Biology Reference
In-Depth Information
climate, and consequently in vegetation; of possible magnetic and electric infl u-
ences. Many held that no visible or perceptible ef ect would in any manner be pro-
duced. While such discussions were going on, their subject gradually approached,
growing larger in apparent diameter, and of more brilliant luster. Mankind grew
paler as it came. All human operations were suspended.
—EDGAR ALLAN POE,
“The Conversation of Eiros and Charmion,” 1850
Sixty-fi ve million years ago, the Cretaceous period closed with one of the
most geologically spectacular and biologically altering events in Earth his-
tory, a dark and stormy night of some three years duration. About 160 Ma
the explosion of a parent asteroid called Baptistina, 170 km in diameter
and located in the inner asteroid belt, created a fl ux of terrestrial impacts
(Bottke et al. 2007). One of the fragments entered the Earth's atmosphere
from the southeast traveling at 90,000 km per hour. It landed in the shal-
low waters of the Yucatán Peninsula with a force of about 100 million
megatons of TNT, or 2 million times the largest H-bomb, leaving a crater
180 km in diameter around the present village of Chicxulub. The local ef-
fects were obviously catastrophic, but shocked (striated) quartz was splashed
out as far as Haiti, and a tsunami over 1 km in height deposited debris near
Tampico, Mexico, and at Recife in northeastern Brazil. Farther afi eld there
were wildfi res, acid rain, near darkness, reduced rates of photosynthesis,
and diminished plant biomass for the large herbivores and carnivores at the
top of the food chain. The dinosaurs became extinct, except for their avian
descendents, and smaller animals were left to diversify, eventually chang-
ing the Mesozoic age of reptiles into the Cenozoic age of mammals (but
see Keller et al. 2009). The Cretaceous-Tertiary contact (K/T boundary)
is marked by what John McPhee (1981) calls “an unearthly concentration
of iridium”—an element rare in Earth deposits, but common in meteor-
ites and widespread in K/T boundary sediments at concentrations up to
160 times that in strata above and below. Immediately after the impact,
abundant organic material became available for saprophytic fungi. Early
colonizers and recovery vegetation followed and included numerous ferns.
These events are recorded in the Raton Basin of New Mexico and Colorado
(fi g. 2.18) as a sequence of K/T beds containing dinosaurs, a clay layer rich
in iridium, Tertiary beds without dinosaurs, a coal layer of organic debris
with abundant saprophytic fungi, and a “fern spike” of recovery vegeta-
tion. Near ground zero, the destruction of terrestrial and fl oating marine
communities was complete, and the history of the modern Gulf-Caribbean
biota begins with the Tertiary period at 65 Ma.
The confi guration of the New World and adjacent plates at the time