Museum Research Stone Lace

New Mammal Research at Florissant

Research by Marie Worley, Graduate Student, Museum and Field Studies Program, CU Museum

image of Ischyromyid rodent tooth

FLFO #5807, Ischyromys cf I. typus, Common Name: Ischyromyid rodent (scale in mm). Florissant Formation.

After learning of the discovery of a fossil horse jaw, Mesohippus, in 1993 at Florissant Fossil Beds National Monument in Colorado, graduate student Marie Worley became intrigued with the possibility of finding other fossil mammal remains. In the summer of 2003, Marie was a paleontology intern sponsored by the GeoCorps program of the Geological Society of America, with the hope of finding tiny bones and teeth from small mammals like rodents, rabbits and insectivores.

Marie collected fossils from Florissant in a systematic study that consisted of two methods. First, she carefully crawled over the surface of the area, on her hands and knees, picking up any fossil bone fragments or teeth that she could find. This resulted in nine identifiable teeth from the same area in which the Mesohippus jaw was discovered. Later, Marie performed what is called dry screening. Dirt shoveled from the surface of the site is sifted through a series of progressively finer screens, to trap sediment to be sorted later, under magnification. This method added six teeth (all from different animals) and four jaws with teeth in place.

image of Eutypomyid rodent tooth

FLFO #5805, Eutypomys parvus, Common Name: Eutypomyid rodent (scale in mm). Florissant Formation

Fossils found with these methods are often called "float" because they are found "floating" on or near the surface. Float fossils have likely moved out of the rock in which they were originally embedded. This is in contrast to fossils found "in situ" which means they were found still embedded in the rock in which they were fossilized. While fossils are ideally found in situ, float fossils can still provide important information about the diversity of animals from a time in the past.

The fossils found at Florissant tell an important part of the story of life in the Eocene. Most of the mammal species found at Florissant have previously only been found at more northern sites in Montana, Nebraska, and Saskatchewan. Notably, these species have not been found at sites of the same age in west Texas, which is geographically closer to Florissant. This suggests that 35 million years ago, the area of Florissant had something in common with Montana, Nebraska and Saskatchewan, and that these areas differed significantly from the existing environment in Texas. Possibilities include differences in climate or elevation, or perhaps even a geographic barrier.

image of a brontothere tooth image of Eomyid rodent tooth image of shrew tooth

FLFO #5801, Megacerops sp., Common Name: Brontothere (scale in cm). Florissant Formation.

FLFO #5808, Adijdaumo minimus, Common Name: Eomyid rodent (scale in mm). Florissant Formation.

FLFO #5816, Domnina cf D. thompsoni, Common Name: Shrew (scale in mm). Florissant Formation.

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