The dramatic extinction of all dinosaurs (except those which led to birds) marked the end
of the Cretaceous period. A worldwide iridium-rich layer, dated at 65.5 million years ago,
provides evidence for a dramatic cause for their ultimate extinction. Iridium is rare in the
Earth’s crust, but common in comets and asteroids. Scientists associate this layer with a
huge crater in the Yucatan and Gulf of Mexico. A collision/explosion between the Earth
and a comet or asteroid could have spread debris which set off tsunamis, altered the climate
(including acid rain), and reduced sunlight 10-20%. A consequent reduction in photosyn-
thesis would have caused a drastic decrease in food chains, leading to the extinction of the
dinosaurs. The fossil record obviously depicts the presence of dinosaurs on Earth, and the
absence of dinosaur fossils after this extinction event demonstrates the relationship between
the fossil record and evolution.
Figure 7.38: The fossil record demonstrates the presence of dinosaurs, which went extinct
over 65 million years ago. ( 39 )
After each mass extinction, open ecological niches are quickly filled by other species. This
is well documented in the fossil record. This episodic speciation following an event such as
a mass extinction also shows the relationship between evolution and the fossil record.
Lesson Summary
- During the 1800s, geologists, paleontologists and naturalists found several forms of
physical evidence that confirmed that the earth is very old.