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Tabl e 6. 2
Timeline of major extinctions
Million years ago
Major extinction
Paleozoic
Cambrian
543
Ordovician
510
Silurian
438
Devonian
408
Mississippian
360
Pennsylvanian
320
Permian
286
Permian extinction
Mesozoic
Triassic
248
Jurassic
208
Cretaceous
144
K-T extinction
Cenozoic
Tertiary
66
Quaternary
2
before the Cretaceous-Tertiary boundary - known as the K-T boundary. Strangely,
turtles, crocodilians, lizards, and snakes were not affected or were affected only
slightly. Whatever factor or factors caused it, there was a major, worldwide biotic
change at about the end of the Cretaceous. But the extinction of dinosaurs is the
best-known change by far and has been a puzzle to paleontologists, geologists,
and biologists for two centuries. Many theories have been offered over the years to
explain dinosaur extinction, but few have received serious consideration. Proposed
causes have included everything from disease, heat waves and resulting sterility,
freezing cold spells, and the rise of egg-eating mammals, to X rays from a supernova
exploding nearby. Since the early 1980s, attention has focused on the impact theory
by the American geologist Walter Alvarez, his father, the physicist Nobel Prize
winner Luis Alvarez, and their colleagues.
There have been over 80 theories of what caused the extinction of dinosaurs,
also known as the KT debate. Paleontologists, geologists, physicists, astronomers,
nuclear chemists, and many others all have been involved in this debate (Alvarez
1997 ). Throughout the 1980s the KT debate was largely between the impact camp
and the volcanism camp. The impact camp argued that the KT extinction was due to
the impact of a gigantic asteroid or comet, suggesting a catastrophic nature of the KT
extinction. The volcanism camp, on the other hand, insisted that the mass extinction
was due to massive volcanism over a much longer period of time, implying a gradual
nature of the KT event. The impact camp had evidence for the impact of an asteroid
or a comet, such as the anomalous iridium, spherules, and shocked quartz in the
KT boundary layer, whereas the volcanism camp had the Deccan Traps, which was
connected to a huge volcanic outpouring in India 65 million years ago.
The first thoroughly documented account of the asteroid theory of dinosaur
extinction, by the original proponents, can be found in Luis W. Alvarez et al.,
“Extraterrestrial Cause for the Cretaceous-Tertiary Extinction: Experimental
Results and Theoretical Interpretation,” Science , 208(4448):1095-1108 (June 6,
1980), a highly technical paper. Popular reviews of the general issue include Dale
 
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