American paddlefish (''Polyodon spathula''), also known as a Mississippi paddlefish, spoon-billed cat, or spoonbill, is a species of ray-finned fish. It is the last extant taxon, living species of paddlefish (Polyodontidae). This family is ...

, belonging to the

paddlefish Paddlefish (family Polyodontidae) are a family of ray-finned fish belonging to order Acipenseriformes, and one of two living groups of the order alongside sturgeons (Acipenseridae). They are distinguished from other fish by their elongated rost ...

family Polyodontidae.

History of discovery

First described by

Edward Drinker Cope Edward Drinker Cope (July 28, 1840 – April 12, 1897) was an American zoologist, paleontology, paleontologist, comparative anatomy, comparative anatomist, herpetology, herpetologist, and ichthyology, ichthyologist. Born to a wealthy Quaker fam ...

in 1883, the first specimen consisted of an incomplete section of the fish's body and tail. In 1886, a partial skull was recovered by Cope. A nearly complete fossil wasn't recorded until 1980. This was due, in part, to the comparative rarity of the fossil as well as the similarities to other species found within the site.

Classification

A member of the family

Polyodontidae Paddlefish (family Polyodontidae) are a family of ray-finned fish belonging to order Acipenseriformes, and one of two living groups of the order alongside sturgeons (Acipenseridae). They are distinguished from other fish by their elongated rost ...

, ''Crossopholis'' is most closely related to the

. ''Crossopholis'' means "fringed scales"; a reference to the thousands of tiny (less than 0.5mm) scales which covered the body of the animal.

Relationships of recent and fossil paddlefish genera, after Grande ''et al.'' (2002).

Paleobiology

The fossils of this fish are found in the Fossil Lake area of the

Green River Formation The Green River Formation is an Eocene geologic formation that records the sedimentation in a group of intermountain lakes in three basins along the present-day Green River (Colorado River), Green River in Colorado, Wyoming, and Utah. The sedimen ...

. It is more commonly found in shallower deposits of the Thompson Ranch sandwich bed in the Northeast corner of the site than in the deeper midlake sediment deposits. As the species comprises less than .02% of fossils found in the formation, it is probable that ''Crossopholis'' spent much of its life in the connecting rivers that existed to the North. This is further evidenced by that lack of juvenile specimens found in the lake area. ''Crossopholis'' was a predator, with fossil evidence of it consuming small schooling fish such as '' Knightia eocaena''. This is in contrast to the American paddlefish, which primarily consumes

zooplankton Zooplankton are the heterotrophic component of the planktonic community (the " zoo-" prefix comes from ), having to consume other organisms to thrive. Plankton are aquatic organisms that are unable to swim effectively against currents. Consequent ...

. Research has indicated that the

rostrum Rostrum may refer to: * Any kind of a platform for a speaker: **dais **pulpit ** podium * Rostrum (anatomy), a beak, or anatomical structure resembling a beak, as in the mouthparts of many sucking insects * Rostrum (ship), a form of bow on naval ...

was an electro-sensory organ, similar to the function in extant relatives. This allowed it to find prey in poor lighting or murky water. With a maximum recorded length of 1.5m, it is smaller than its modern relatives.

References

Polyodontidae Prehistoric chondrostei Prehistoric ray-finned fish genera Eocene fish of North America Ypresian genera Fossils of the United States Fossils of Wyoming Taxa named by Edward Drinker Cope Fossil taxa described in 1883 {{paleo-chondrostei-stub