Page 28 - The Restless Earth Fossils

the tortuous road to Fossilhood 27

mining the famous La Brea tar pit traps in the center of Los
Angeles, California. Many insects, birds, turtles, and plant parts
also found their way into the tar. Quick burial and low oxygen
preserved their remains intact—in this case for tens of thousands
of years.

carBonization: Fossil “road Kill”

A paleontologist visiting Douglas Pass, Colorado, high in the
Rocky Mountains, may use her rock hammer to pop apart a thick
slab of shale and find the dark brown image of a Macginitiea
(sycamore) leaf or a fossil cranefly. A musty smell reminds her
of fish and rotting vegetation on a lakeshore. The leaf or the
insect has been carbonized: partially decayed, wrapped in slime,
crushed, and heated.
Herbert W. Meyer, a paleontologist with the U.S. National
Park Service, has studied this process in some detail at famous
fossil beds (the site of ancient lakes) near Florissant, Colorado.
The layers of shale at Florissant and Douglas Pass, as well as other
locations in Utah and Wyoming, alternate with the ash from vol-
canic eruptions. Many of the early workers at these sites assumed
that animals and plants in the lakes died and were preserved in
mass dying events during volcanic eruptions, but careful studies
have shown that much of this preservation happened in the peri-
ods between these violent events. “The thin shale layers formed
slowly over many decades,” says Meyer, “whereas the layers of
volcanic ash accumulated much more rapidly.”
Images taken of Florissant fossils with a scanning electron
microscope have shown that leaves and delicate insect wings
are covered with a thin layer of billions of tiny plant cells called
diatoms. Growing abundantly in lake water enriched with silica
from volcanic ash, these diatoms and bacteria covered dead
insects, fish, and leaves floating on or in the lake with a thin
film that is almost like plastic wrap. This film helped preserve
the organisms long enough to be buried by successive layers
of silt and volcanic ash. The weight of these layers ultimately

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