Moreover, those species already in decline, including the dinosaurs and
pterosaurs, might have been dealt a final death blow from above.
AN ASTEROID IMPACT
At the end of the Cretaceous, the dinosaurs along with 70 percent of all
known species became extinct. Because the division between the Cretaceous
and the Tertiary, called the K-T boundary, is not a sharp break but might rep-
resent up to 1 million years or more, this extinction was not necessarily sud-
den. It could have taken place over an extended period. Many dinosaurs along
with other species were already in decline several million years before the end
of the Cretaceous. Triceratops (Fig. 171), whose vast herds covered the entire
globe and might have contributed to the decline of other dinosaur species,
were among the last dinosaurs to die.
One theory attempts to explain the extinction of the dinosaurs and more
than 70 percent of other species at the end of the Cretaceous.One or more large
asteroids or comets struck Earth with an explosive force equivalent to 100 tril-
lion tons of dynamite, which is equal to 1 million eruptions of Mount St. Helens.
Sphereule layers up to 3 feet thick found in the Gulf of Mexico are related to the
65-million-year-old Chicxulub impact structure off the Yucatan Peninsula of
Mexico.The spherules resemble the glassy chondrules (rounded granules) in car-
bonaceous chondrites,which are carbon-rich meteorites,and in lunar soils.
Figure 171Vast herds of
triceratops roamed all parts
of the world toward the
end of the Cretaceous.CRETACEOUS CORALS