Florissant Fossil Beds National Monument (Part 2)

This unnamed stump might be the largest petrified stump in the world by diameter, but it cannot be determined for certain because the top of the stump is slightly below breast height, where the standard diameter measurement would be taken. Using an equation, the diameter at breast height of this stump is estimated at about 16 feet so the tree may once have stood as tall as 300 feet. Buried petrified stumps can be located using a magnetometer, which measures the strength of the magnetic field of the minerals in rocks. The mudstone that surrounds the stumps contains magnetite which produces a strong magnetic field. Since the stumps are mostly composed of silica, they have weak magnetic fields that are easy to identify in contrast.

In 1950, H. Dale Miller purchased Henderson Petrified Forest and renamed it Pike Petrified Forest. John Baker operated the site and later bought out Miller to become the owner. In 1956, Walt Disney purchased a petrified stump from the site and put it on display at Frontierland in Disneyland. Pike Petrified Forest closed only five years later.

At Florissant, fossils of plant leaves, fruits, flowers, cones, seeds, pollen, and tree stumps were preserved. In the late 1950s and early 1960s, paleopalynologist Estella Bergere Leopold, daughter of renowned naturalist Aldo Leopold, collected pollen fossil specimens at Florissant. In 1961, Agnes Singer requested the Florissant valley be considered for protection as a national monument and the following year the National Park Service prepared the proposal. Congress drafted the first bill in 1964 but it was unsuccessful as were similar bills in 1965 and 1967. Following years of inaction, the Park Land Company began planning for development in the eastern part of the fossil beds in 1968. In 1969, Colorado senator Gordon Allott presented a bill for the national monument to the Senate. Upon learning that nearly half of the land proposed for the monument was set for development, Leopold and fellow scientist Bettie Willard hired attorney Victor Yannacone. He had them form the Defenders of Florissant, Inc. along with Vim Wright and other concerned citizens and they brought a case for a restraining order against development on the land. It was one of the nation's first explicitly environmental cases. Judge Chilson of the Federal District Court in Denver, CO denied their case as well as their first appeal, but Chief Justice Alfred P. Murrah of the Tenth Circuit Court of Appeals granted a temporary injunction on development until Congress made a decision on the monument. Congress then passed the monument legislation and President Richard Nixon signed the bill to establish Florissant Fossil Beds National Monument.

This map shows where some modern relatives of the Florissant fossil plants can be found. During the early Oligocene, the climate at Florissant cooled, which forced plants to adapt, go extinct, or disperse. By 33 million years ago, the plants on this map that once lived together dispersed widely across the country and the world.

Over 130 plant species have been identified from pollen fossils at Florissant. Estella Bergere Leopold compared Florissant pollen fossils to the pollen of modern plants to plot the temperature distributions of living relatives. The range of overlap provided a mean annual temperature between about 57-59°F (15-16°C) for Late Eocene Florissant.

In 1973, the National Park Service purchased the Colorado Petrified Forest Ranch from Agnes Singer and the buildings were torn down shortly thereafter. In 1975, the former museum of Pikes Petrified Forest was renovated into a ranger station and visitor center, and the Hornbek Homestead was restored the following year. Between 1976-1993, Frederick Martin Brown volunteered as a paleontologist at the monument, and in 1994, Herb Meyer was hired as the first permanent paleontologist. In 2013, the current visitor center, museum, and research facility was completed. In 2021, Florissant was named an International Dark Sky Park by the International Dark-Sky Association.

Mesohippus, or “middle horse,” is an extinct genus of primitive horses that lived between 37-32 million years ago from the Late Eocene to Early Oligocene. It had three toes per foot instead of hooves and grew about two feet tall. It was also a browser unlike modern horses which are grazers. Megacerops, or “large-horned face,” is an extinct brontothere genus that lived about 38-33.9 million years ago during the Late Eocene. Brontotheres were odd-toed rhinoceros-like browsers distantly related to horses. They are the largest animals known from Florissant fossils, growing about eight feet tall, 16 feet long, and weighing up to two tons. They also possessed blunt Y-shaped horny protrusions that could measure over 3.3 feet long. Merycoidodon and Leptomeryxare also known from Florissant fossils. Merycoidodon, or "ruminating teeth," is an extinct genus of oreodont that lived about 46-16 million years ago from the Middle Eocene to Middle Miocene. Oreodonts were even-toed hoofed mammals related to camels but more similar in appearance to sheep or pigs. 

The work of Jaelyn Eberle and her students tripled the number of mammals described from Florissant. About 20 mammals are now known and many were identified from scant evidence such as teeth alone. Leptomeryx is an extinct genus of deer-like ruminant that lived 38-24.8 million years ago from the Late Eocene to the Late Oligocene. Megalagus brachyodon is a primitive rabbit species that lived 37.2-33.9 million years ago during the Late Eocene and migrated from Asia to North America. Domnina is an extinct genus of primitive insectivorous shrew that lived 46.2-23.0 million years ago from the Middle Eocene to the Early Miocene. Pelycomys is an extinct genus of mountain beaver that lived 37.2-33.9 million years ago during the Late Eocene. Oreotalpa florissantensis, or “mountain mole originating in Florissant,” is an extinct mole species that lived between 37.2-33.9 million years ago during the Late Eocene. It is the oldest known mole from North America and the original fossil resides at the University of Colorado Museum of Natural History. A model of the fossil was enlarged 20 times its actual size and 3D printed to produce the replica jaw here.

Shales were deposited in alternating layers with volcanic tuff composed of ash and pumice at the bottom of Lake Florissant.

Paper shales contain most of the fossils discovered at Florissant. Silica-rich volcanic material deposited into Lake Florissant caused cyclical blooms of freshwater diatoms, which are a type of algae. The diatom blooms reduced available nutrients, oxygen, and light in the lake and combined with the stress of volcanic activity to cause large die-offs of organisms, whose bodies would settle on the lake bottom. When the diatom colonies died, their silica shells covered the organic matter that accumulated on the lake bottom and bonded with clay from weathered ash to form couplets. As sediments built up, the couplets compacted to form thin layers of paper shale about 0.1-1 mm thick that preserved many fossils. The paper shales produced from this process can be split to reveal two halves of the same fossil, the part and counterpart, which may display different aspects such as the legs on one half and the back on the other. If the couplets represent annual cycles of diatom blooms and die-offs, then Lake Florissant may have existed for 2,500-5,000 years. Florissant preserves one of the earliest known examples of freshwater diatoms and boasts the most diverse early diatom flora.

Multiple fossilization processes occurred to preserve organisms and signs of their activity at Florissant. Tree stumps and vertebrate bones fossilized via permineralization while invertebrates and plant parts fossilized as compressions or impressions. Compression occurs when an organism dies, gets buried by sediment, and decays until only a black film of carbon is preserved. Impressions form when the hard parts of the organism make an imprint in sediment before decaying. Fossils may be made up of both compressions and impressions such as with the insects, whose bodies were compressed while their wings left impressions. Other fossilization processes include molds, casts, and trace fossils. When an organism is buried in soft sediment, decays entirely, and leaves an impression, a mold is formed. When the impression is filled with other sediments to form rock in the shape of the organism, a cast is formed. A trace fossil, also known as an ichnofossil, is a fossil of the activity of an organism such as a footprint, trail, burrow, or some other trace evidence. Over 10,000 fossils are held in the secured collections room of the monument.

This is a replica of the brush-footed butterfly (Prodryas persephone) fossil found by Charlotte Hill. It was described by Samuel Scudder in 1878, the first butterfly fossil described from North America. Twelve butterfly species have been identified from fossils at Florissant, the most of any fossil insect site.

In 1907, Theodore Dru Alison Cockerell described the first known fossil tsetse fly (Glossina oligocenus) from Florissant based on a specimen that had been incorrectly identified as a bot fly by Samuel Scudder in 1892. This showed that the Eocene climate at Florissant was closer to that of tropical Africa. Cockerell would later describe three more species of fossil tsetse flies from Florissant.

This is a replica of a cuckoo (Eocuculus cherpinae) fossil collected at Florissant by Colette Cherpin and Jeffery Carpenter and described by Robert M. Chandler in 1999. Other fossil birds identified from Florissant include rails, a roller, and a pootoo.

Almost all the mammal fossils from Florissant were found in the lower mudstone unit. The only one recovered from the shale units is that of Nanodelphys huntii, a small 4-5 inch “mouse opossum” species.

On the left are plant fossils featuring different forms of insect damage. On the right are leaf and flower fossils of Populus sp.