Part 8

Except for these insects preserved as whole "mummies" in the oil shales, the majority of the insect fossils are preserved as distilled outlines. This distillation process resulted from the weight and heat of overlying sediments of driving off the volatile substances from the buried insects, leaving a hydrocarbon outline. The process is so precise that the fine hairs and wing veins and even body-color markings are preserved.

Vertebrates

FISH.

The single factor that has made Fossil Butte world famous is the occurrence of literally millions of beautifully preserved fossil fish. In one layer approximately 14 inches thick, the fish are nearly black and preserved in a nearly white shale. Most of the fish are perfectly preserved, retaining every detail of the skeleton and even undisturbed scales. Fish from Fossil Butte have been collected and sold since the 1870s and may be found on sales counters, in museums, and on living room walls throughout the world.

The fossil fish from the Green River Formation have a history of discovery stretching back to 1856. At that time Leidy (1856:256) described a fish given to him by a Dr. John Evans. Leidy called the fish Clupea humilis, a type of herring. Exactly where Dr. Evans obtained the specimen is not known. He passed through the area of the Green River Formation several times and could have collected it during any one of those trips (Knight 1955:12).

During the construction of the Union Pacific Railroad in the 1860s, workmen blasting a cut through Green River shales, about 2 miles west of Green River, Wyoming, came upon remains of well-preserved fish. Hayden (1871:742) first mentions this cut as "Petrified Fish Cut." Many of the fish from this cut were given to Cope who described them in Hayden’s (1871) report. Insects, plants, and a bird feather were also obtained from "Petrified Fish Cut" at this time.

Sometime in the 1870s the first fish were obtained from Fossil Butte itself. Cope (1877, 1878) described fish from the Green River Formation which may be from Fossil Butte. His locality is "nearer the mainline of Wasatch Mountains" (Cope 1877:807). This may be Fossil Butte. His 1878 locality information is just as poor. He does mention a specimen of Priscacara peali given to him by A. C. Peale. Since it was Peale who first wrote about Fossil Butte, the specimen may have come from the butte.

Peale (1879) first mentioned the quarrying of fish at Fossil Butte, but he had no specific date as to the discovery of the butte or the beginning of quarrying of fish.

Cope (1884) published two large volumes about the Tertiary vertebrates of the West. Here he mentioned three fossil localities for his Green River Fish: "Petrified Fish Cut," "The Mouth of Labarge Creek," and "Twin Creek." Twin Creek is now known as the Fossil Butte site.

Since that time, the quarries at Fossil Butte have been extensively worked, mostly by commercial collectors.

The fish from Fossil Butte form an unusual array of genera (Table 1) unlike any now found living together. Included in the assemblage are forms usually found in marine waters and tropical, fresh-water fish (Schaeffer and Mangus 1965).

TABLE 1. Simplified classification of fish from Fossil Basin.

Elasmobranchii Trygonidae Xiphotrygon (sting ray) Chondrostei Polyodontidae Polyodon (paddle fish) Holostei Lepisosteidae Lepisosteus (gar) Amiidae Amia (bowfin) Isospondyli Osteoglossidae Phareodus Gonorhynchidae Notogoneus (sand fish) Clupeidae Diplomystus (herrings, shad) Knightia Ostariophysi Siluridae Ameiurus (catfish) Xenarchi Aphredoderidae (pirate perches) Erismatopterus Asineopidae Asineops Acanthoptergii Percidae Mioplosus (perches) Serronidae Priscacara

More than 40 species of fish have been described from the Green River shales, but authorities do not agree on how many of these are valid. Because of this, only super specific groups will be considered in the following discussion.

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.

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.

Phareodus (Fig. 21) belongs to a family of fish, the Osteoglossidae, which began as marine forms and has since become a fresh-water family. At present, they are restricted to tropical rivers and lakes of South America, Africa, and Australia. In the past they had a much greater distribution, as its occurrence at Fossil Butte indicates, including North America. It is a deep-bodied fish with a large head. The anal and dorsal fins are close to the tail fins. The many sharp teeth in the mouth attest to the carnivorous habits of Phareodus.

The family Gonorhynchidae, the living sandfish of the Indo-Pacific area, is represented at Fossil Butte by Notogoneus (Fig. 22). Notogoneus is very long-bodied and quite slender. The body width does not vary much from head to tail, although narrowing does occur in the tail region. The tail fin is symmetrical, forming a good rudder. The anal, dorsal, pectoral, and pelvic fins are all small. The body is covered with small, elongate scales. Notogoneus was apparently a bottom feeder, living on small organisms picked off or out of the bottom of Fossil Lake.

The Recent catfish, Ameuirus, has also been found at Fossil Butte in the Green River shales. The form of the Eocene catfish was very much like the Recent one. The habits of the Eocene catfish were probably like those of the modern Ameuirus, i.e., adaptations to a scavenger-type existence.

Rather common in the sediments of Lake Gosiute, but not yet known from the Fossil Basin, is Erismatopterus, a member of the family Aphredederidae or pirate perches. This small, elongate fish has few distinguishing characters, the rounded front edge of the skull being most distinguishable.

Asineops is an Eocene relative of Erismatopterus and also one of the pirate perches. Asineops is, however, placed in a different family, the Asineopidae. Asineops, like Erismatopterus, is a rather plain-looking fish. The dorsal fin is long relative to the rest of the body. The body is slightly deeper behind the gills than elsewhere, giving Asineops a rather common appearance.

The Percidae, or perches, are represented at Fossil Butte by Mioplosus (Fig. 23). Mioplosus, as indicated by its well-developed teeth, was a carnivore that probably preyed on its piscine relatives in Fossil Lake. The large head blends in well with a strongly built, long body. The anal and dorsal fins are subequal in size and positioned opposite each other. The tail fin is large and fan-shaped. It is easily identified by the presence of two dorsal fins.

The one genus for which no close living relatives can be found is Priscacara (Fig. 24). The family Priscacaridae is thought to be related to the bass. The family is known only from the Eocene. The size of Priscacara is extremely variable. The body is deep and nearly round like that of the common sunfish, the head blending in well with the body and its contours. The most distinguishing feature of Priscacara is the series of strong, stout spines supporting the anal and dorsal fins. These spines may have acted as cut-waters for better swimming and/or they may have protected Priscacara from its more voracious relatives.