Fig. 14. Xiphotrygon, a fossil ray. These forms are present but rather rare in the fish deposits of Fossil Butte. Length of original, 39 cm. Collection of University of Wyoming.
One of the strangest forms to come from Fossil Butte is that of Xiphotrygon ([Fig. 14]). This is not a true fish, but is a sting ray, related to sharks. Closely related forms still survive, with little change, in coastal waters in many parts of the world. Like sharks, sting rays have a skeleton composed of cartilage. Normally, cartilage is not preserved as a fossil as it disintegrates readily. The excellent skeletons of Xiphotrygon are good evidence of how well the shales of the Green River Formation preserve the fossils. Xiphotrygon is characterized by being shortened from back to front and flattened from top to bottom. It possesses a long, whip-like tail and bears enormous, flat pectoral (anterior) fins. The mouth is on the underside of the body indicating that Xiphotrygon, like its modern relatives, fed along the bottom of the lake. The strong flattened teeth form something like a pavement in the mouth, suggesting that it fed on clams and other hard-shelled invertebrates. Close relatives of Xiphotrygon live mostly in marine waters, but occasionally enter fresh water.
The paddlefish, Polyodon, is rare but present at Fossil Butte. Similar forms still survive in China and the United States. The prominent features of Polyodon include a virtually scaleless body, a long, depressed snout, and long gill rakers. The skeleton, unlike that of most modern fish, is composed of cartilage, hence skeletons of Polyodon are rare. The difference between the modern and Eocene paddlefish are so slight that the two probably filled similar or the same ecologic niches. This consists of feeding on plankton and other small organisms which can be obtained from the water. The long gill rakers would provide a large surface area on which food could be trapped as the water passed out through the gill slits. The large mouth would also provide a large surface area for catching and trapping food.
Quite common to streams and rivers of North America from the Cretaceous to the Recent is the garfish, Lepisosteus ([Fig. 15]). The most striking feature of the gar is its diamond-shaped scales, which are extremely hard and shiny. The body is long, essentially of the same depth throughout, and ending in a broad-based tail that is slightly asymmetrical. The mouth is armed with many small, sharp teeth for catching prey. One specimen over 5 ft long has been found in the Fossil Butte area.
Fig. 15. The fossil gar Lepisosteus. Only a few gar have been found at Fossil Butte itself. Length of original, 70 cm. Across the highway from the butte, one specimen over 5 ft long was found by Carl Ulrich. Collection of University of Wyoming.
Fossils of the bowfin Amia ([Fig. 16]) are recorded from the Green River shales at Fossil Butte and in stream deposits from Cretaceous to Recent. This fish is still another “living fossil.” Amia is a nocturnal predator. Presumably, the Eocene Amia had similar habits. The body in Amia has become elongate, as has the dorsal fin. The thickness of the scales has been reduced, but they are still rather heavy and cover the body. Specimens are rather large and deep-bodied. The tail fin is nearly symmetrical above and below. Amia is rare in the Fossil Butte fish quarries.
By far the most abundant fish from Fossil Butte is the genus Knightia (Fig. [17], [18]). This fish is a member of the Clupeidae, a family that includes modern herrings, shad, and sardines. The tail fin is small relative to the rest of the body. A row of modified scales extends on the back from the skull to the dorsal fin. Possibly they aided in streamlining the fish. Knightia apparently fed on the large amounts of plankton, especially algae, which lived in the waters of Fossil Lake. Fossils of Knightia are also known from Eocene lake sediments in South America.
Knightia appears to have been susceptible to mass mortalities. Some layers of shale at Fossil Butte contain literally hundreds of thousands of these fish that must have suffered catastrophic mass mortality. Possible reasons for the obvious sudden death of so many fish will be discussed in the section on paleoenvironment and taphonomy.
Another Eocene relative of the herring found at Fossil Butte is Diplomystus (Fig. [19], [20]). Like Knightia, Diplomystus bears a row of modified scales on its back. Diplomystus is the second most abundant fish at Fossil Butte. The jaw in Diplomystus has a rather pronounced oblique angle to it. The deepest portion of the body is directly behind the gill region, with a continuous narrowing of the body toward the tail region. Diplomystus is known from Cretaceous rocks of Brazil and Syria and from Tertiary sediments in Brazil and West Africa (Schaeffer and Mangus 1965). Close relatives of Diplomystus now inhabit the waters off the coasts of Peru and eastern Australia.