The Fossil in the lobby

Fossilized saw palmetto palm frond with a “school” of fossilized fish on a limestone matrix, Kemmerer, Wyoming, 52 million years old (Eocene). Green River Formation.

Fossils

Priscacara liops, genus Priscacara. A member of the family Priscacaridae. The name Priscacara means “primitive head.” Shaped somewhat like a sunfish, the genus has sturdy, protective dorsal and anal spines.

Knightia is an extinct genus of clupeid clupeiform bony fish that lived in the freshwater lakes and rivers of North America and Asia during the Eocene epoch. In Knightia fish, rows of dorsal and ventral scutes run from the back of the head to the medial fins. They had heavy scales and small conical teeth. Knightia is the state fossil of Wyoming.

Serenoa repens, commonly known as saw palmetto, is the sole species currently classified in the genus Serenoa. It is a small palm that grows to a maximum height of around 7–10 feet (2–3 m). It is endemic to the subtropical southeastern United States, most commonly along the south Atlantic and Gulf coastal plains and sand hills. It grows in clumps or dense thickets in sandy coastal areas and as undergrowth in pine woods or hardwood hammocks.

Geology

The Green River Formation is an Eocene geologic formation that records sedimentation in a group of intermountain lakes in three basins along the present-day Green River in Colorado, Wyoming, and Utah. The sediments were deposited in very fine layers: a dark layer during the growing season and a lighter inorganic layer during the dry season. Each pair of layers is called a varve and represents one year.

The sediments of the Green River Formation present a continuous record of six million years. The mean thickness of a varve here is 0.18 mm, with a minimum thickness of 0.014 mm and a maximum of 9.8 mm.

[https://en.wikipedia.org/wiki/Green_River_Formation#cite_note-brad-1]

The sedimentary layers were formed in a large area named for the Green River, a tributary of the Colorado River. The fossil beds of the Green River Formation span a five-million-year period, dating to between 53.5 and 48.5 million years ago. This span includes the transition between the moist early Eocene climate and the slightly drier mid-Eocene.

The climate was moist and mild enough to support crocodiles, which do not tolerate frost, and the lakes were surrounded by sycamore forests. As lake configurations shifted, each Green River location developed distinct characteristics over time. The lake system formed over underlying river deltas and shifted in the flat landscape with slight tectonic movements, receiving sediments from the Uinta Highlands and the Rocky Mountains to the east and north.

The fossil deposits formed in anoxic conditions in fine carbonate muds that accumulated in the lakebeds. The lack of oxygen slowed bacterial decomposition and kept scavengers away. As a result, leaves of palms, ferns, and sycamores—some showing insect damage sustained during growth—were covered with fine-grained sediment and preserved. Insects were preserved whole, even delicate wing membranes and spider spinnerets.

Knightia and Priscacara liops, genus Priscacara, pictured in the images below.

The paleoecology of Fossil Lake: what was life like 52 million years ago in southwestern Wyoming?

The fossils in the FBM represent a collage of 52-million-year-old “snapshots” documenting a biologically diverse freshwater lake surrounded by subtropical lowlands and more distant temperate uplands. The plants and animals were transitional species between the age of the dinosaurs and modern ecosystems.

The early Eocene included the warmest global temperatures of the Cenozoic era (Wilf 2000). The warm, humid climate of Fossil Lake was similar to the present climate of the Gulf Coast and southern Atlantic regions of the United States, with annual rainfall of 30 to 40 inches and nearly frostless winters (Bradley 1929, 1948; MacGinitie 1969). This ecosystem contrasts sharply with the cool mountain-desert climate that exists in the region today.

There were active volcanoes about 120 miles north of the lake that produced several major eruptions during the lake’s existence. These eruptions left thin ash layers within the FBM. Occasionally, eruptions caused massive forest fires and catastrophic mass kills in the lake, evidenced by layers heavily blackened with carbon and charcoal occurring just above or below fish mass-mortality layers.

A natural balance of prey and predator existed in the lake for millennia as part of a complex food chain. Algae such as Pediastrum sp. were the primary producers that converted sunlight and carbon dioxide into sugars and served as the primary food source of the lake ecosystem.

Primary consumers included vast schools of the filter-feeding fish Knightia eocaena. This species existed in the lake in enormous numbers and fed on algae and other microorganisms. It was the most common fish species in the lake and reproduced in great numbers, providing one of the most abundant food sources for higher levels of the food chain.

Secondary consumers included predaceous fishes such as Crossopholis, Lepisosteus, Amia, Atractosteus, Phareodus, Diplomystus, Priscacara, and Mioplosus, as well as frigatebirds, bats, trionychid turtles, aquatic lizards, small crocodilians, the otter-like pantolestid, and many other animals. Knightia was the primary link in the food chain supporting this level of the ecosystem.

At the top were tertiary consumers such as large crocodiles, alligators, monitor lizards, and giant trionychid turtles that preyed on secondary consumers.

On shore, the primary producers were land plants and shallow-water vegetation that were fed upon by herbivores ranging from insects to the small horse Protorohippus venticolus. Insects were in turn eaten by secondary consumers such as birds, bats, fishes, and insectivorous mammals. Protorohippus may occasionally have fallen prey to large crocodiles in the lake.

Near the shore were aquatic plants such as water lilies, floating ferns, ceratophyllum, and many kinds of swimming insect larvae and nymphs. Along the shoreline grew cattails, horsetail, elephant ear plants, ferns, sumac, balloon vines, and palms. An abundance of very large, well-preserved palm fronds in the FBM indicates that large groves of palm trees likely grew close to the water’s edge.

Dragonflies and damselflies filled the air along the shoreline while clouds of march flies and small biting gnats swarmed over the water. The nearshore aquatic plants provided nursery grounds for many schools of young fish.

The lake was filled by water flowing down from the uplands, including a major river to the northeast. As a result, some river inhabitants occasionally appeared in the northern nearshore FBM quarries, including the pickerel Esox kronneri, the Eocene mooneye Hiodon falcatus, the trout-perch Amphiplaga brachyptera, clams, the otter-like Palaeosinopa, as well as freshwater shrimp and crayfish.

The river also carried cooler-climate vegetation into the lake. Plants from the distant highlands—including conifers, plane trees, and sweetgum—shed their leaves and fruits into streams and rivers flowing down the mountains.

Around the lake were small groups of the three-toed “dawn horse,” Protorohippus venticolus, which grazed on vegetation. The palms and other trees were filled with a diverse array of birds, insects, and small carnivorous mammals.

Large mammalian carnivores of the order Carnivora had not yet evolved. Mammals occupying that ecological niche were very different from those of today. One such animal was a small carnivorous tree-climbing mammal that had developed a prehensile tail, allowing it an agile life in the trees. It moved through the canopy with its long tail and graceful limbs, feeding on small animals.

The top predators of the time continued to be reptiles such as large crocodiles and monitor lizards, as well as the giant two-meter-tall flightless bird Gastornis. The daytime skies contained frigatebirds that swooped down to feed on fish and other small vertebrates in the lake. Rails, rollers, and birds resembling a cross between ducks and flamingos were common shorebirds, and many species of parrots inhabited the surrounding trees.

It is surprising today to think of Wyoming as a hotspot for parrot diversity.

Colonies of bats lived near the north end of the lake, where most of the approximately 35 fossil bat specimens have been discovered. Today, bats use echolocation to navigate and locate food in the dark. One species that lived near Fossil Lake, Onychonycteris finneyi, appears to have lacked echolocation and may have been active during the day, inhabiting nearshore trees.

From palm trees to crocodiles, the FBM fossils provide a quasi-photographic record of a lost world. Rather than isolated plant and animal remains, they preserve a complex and vibrant ecosystem locked in stone.