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Fossilized Saw Palmetto palm frond w “school” of fossilized fish on 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 rather like a sunfish, the genus sports sturdy, protective dorsal and anal spines.

Knightia is an extinct genus of clupeid clupeiform bony fish that lived in the fresh water 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. 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, growing to a maximum height around 7–10 ft (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 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 layers, a dark layer during the growing season and a light-hue inorganic layer in 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 maximum of 9.8 mm.[1]<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 various fossil beds of the Green River Formation span a 5 million year period, dating to between 53.5 and 48.5 million years old. This span of time 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 the lake configurations shifted, each Green River location is distinct in character and time. The lake system formed over underlying river deltas and shifted in the flat landscape with slight tectonic movements, receiving sediments from the Uinta highland and the Rocky Mountains to the east and north. The lagerstätten formed in anoxic conditions in the fine carbonate muds that formed in the lakebeds. Lack of oxygen slowed bacterial decomposition and kept scavengers away, so leaves of palms, ferns and sycamores, some showing the insect damage they had sustained during their growth, were covered with fine-grained sediment and preserved. Insects were preserved whole, even delicate wing membranes and spider spinnerets.

"Green River Formation." Wikipedia. Wikimedia Foundation, 07 June 2017. Web. 14 June 2017. <https://en.wikipedia.org/wiki/Green_River_Formation>.

Knightia and Priscacara liops Genus Priscacara pictured below.


The Paleoecology of Fossil Lake: What Was Life Like 52 Million Years Ago in Southwestern Wyoming?

The fossils in the FBM make up a collage of 52-million-year-old "snapshots" documenting a biologically diverse freshwater lake surrounded by subtropical lowlands and more distant temperate upland regions. The plants and animals were transitional species between the age of the dinosaurs and modern day. The early Eocene included the warmest global temperatures of the Cenozoic era (Wilf 2000); and 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 an annual rainfall of 30 to 40 inches and nearly frostless winters Bradley 1929, 1948; MacGinitie 1969). That ecosystem and ecology contrast sharply with the cool mountain-desert climate that exists in the region today (fig. 213). There were active volcanoes about 120 miles north of the lake that had several major eruptions during the lake's existence and left thin ash layers within the FBM. Occasionally these eruptions caused massive forest fires and catastrophic mass kills in the lake, as evidenced by occasional beds that are 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 sugar and served as the primaly food source of the lake. PRIMARY CONSUMERS included vast schools of the filter-feeding fish †Knightia eocaena. This species existed in the lake by the billions and fed on primary producers such as algae and other microorganisms.

It was the most common of all the fish species in the lake, and it reproduced in great numbers, providing one of the most abundant food sources for higher links in the food chain. SECONDARY CONSUMERS included predaceous fishes (†Crossopholis, Lepisosteus, Amia, Atractosteus, †Phareodus, †Diplomystus, †Priscacara, and †Mioplosus), as well as frigatebirds, bats, trionychid turtles, aquatic lizards, small crocodilians, the otter-like tpantolestid, and many other animals. "†Knightia was the primary link in the food chain that fueled this level of the ecosystem. And at the top there were TERTIARY CONSUMERS such as the large crocodiles, alligators, monitor lizards, and giant trionychid turtles that preyed on the secondary consumers. On shore the primary producers were land plants and shallow-water plants that were fed upon by herbivores ranging from insects to the small horse †Protorohippus venticolus. Insects were, in turn, fed upon by secondary consumers like birds, bats, fishes, and insectivorous mammals; and perhaps †Protorohippus occasionally fell prey to the large crocodiles in the lake.

Near the shore there were aquatic organisms such as water lilies, floating ferns, ceratophyllum, and many kinds of swimming insect larvae and nymphs. Along the shoreline were cattails, horsetail, elephant ear plants, ferns, sumac, balloon vines, and palms. An abundance of very large well-preserved palm fronds in the FBM indicate that large groves of palm trees must have grown 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 helped provide nursery grounds for the many schools of baby fishes that dwelt there. The lake was filled by water flowing down from the uplands, including a major river to the northeast. This resulted in some river inhabitants showing up in the northern nearshore FBM quarries from time to time, like the pickerel †Esox kronneri, the Eocene mooneye †Hiodonfalcatus, 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. The more temperate, cooler-climate plants in the distant highlands—including conifer, 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. The large true carnivores of today of the mammalian order Carnivora had not yet evolved, and those mammalian forms occupying that niche were very different than those of today. One such form was a small carnivorous tree-climbing mammal, the first to have developed a prehensile tail allowing it an agile life in the trees (fig. 144). It swung freely through the tree canopy with its long tail and graceful arms and legs, feeding on small animals along the way. The top predators of the time continued to be reptilian, such as the large crocodiles and monitor lizards, as well as the giant 2-meter-tall flightless bird †Gastornis. The daytime skies contained frigatebirds that swooped down from the sky to feed on fishes and other small vertebrates in the lake. Rails, rollers, and birds that resembled a cross between ducks and flamingos were common shorebirds, and many species of parrots inhabited the trees surrounding the lake. It is hard today to think of Wyoming as a hotspot for parrot diversity! There were colonies of bats near the north end of the lake, where most of the 35 or so fossil bat specimens have been discovered. Today, bats use echolocation as a way to navigate and find food in the dark. One of the bat species that lived near Fossil Lake, †Onychonycteris finneyi, appears to have lacked the ability to echolocate and may have been a day flier also inhabiting nearshore trees. From palm trees to crocodiles, the FBM fossils provide a quasi-photographic record of a lost world. There are more than just a few isolated plant and animal species preserved here; there is a complex and vibrant ecosystem locked in stone.

† = Extinct

Grande, Lance. “Reading the Pages of Deep History.” The Lost World of Fossil Lake. Ed. Lance Grande. Chicago and London: The University of Chicago Press, 2013. 351–354. Print.
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