A wide
range of organisms became extinct at the end of the Cretaceous
period. The most conspicuous, of course, were the dinosaurs. While
there is evidence that dinosaur diversity declined in the Late
Cretaceous of North America, many species are known from the Hell
Creek and Lance Formations of the Late Cretaceous. These include
six or seven families of theropods and a similar number of ornithischians.
Among the Dinosauria, the only survivors were the birds, but birds
suffered heavy losses. A number of diverse groups became extinct,
including the Enantiornithes and Hesperornithiformes; the last
of the pterosaurs also went extinct. A number of mammal groups
also became extinct. In the sea, many species of phytoplankton
were wiped out. The great sea reptiles of the Cretaceous, the
mosasaurs and plesiosaurs, also fell victim to extinction. Among
molluscs, the ammonites, a diverse group of coiled cephalopods,
were exterminated, as were the specialized rudist and inoceramid
clams. Freshwater mussels and snails also suffered heavy losses
in North America. Much less is known about how the K-T event affected
the rest of the world. It should be emphasized that the survival
of a group does not mean that the group was unaffected: a species
might have been 99% annihilated by the asteroid strike, yet still
manage to survive.
In
1980, a team of researchers led by Nobel-prize-winning physicist
Luis Alvarez, his son, geologist Walter Alvarez, and a group of
colleagues discovered that fossilized sedimentary layers found
all over the world at the Cretaceous-Tertiary boundary, 65.5 million
years ago, contain a relatively high concentration of iridium,
hundreds of times greater than normal. The end of the Cretaceous
coincided with the end of the dinosaurs and was in general a period
of extraordinary mass extinction, leading to the Tertiary Period
of the Cenozoic Era, in which mammals came to dominate on Earth.
The paper suggested that the dinosaurs had been killed off by
the impact of a ten-kilometer-wide asteroid on Earth (see impact
event). Two facts supporting this conclusion are that
* iridium is relatively
abundant in asteroids, and
* the isotopic composition of iridium in K-T layers resembles
that of asteroids more closely than that of terrestrial iridium.
Iridium
is very rare on Earth's surface, but much more common in the Earth's
interior as well as in extraterrestrial objects, such as asteroids
and comets. Furthermore, chromium isotopic anomalies are found
in Cretaceous-Tertiary boundary sediments which strongly supports
the impact theory and suggests that the impactor must have been
an asteroid or a comet composed of material similar to carbonaceous
chondrites.
The
resulting blast would have been hundreds of millions of times
more devastating than the most powerful nuclear weapon ever detonated,
may have created a hurricane of unimaginable fury, and certainly
would have thrown massive amounts of dust and vapor into the upper
atmosphere and even into space. A global firestorm may have resulted
as the incendiary fragments from the blast fell back to Earth.
Analyses of fluid inclusions in ancient amber suggest that the
oxygen content of the atmosphere was very high (30 - 35%) during
the late Cretaceous [1]. This high O2 atmospheric content would
have supported massive combustion. The level of atmospheric O2
plummeted in the early Tertiary. In addition the worldwide cloud
would have choked off sunlight for years, resulting in a "long
winter" that wiped out many existing species, as well as
creating "acid rains" that would have inflicted further
hardship on the environment.
One
problem was that no known crater matched the event. This was not
a lethal blow to the theory. Although the crater resulting from
the impact would have been 150 to 200 kilometers in diameter,
Earth's geological processes tend to hide or destroy craters over
time. Still, finding a crater would have buttressed the "Alvarez
hypothesis", as it came to be known. The discovery by Alan
K. Hildebrand and Glen Penfield of a crater buried under Chicxulub
in the Yucatan as well as various types of debris in North America
and Haiti have lent credibility to this theory (see Chicxulub
Crater). Most paleontologists now agree that an asteroid did hit
the Earth 65 million years ago, but many dispute whether the impact
was the sole cause of the extinctions.
Several
paleontologists remained skeptical about the impact theory, as
their reading of the fossil record suggested that the mass extinctions
did not take place over a period as short as a few years, but
instead occurred gradually over about ten million years, a time
frame more consistent with longer term events such as massive
volcanism. Several scientists think the extensive volcanic activity
in India known as the Deccan Traps may have been responsible for,
or contributed to, the extinction. A partial reason for the rejection
of the impact theory may have been a certain general distrust
of a group of physicists intruding into the paleontologists' domain
of expertise.
Luis
Alvarez, who died in 1988, replied that paleontologists were being
misled by sparse data. His assertion did not go over well at first,
but later intensive field studies of fossil beds lent weight to
his claim. Eventually, most paleontologists began to accept the
idea that the mass extinctions at the end of the Cretaceous were
largely or at least partly due to a massive Earth impact. However,
even Walter Alvarez has acknowledged that there were other major
changes on Earth even before the impact, such as a drop in sea
level and massive volcanic eruptions in India (Deccan Traps sequence),
and these may have contributed to the extinctions.
It
is worth noting that the Cretaceous extinction is neither the
only mass extinction in Earth's history, nor even the worst. Previous
extinction events have included the Triassic-Jurassic extinction
event and the Permian-Triassic extinction event, which is the
largest extinction event ever recorded.
