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''Crossopholis'' is an extinct bony fish known from the early
Eocene The Eocene ( ) Epoch is a geological epoch that lasted from about 56 to 33.9 million years ago (mya). It is the second epoch of the Paleogene Period in the modern Cenozoic Era. The name ''Eocene'' comes from the Ancient Greek (''ēṓs'', ...
(
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 only living species of paddlefish (Polyodontidae). This family is most closely r ...
, 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 titular elongla ...
family.


History of discovery

First described by
Edward Drinker Cope Edward Drinker Cope (July 28, 1840 – April 12, 1897) was an American zoologist, paleontologist, comparative anatomist, herpetologist, and ichthyologist. Born to a wealthy Quaker family, Cope distinguished himself as a child prodigy interested ...
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 titular elongla ...
, ''Crossopholis'' is most closely related to the
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 only living species of paddlefish (Polyodontidae). This family is most closely r ...
. ''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 Fossil Butte National Monument is a United States National Monument managed by the National Park Service, located west of Kemmerer, Wyoming, United States. It centers on an assemblage of Eocene Epoch (56 to 34 million years ago) animal and pla ...
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 in Colorado, Wyoming, and Utah. The sediments are deposited in very fine ...
. 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 ''Knightia'' is an extinct genus of clupeid bony fish that lived in the freshwater lakes and rivers of North America and Asia during the Eocene epoch. The genus was erected by David Starr Jordan in 1907, in honor of the late University of ...
''. This is in contrast to the American paddlefish, which primarily consumes
zooplankton Zooplankton are the animal component of the planktonic community ("zoo" comes from the Greek word for ''animal''). Plankton are aquatic organisms that are unable to swim effectively against currents, and consequently drift or are carried along by ...
. Research has indicated that the
rostrum Rostrum may refer to: * Any kind of a platform for a speaker: **dais **pulpit * 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 ships * Ros ...
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 ray-finned fish genera Eocene fish of North America Taxa named by Edward Drinker Cope Fossil taxa described in 1883 {{paleo-chondrostei-stub