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The
Florissant Formation: A Virtual Tour
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Fossil Preservation in the Paper Shales of Florissant
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Cross-Section of Paper Shale from Lower Shale Unit (1.8 cm thick) |
Fossil
Preservation in the Lake Shales
The lacustrine or lake paper shales are made of alternating layers
of diatoms and ash-clay. O’Brien et al (2002) used scanning
electron microscopy and energy dispersive X-ray analysis to examine
these alternating layers. The diatomite layer is lighter in color
and consists of the frustules of diatoms. Diatoms are single-celled
protists that make their protective shells or frustules of silica.
Fossils are almost exclusively found imbedded within the diatomite
layers. Furthermore, these diatomite layers are covered by a mucous
layer, which was secreted by diatoms and bacteria. Ash-clay layers
appear darker in color. The diatomite and ash-clay layers form
what is known as a couplet 0.1 to 1.0 mm in thickness. Typically,
layers of 3 to 10 couplets are sandwiched between pumice layers,
which are several millimeters to centimeters in thickness (pp.
3 & 4). O’Brien et al (2002), proposed the following
events for the deposition of the Florissant Fossiliferous shale:
1. Volcanic ash weathering into clay washed into Lake Florissant
from the surrounding terrain.
2. The volcanic sediment was deposited as a thin layer of ash-clay,
enriching the lake water with silicon.
3. Diatoms bloomed as a result of the added silicon and formed
polysaccharide mucus mats.
4. It is hypothesized that the diatom and bacterial mucous
film sealed and protected the organisms from decomposition.
5. Mucus mats sank to the lake bottom and formed thin, diatom-rich
laminae.
6. The organisms were subsequently fossilized (p. 6).
The paper shale is made of many couplet layers suggesting seasonality
to the cycle above. The couplets may be varves representing
annual layers. The pumice layers interbedded at irregular
intervals within the paper shales record sporadic volcanic
eruptions that produced volumes of ash and pumice. These
layers were formed rapidly and are much thicker than the
couplets.
Both
insects and leaves preserved in the Florissant beds are often
carbonized Insects and leaves entangled
in the diatom mucus mats were incorporated into layers of sediments
and volcanic ash at the bottom of Lake Florissant. Many of
these insects and leaves decomposed leaving imprints. As the
sediments compacted and hardened into shale the imprints became
impression fossils. Some organisms only partially decayed retaining
a dark colored carbon residue to become compression fossils
(carbonization). Many insects have their wings preserved as
impressions (no organic residue) while their bodies retain
organic residue forming dark compressions. Compressions are
often flattened, having a two-dimensional appearance. However,
the preservation in diatom layers allows some organisms to
retain their three-dimensional character. Some insects are
found with organs and appendages. Some leaves can be found
with internal structures (Meyer, 2003, pp. 35-37).
The
paper shales act as nature’s “plant and insect
press” and make Florissant a fossil lagerstätten.
Florissant has produced roughly 1700 described species of plants
and animals (Meyer, Veatch & Cook, 2004, p. 151). Impressions
of leaves, fruits, seeds and flowers account for about 120
species (Manchester, 2001, p. p. 137). Palynology, the study
of microscopic plant fossils, such as pollen and spores, adds
another 25 genera to the fossil flora of Florissant (Leopold & Clay-Poole,
2001, p. 17). Over 1500 of the 1700 described species at Florissant
are insects and spiders (Meyer et al., 2004, p. 158). The most
common vertebrate found in the fossil shale are fish, with
the majority representing bottom dwellers. Four genera representing
catfish, suckers, a bowfin and a pirate perch have been described.
Although rare, birds are represented by a small plover, roller,
and cuckoo. Only one mammal, a small opossum, has been found
within the shale beds. Interestingly, no amphibians or reptiles
have been found at Florissant (Nudds & Shelden, 2008, pp.
227 & 228).
It is clear that the fossil bearing shale at Florissant has
received a lot of attention from paleontologists over the years.
However, within the shale units different depositional environments
are also represented by layers of mudstone and siltstone. A
recent study comparing shale, mudstone, and siltstone within
the middle shale unit at Florissant found that the abundance
and preservation quality of the most commonly found insect
orders in lacustrine settings did not differ across these different
sedimentary environments (Henning, Smith, Nufio, and Meyer,
2012, p. 481).
The findings of this study are surprising for a couple of
reasons. Diatoms have been thought to play a major role in
the preservation quality of insects found within the shale.
However, mudstone is not typically associated with diatom layers
indicating that the presence of diatoms may not enhance preservation
(Henning et al., 2012, p. 487). Siltstone represents a higher
energy depositional environment with larger grain sizes than
mudstone or shale. Interestingly, the preservation quality
and abundance of fossils found within the siltstone was equal
to mudstone and shale. Perhaps the paper shale has been favored
over the years because it is easier to split open. The surprising
results of this study suggest paleontologists broaden their
search for fossils by including the mudstone and siltstone
layers found within the shale units at Florissant.
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Fagopsis Lower Shale Unit |
Crane Fly Lower Shale Unit |
Bibliography
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Henning,
J.T., Smith, D.M., Nufio, C.R. and Meyer, H.W. (2012).
Depositional setting and fossil insect preservation: a
study of the late Eocene Florissant Formation, Colorado.
Palaios 27: 481-488.
O’Brien,
N.R., Meyer, H.W., Reilly, K., Ross, A.M., and Maguire, S.,
(2002). Microbial taphonomic processes in
the fossilization
of insects and plants in the late Eocene Florissant Formation,
Colorado: Rocky Mountain Geology, v. 17, pp. 1-11).
Leopold, E.B. and Clay-Poole, S.T. (2001). Fossil leaf and
pollen floras of Colorado compared: climatic implications.
In Evanoff, E., Gregory-Wodzicki K.M. and Johnson, K.R. [Eds.]
Fossil Flora and Stratigraphy of the Florissant Formation,
Colorado. (pp. 17-55). Proceedings of the Denver Museum of
Nature and Science, series 4, number 1. Meyer, H.W. (2003). The
Fossils of Florissant. Washington: Smithsonian
Books.
Meyer, H.W., Veatch, S.W. and Cook, A. (2004). Field guide to
the paleontology and volcanic setting of Florissant fossil beds,
Colorado (pp. 151-166). In Nelson, E.P. and Erslev, E.A. [Eds.]
Field Trips in the Southern Rocky Mountains, USA. Geological
Society of America Field Guide 5.
Nudds,
J.R. & Selden,
P.A. (2008). Fossil Ecosystems of North America: A Guide
to the Sites and Their Extraordinary Biotas.
Chicago: The University of Chicago Press.
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