plankton disappeared along with most marine life in the upper portions of
the ocean.
Sixty-five-million-year-old sediments found at the boundary between the
Cretaceous and Tertiary periods throughout the world (Fig. 172) contain shocked
quartz grains with distinctive lamellae, common soot from global forest fires, rare
amino acids known to exist only on meteorites,the mineral stishovite—a dense
form of silica found nowhere except at known impact sites, and unique concen-
trations of iridium—a rare isotope of platinum relatively abundant on meteorites
and comets but practically nonexistent in Earth’s crust.
The geologic record holds clues to other giant meteorite impacts with
anomalous iridium concentrations that coincide with extinction episodes.
However, they are not nearly as intense as the iridium concentrations in beds
marking the end of the Cretaceous, which are as high as 1,000 times back-
ground levels. This suggests that the end-Cretaceous extinction might have
been a unique event in the history of life on Earth.
After discussing life living in the warm Cretaceous, the next chapter will
examine the evolution of the mammals and the geology of the Tertiary period.
Figure 172The
southwest slope of South
Table Mountain, Golden,
Colorado.The boundary
between the Cretaceous
and Tertiary periods lies
10 feet below where the
man is standing.
(Photo by R.W. Brown,
courtesy USGS)CRETACEOUS CORALS