Fossil Fish Species

Fossil fish are the most common fossils from the Fossil Butte Member (FBM). So far, 27 species of fish have been discovered in the FBM. The diversity of species and the abundance of fossils, together allow scientists to understand the ecosystem of Fossil Lake.

See what fish fossils are on display in our museum exhibits.
Asterotrygon maloneyi fossil stingray with claspers visible at base of tail. From Green River Formation.
Asterotrygon maloneyi, catalog number FOBU408

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Stingrays - 2 Species Identified: Heliobatis radians & Asterotrygon maloneyi

Order Myliobatiformes

  • Family Heliobatidae - H. radians
  • Family Asterotrygonidae - A. maloneyi
As stingrays have cartilaginous skeletons (lacking bones), it is uncommon to find more than their teeth or stings fossilized. Currently, the FBM is the only place in the world where articulated (complete) freshwater stingray fossils have been discovered. Due to the number of specimens that have been found, from unborn embryos to adults, the life cycles of the ancient species can be studied. One A. maloneyi specimen was found with several baby stingrays scattered around it and another with an embryo inside.

There are almost 200 modern stingray species. Most are bottom-dwellers and feed on bottom-dwelling invertebrates, such as shrimp and crayfish. Stingray fossils are found in association with these invertebrates, indicating their ancient diet was similar to their modern relatives'. Modern freshwater stingrays are found in tropical river systems of South America, Asia, Africa, and Australia.
Heliobatis radians stingray fossil with portion of bottom ray missing and claspers to the left of the tail.
Heliobatis radians, catalog number FOBU17652

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Male stingrays can be identified by the claspers, male reproductive structures, located at the base of the tail.
Large Crossopholis magnicaudatus paddlefish fossil with smaller fossil fish surrounding it and inside its stomach. From Green River Formation.
Crossopholis magnicaudatus, catalog number FOBU7217

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Paddlefish - Crossopholis magnicaudatus

Order Acipenseriformes, Family Polydontidae
C. magnicaudatus is often preserved with fish in the stomach, indicating this species was a predator. This fossil species is most commonly found in shallow-water deposits, but very few juveniles have been discovered. This could indicate that C. magnicaudatus matured and spawned in connecting rivers before migrating to Fossil Lake as an adult.

This species' name, crossopholis, meaning 'fringed scale,' comes from the thousands of tiny, fringed scales covering its body. C. magnicaudatus reaches a maximum of five feet in length.

Both modern paddlefish and C. magnicaudatus' snouts, or paddles, are lined with electroreceptor organs that allow them to navigate in dark or cloudy waters. There is one living species of paddlefish, Polyodon spathula, found only in North America.
Fossil gar with glossy diamond-shaped scales facing left.
Masillosteus janei, catalog number FOBU14016

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Gars - 4 Species Identified: Lepisosteus bemisi, Atractosteus simplex, Atractosteus atrox & Masillosteus janei

Order Lepisosteiformes, Family Lepisosteidae
Seven living gar species are found in freshwaters of North America, Central America, and Cuba. Of the 4 Fossil Lake gar species, all but one are very similar to modern gars. M. janei lacked the long jaw and sharp teeth characteristic of modern gars. M. janei teeth were instead rounded and flat, ideal for crushing small invertebrates like snails and crayfish. This gar species is primarily found in association with these fossil invertebrates.
Atractosteus simplex fossil gar with diamond shape scales covering entire body. From Green River Formation.
Atractosteus simplex, catalog number FOBU788

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Outside of the FBM, gar scales are much more common fossils than complete skeletons. These tough scales offer significant protection from would be predators.

Modern gar species often migrate up rivers to spawn. This is a possible explanation for the lack of juvenile gar fossils found in the FBM.
Lepisosteus bemisi gar fossil curved with nose and tail upwards and covered in diamond scales.
Lepisosteus bemisi, catalog number FOBU13648

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Lepisosteus bemisi has a long snout with needle-like teeth. It has 13-14 tail fin rays as opposed to the 12 found in the other gar species.
Atractosteus atrox gar fossil compressed so it is viewed from its back. Its diamond-shaped scales were preserved.
Atractosteus atrox, University of Wyoming specimen

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Atractosteus atrox is the largest of the GRF gar species reaching up to 6 feet long.

Atractosteus atrox is not on display.
Amia pattersoni bowfin fossil with a black head and dark brown body. From Green River Formation.
Amia pattersoni, catalog number FOBU7218

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Bowfins - 2 Species Identified: Cyclurus gurleyi & Amia pattersoni

Order Amiiformes, Family Amiidae
The genus Cyclurus is not known to have survived past the Eocene Epoch, possibly dying out about 35 million years ago. The closely related Amia genus survives today with the North American A. calva species.

The long jaw and pointed teeth found on A. pattersoni specimens are indicators of a possible apex predator.
A fossil fish with a fan tail and dorsal rays covering the back two thirds of its body. Its body is a light brown and the short head is black.
Cyclurus gurleyi, catalog number FOBU14192

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The crushing jaws found on C. gurleyi specimens suggest an invertebrate diet.
Diplomystus dentatus fossil fish with a deep belly and forked tail. From Green River Formation.
Diplomystus dentatus, catalog number FOBU791

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Ray-finned Fish - Diplomystus dentatus

Order Ellimmichthyiformes, Family Paraclupeidae
D. dentatus is a primitive relative of the modern day herring. The genus Diplomystus is also known from fossil deposits in China. The last known species of the Ellimmichthyiformes order went extinct sometime in the middle Eocene.

D. dentatus is the 2nd most common fossil fish found from Fossil Lake. Specimens ranging from embryonic size (about 0.7 inches) to full-grown adults (about 26 inches) are common. Smaller-sized specimens are more commonly found in mid-lake than near-shore deposits. This suggests that D. dentatus spawned in open water. This species' upturned mouth indicates it fed at the surface of Fossil Lake. D. dentatus specimens are commonly found with other fish stuck in their mouths, including other D. dentatus specimens.
A long, thin orange-brown fish fossil with a forked tail on a rock where the top and bottom edges are visible.
Knightia eocaena, catalog number FOBU13406

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Herrings - 2 Species Identified: Knightia eocaena & Knightia alta

Order Clupeiformes, Family Clupeidae
The Clupeidae family has an estimated 50 modern freshwater species. Clupeidae species can lay as many as 200,000 eggs at once, allowing species to multiply quickly. Modern species are also prone to mass die offs.
K. eocaena is:
  • The most common fish found from Fossil Lake.
  • The most commonly found articulated (complete) vertebrate fossil in the world.
  • The Wyoming state fossil.
Knightia alta fossil fish with a deep belly and forked tail. From Green River Formation.
Knightia alta, catalog number FOBU13444

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Both K. eocaena and K. alta are frequently discovered in mass mortality plates, some containing up to 100 fish per square meter. This suggests that, like their modern relatives, the Knightia species were sensitive to environmental changes.

K. eocaena specimens of all sizes are found in mass mortality plates, indicating this species schooled as both a juvenile and adult. The average adult size of K. eocaena specimens is about 6 inches, though specimens as large as 10 inches have been found.
A long, thin Notogoneus osculus fossil fish with down-turned mouth. From Green River Formation.
Notogoneus osculus, Fossil Butte NM specimen

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Beaked Sandfish - Notogoneus osculus

Order Gonorynchiformes, Family Gonorynchidae
N. osculus was the last survivor of the Gonorynchiformes order in North America. There are 5 modern species in the Gonorynchidae family that inhabit tropical marine regions of the Indo-Pacific Oceans.

Very young juveniles and large adults are the only N. osculus specimens found from Fossil Lake. This could indicate that N. osculus was a migratory species. Their lifecycle may have consisted of: hatching in Fossil Lake, migrating to connecting streams to mature, and returning to Fossil Lake as an adult to spawn and die.

N. osculus had a down-turned, sucker-like mouth. Its toothless mouth suggests that this was a bottom-feeding species. The highly salty water conditions at the bottom of Fossil Lake should have excluded most scavengers and bottom-feeders. Yet, N. osculus is found exclusively in deep-water deposits. This suggests that N. osculus spent most of its life outside of Fossil Lake in connecting rivers and streams.
A long orange-brown fish fossil that tapers significaly to the tail which is forked.
Mioplosus labracoides, BYU Idaho specimen

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Perches - 7 Species Identified: Mioplosus labracoides, Cockerellites liops, Priscacara serrata, Priscacara sp. a, Priscacara sp. b, Hypsiprisca hypsacantha & Hypsiprisca sp.

Order Perciformes

  • Family Latidae - M. labracoides
  • Family Moronidae - C. liops, P. serrata, Priscacara sp. a, Priscacara sp. b, H. hypsacantha, and Hypsiprisca sp.
M. labracoides specimens are characterized by 2 dorsal fins and a forked tail and are known to reach 20 inches. Juvenile M. Labracoides are commonly found in mass mortalities while adults are found alone, indicating it traveled in schools as a juvenile and became solitary as an adult. Juvenile and adult specimens are often found preserved with smaller fish in the jaw or stomach.
Cockerellites liops a round-shaped fossil fish with spiky fins on top. From Green River Formation.
Cockerellites liops, catalog number FOBU428

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C. liops specimens are known to reach a maximum size of 6 inches. C. liops are found as isolated specimens and in mass mortalities, indicating that it traveled in schools throughout its life. Its body shape is similar to members of the modern Centrarchidae family (the sunfish family). When you visit Fossil Butte National Monument, look for an enlarged bronze sculpture of C. liops on our entrance sign!
A football-shaped fossil fish with prominent spines along the back and a fan-shaped tail.
Priscacara serrata, catalog number FOBU14318

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Priscacara serrata had crushing teeth which would allow it to each arthropods. Some specimens have fish bones in their stomach or asphyxiated with fish in their mouths. The maximum known size of P. serrata is 16 inches. It did not school like the smaller, similar-looking C. liops.
Priscacara sp. a has a much rounder body with eleven short dorsal spines.
Priscacara sp. b has four anal spines as opposed to the normal three. For a picture see Lance Grande's The Lost World of Fossil Lake: Snapshots from Deep Time page 168.
An orange-brown fossil fish on speckled tan stone.
Hypsiprisca cf hypsacantha, catalog number FOBU13696

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Hypsiprisca hypsacantha was originally described as Priscacara hypsacantha by Cope.
A brown fossil fish with a fan tail and fine bones. The edge of the rock can be seen at upper right and lower left.
Hypsiprisca sp, catalog number FOBU13413

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Hypsiprisca sp. is more slender than H. hypsacantha and has a more convex tail fin margin. Its maximum known size is 3.5 inches.
Asineops sp. orange fossil fish with splayed tail. From Green River Formation.
Asineops sp., catalog number FOBU13556

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Mystery Fish - 2 species Identified: Asineops squamifrons & Asineops sp.

Order Unknown, Family Asineopidae
A.squamifrons is known as the "mystery fish" as it lacks identifiable traits to place it in an order. It was assigned its own family and has since been joined by Asineops sp. Asineops appears to have gone extinct in the middle Eocene.
A football-shaped dark-brown fossil fish with dorsal and anal fins close to the tail.
Asineops squamifrons, catalog number 13729

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A.squamifrons is characterized by short dorsal spines, rounded tail margin, tiny sharp teeth, and large mouth. It is known from isolated specimens and mass mortalities. It may be a primitive, spiny-rayed fish.
Phareodus encaustus fossil fish with large dorsal and ventral fins near the tail. From Green River Formation.
Phareodus testis, catalog number FOBU17694

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Bony-Tongue Fish - 2 Species Identified: Phareodus encaustus & Phareodus testis

Order Osteoglossiformes, Family Osteoglossidae
Living members of the Osteoglossidae family include 10 species that live exclusively in tropical freshwaters in South America, Africa, Asia, and Australia. Modern Osteoglossidae are mouth-brooders (parents hold eggs and hatchlings in their mouths)
A football-shaped dark brown fossil fish with sharp teeth. Its anal and dorsal fins are close to its forked tail.
Phareodus encaustus, catalog number FOBU790

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Phareodus adult fossils are usually found alone, but juveniles are known from mass mortalities. This indicates that Phareodus schooled as a juvenile and became solitary as an adult. Fossils are often preserved with smaller fish in their jaws and stomach, this as well as their large, sharp teeth indicates they were predators. Their rearward oriented fins on back and underside were an adaptation for speed. The maximum known size of P. encaustus is 30 inches and the maximum known size of P. testis is 20 inches.
A juvenile Amphiplaga brachyptera with a large head and scales visible. From Green River Formation.
Amphiplaga brachyptera, catalog number FOBU11600

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Trout-Perch - Amphiplaga brachyptera

Order Percopsiformes, Family Percopsidae
The Percopsidae family has one modern genus with 2 species. They are restricted to freshwater rivers and lakes of North America.

A. brachyptera is the earliest known member of the Percopsidae family, and is known exclusively from Fossil Lake deposits. Larval-size A. brachyptera specimens are almost completely absent from the FBM. This suggests that A. brachyptera spawned in rivers. A. brachyptera is also never found in mass mortality plates, indicating this species was solitary and not a schooling fish.
Hiodon falcatus fish fossil with a small black head and a forked tail. From Green River Formation.
Hiodon falcatus, catalog number FOBU13573

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Mooneye - Hiodon falcatus

Order Hiodontiformes, Family Hiodontidae
There are 2 living species of the Hiodon genus that are found in freshwater in North America. They reach about 20 inches in length and feed primarily on insects.

Fossil specimens of the Hiodon genus are known from both North America and Asia. H. falcatus is almost exclusively found in near-shore sediment from Fossil Lake. Most specimens are between 4 and 6 inches long, though some are as large as 10 inches.
A long, thin red-brown fish fossil with a forked tail and a darker head.
Esox kronneri, Field Museum of Natural History specimen

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Pikes and Pickerels - Esox kronneri

Order Exociformes, Family Esocidae
Only a single specimen of Esox kronneri has been found. It may have lived in tributary streams, making it uncommon in lake sediments.

The family Esocidae is a group of freshwater fishing known only to live in the Northern Hemisphere. There are seven species of Esox living today in North America, Europe, and Asia in cooler temperate regions. Pikes and pickerals are primarily piscavores but will occasionally eat other vertebrates and invetebrates.

This species is not on display.


Grande, Lance. 2013. The Lost World of Fossil Lake: Snapshots from Deep Time. Chicago: The University of Chicago Press.

Ross, Stephen T., Bernard R. Kuhajda, Anthony A. Echelle, Joseph R. Tomelleri, Brooks M. Burr, and Melvin L. Warren, Jr. "Esociformes: Esocidae, Pikes, and Umbridae (Mudminnows)." In Freshwater Fishes of North America: Volume 2: Characidae to Poeciliidae, 193-260. Baltimore: Johns Hopkins University Press, 2020.,

Last updated: February 29, 2024

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