"Scenic view from atop Twin Rock" by U.S. National Park Service , public domain
![]() | Florissant Fossil BedsMicroscopic World of Florissant |
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Florissant Fossil Beds
Microscopic World of Florissant
Imagine walking along the shore of ancient Lake Florissant, almost 34 million years ago. What would
Florissant look like? During the late Eocene epoch, the world was a much warmer place. You’d see a lush,
thriving habitat—water lilies and cattails lying along the beach, willows leaning into the lake, the air thick
with hundreds of swarming insects. But unless you had a microscope, you’d inevitably miss the world
invisible to the naked eye. Pollen, algae and microscopic invertebrates are preserved in the fossil beds,
providing a window into the environment around Lake Florissant.
Fossil Pollen and Spores
Pollen and spores are regularly released by plants and are
carried by the wind or by pollinators. During the late
Eocene, pollen from the plants in Florissant ended up
either settling on the surface of the lake or washing into the
lake by streams. The pollen then settled to the lake bottom,
combining with dead diatoms (algae) in the mud that had
bloomed from the ash from the nearby Guffey volcano.
When these fossils are excavated, the rock might not show
signs of containing fossils at all. This
might explain why early naturalists to
the area rarely documented the
presence of such microfossils. In
order to see the identifying
characteristics, a micropaleontologist
needs to isolate the individual pollen
grains and spores by dissolving the
rock in acid. The extracted pollen
grains are then stained with dye and
mounted on slides to view under a
Longest dimension: 0.128 mm microscope. Pictured to the left is a
fossilized pollen grain found at
Florissant from a fir tree.
Pollen can be identified by size, shape, and the number and
location of pores and furrows. Florissant’s collection has
been identified to contain over 130 different species of pollen and
spores. Moreover, 25 of the plant genera at Florissant are known
only by their pollen. These microfossils have been critical to
defining the plant communities surrounding the lake and the
climate necessary to support these plant species as seen in the figure
below.
Fossil Diatoms
Diatoms are a type of unicellular algae that flourished in
Lake Florissant that have cell walls made of silica. The
exterior is typically covered in pores and unique
ornamentation and can be used to identify different genera
of diatoms. Like pollen, diatoms are carefully extracted from
the rock matrix and then mounted on slides. A scanning
electron microscope (SEM) is used to be able to view the sub
-millimeter level of detail on the diatoms. The photo to right
displays an SEM image of two exceptionally preserved of
Florissant’s diatoms.
Diatoms were critical to the preservation of the fossils at
Florissant due to the mucus they produce when stressed.
The silicon-rich volcanic ash from the nearby volcanoes
would be deposited into the lake that acted as a fertilizer,
causing algal blooms to form large mats on the surface of the
lake. When the silicon was exhausted, the diatoms would
become stressed
and exude mucus
that would
eventually settle
to the bottom of
the lake. The
mucus slowed the
decay of dead
organisms in the
lake so they could
fossilize.
Fossil Ostracods
Ostracods are a type of small crustacean, almost like a shrimp enclosed in a
shell, which resided in Lake Florissant. The top left photo depicts a modern
ostracod within its shell. They likely lived in the lake’s shallow areas, feeding
on detrital remains that settled to the bottom. As they fed on detritus, they
absorbed dissolved calcium and other elements from the lake water to
augment to their shells. These ostracods were then fossilized, embedded in the
resulting shale in their original assemblages.
Scale: 1mm in length
Like diatoms, ostracods are typically observed using SEM. The bottom left
photo shows an ostracod shell that was extracted from the Florissant shale
viewed under an SEM. Once identified, ostracods can reveal clues about their
original habitat in Lake Florissant. Various elements in the shell record water
quality during the ostracod’s life, like the water current, depth and salinity.
Furthermore, their presence across the fossil record can be related to
temperature. Ostracods can even be used to estimate the temperature at the
time they were buried. Knowing this, scientists can approximate the changes in
the paleoclimate, and help us better understand modern climate change.
Scale: 1.11 mm in length
Fossil Charophytes
The most recent fossil finds at Florissant include freshwater
algae called charophytes. These macro algae are considered
the progenitors of land plants since they have a complex
lifecycle similar to mosses. When their spores are fertilized,
they surround the spores with a lime covering so that upon
release, it will be protected from elements. The capsule, also
called gyrogonite, keeps the spores dormant until favorable
conditions to grow arise. Rarely, the gyrogonites get buried
and become fossilized instead.
Like ostracods, gyrogonites have unique features on the
exterior that is indicative of a certain genus. Charophytes can
be seen with the naked eye but a microscope is needed to
view the intricate details on specimens. Such as the spiral
patterns along the exterior as seen in the photos to the right.
Charophyte fossils can detect paleoenvironment conditions
like water quality and salinity. These algae continue to exist
1 mm
today shallow ponds with high water clarity. Because of this,
lake Florissant may have been shallow lake with low
turbulence for some of its existence.