The end of the Cretaceous Period was also the end of the dinosaurs.
Exactly why the “terrible lizards” died out after dominating the world
for more than 150 million years is not known for sure, but many guesses
have been made.
One likely idea is that widespread uplift and mountain building that
began late in Cretaceous time, accompanied by changes in climate, may
have greatly reduced the supply of soft edible plants. If so, it is easy
to imagine how huge dinosaurs accustomed to a ton or more of lush plant
food each day would soon starve to death.
Many dinosaurs were vegetarians. As they died out, the flesheaters, such
as _Tyrannosaurus_, soon ran short of food also, and probably began to
eat each other. _Tyrannosaurus_ closely resembled the Jurassic
_Allosaurus_ shown in figure 23, except that _Tyrannosaurus_ was much
larger and more formidable—in fact it probably was the most terrible
predator that ever roamed the surface of the Earth. The dinosaurs had
become too highly specialized to their environment to adapt themselves
to changes of this kind.
Another fascinating notion is that the growing population of small
primitive mammals devoured dinosaur eggs (which were left unattended
like those of turtles and alligators) nearly as fast as mamma dinosaur
could lay them. But whatever the reason, it is clear that some worldwide
condition caused the gradual extinction of the ponderous
over-specialized dinosaurs and allowed the rise to power of the next
types of animals destined to rule the Earth—the brainier and more
adaptable mammals.
At this time the rocks were gently bent into upfolds, called anticlines
or arches, and downfolds, called synclines or basins (fig. 27). One
upfold that began to take form was the Uncompahgre arch, the crest of
which shapes Piñon Mesa just south of the Monument. But this gentle
upfold was to grow larger and to have its flanks wrinkled and broken in
the next geologic era—the Cenozoic.