The Friends of the Florissant Fossil Beds, Inc.

Benson’s talk, titled “Exceptional Diversity of Late Eocene Freshwater Diatoms from the Florissant Formation, Teller County, Colorado,” summarized some of her work to date on the previously unstudied fossil diatoms of the Florissant Formation.

Diatoms are microscopic, single-celled, golden-brown algae with hard silica shells (frustules). These shells are extremely tiny, only 10-100 microns (1 micron = 1/1000 millimeter). Scientists use the shapes, sizes, and ornamentation of the frustules to identify diatom species. Diatoms can be divided into two major groups according to shape, centric (round) and pennate (elongated).

Although marine diatoms are known as far back as the Jurassic Period (200 million years ago), confirmed reports of freshwater diatoms in North America are much more recent. The diatoms of the Florissant Formation represent one of the earliest occurrences of freshwater diatoms in North America.

Fossil diatoms are most abundant in thin “paper shale” layers within the Florissant Formation, such as those found at the Scudder Pit Interpretive Site. Photo Credit: Melissa Barton

Benson’s research, which is partially funded by visitor fees from the Federal Lands Recreation Enhancement Act (formerly Fee Demo Program), is currently focused on identifying the fossil diatoms and comparing the Florissant assemblage to other Eocene assemblages from western North America. Additional funding sources include the Paleontological Society, Walker Van Riper Fund at UCM, Evolving Earth Foundation, Colorado Scientific Society, Colorado Mountain Club, and the Sam Van Landingham Fellowship at the California Academy of Sciences.

In order to examine the diatoms with a light microscope, Benson must break up the matrix of the rock, make a slurry, dry the slurry on cover slips, and then mount the slips on slides. However, many important identifying characteristics cannot be seen without the higher-powered scanning electron microscope (SEM). Taking a good micrograph is difficult and time-consuming.

These SEM micrographs of modern diatoms show the whole frustule of a centric diatom (A), two whole pennate diatom frustules (B), a single valve of a centric diatom (C), and another centric diatom frustule (D). Relatives of some of these diatoms are known from the fossil record. Photo Credit: Mary Ann Tiffany, San Diego State University

The Florissant diatoms are extremely diverse compared to other Eocene sites, Benson has found. Many of the Florissant genera have not been recorded in other Eocene or earlier deposits. Benson hopes to find and describe new species within these genera.

“Although the diatoms are relatively well-preserved, they are not as well-preserved [as the plant and insect fossils of Florissant],” Benson said at the talk, “so their claim to fame will be their diversity, not their preservation.”

Since diatom frustules can dissolve in alkaline water, they are not always preserved in lake deposits, even if diatoms lived in the lake. Diatomaceous sediments can also be replaced by chert if additional silica is introduced after burial.

Florissant presents an exceptional opportunity for scientists like Benson to study the evolutionary history of diatoms and what diatoms can tell us about the ecology and chemistry of ancient lakes.

-Melissa Barton

Acknowledgements: Thanks to Mary Ellen Benson and Dr. Dena M. Smith for assistance in writing this article.

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