End of the Dinosaurs
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.