Page 64 - The Restless Earth Fossils

Marking turning points in evolution 63

western Colorado yield fossil insects and leaves. In 2002, volun-
teers helped the Denver Museum of Nature and Science, Utah
State Parks, and the U.S. Bureau of Land Management personnel
excavate more than 300 square feet (28 square meters) of Green
River Formation leaf and insect fossils to line the walls of a new
museum in Vernal, Utah. The work also discovered new speci-
mens to illustrate the Parachute Creek Atlas Project—an Eocene
fossil plant record that complements work done on “flashier”
vertebrate species.

Mass extinctions: liFe on the edge
oF disaster

The fossils associated with the Paleozoic, Mesozoic, and Cenozoic
Eras stand out by their differences. Paleontologists can see those
dramatic differences in the rock. The boundary between the
Mesozoic and Cenozoic Eras—the marker for the end of the reign
of dinosaurs—is a seam of clay that can be covered by the width
of a human hand. In the late 1970s, geochemist Lewis Alvarez and
his father, Walter, a geologist and geophysicist, collected samples
from the boundary layer exposed near Gubbio, Italy, where it is
sandwiched between layers of pink limestone. They found that
the layer contained a very high concentration of iridium, an ele-
ment usually rare on Earth, but common in meteors and aster-
oids—the leftover debris of planet building during the birth of
the solar system. Also present were deformed particles of quartz
called shocked quartz that result from the impacts of meteors
with the Earth.
Other scientists found these same features at sites all over
the world. Radiometric studies dated the layer to 65.5 million
years old—precisely the age of the Cretaceous-Tertiary boundary
(abbreviated as the KT boundary) as determined by other meth-
ods. Alvarez calculated that an object the size of Mount Everest
must have struck the Earth to leave a worldwide trace of this mag-
nitude. That would have left a crater some 110 miles (180 km)
wide. No one was aware of a crater that size until the early 1990s,

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