City of Mesa, AZ

Dinosaur Mountain - Cretaceous Arizona

Sediments from the late Cretaceous Period have been found in scattered localities across southeastern Arizona, and deposits of similar age occur in Utah, New Mexico, and Sonora, Mexico. These areas yield information about the plants and animals of Arizona at this time.

Tryannosaurus rex

photo of T. rex replica on dinosaur mountain

Tyrannosaurus rex was obviously a very large meat eater, but that’s only the beginning of the story. Was T. rex a fierce predator, or did he scavenge the kills of other animals? Most of today’s large carnivores will happily do both. Scientists’ differing interpretations depend on how they reconstruct the anatomy of Tyrannosaurus rex.

In the southwestern United States, Tyrannosaurus is known from New Mexico, Texas, and Colorado. In Arizona, we have found fragments of bone from animals belonging to the tyrannosaur family, but we do not have enough to be sure of their specific identity.
The specimen shown is a juvenile; an adult would be about twice as big.


photo of Pentaceratops replica

Only fragmentary remains of the horned dinosaurs have been recovered in Arizona, but Pentaceratops is the most abundant form in the nearby San Juan Basin of New Mexico. Fully adult Pentaceratops have the largest skull of any vertebrate animal. “Five Horned Face,” as the name means, refers to the nasal horn in the middle of the face, the two brow horns, and the horns in the cheek region known as “jugal horns.”


photo of Paleosaniwa replica

This large lizard is a relative of today’s monitor lizards and Australian goannas. Like the monitors, Paleosaniwa was likely an opportunistic predator who would welcome the chance to raid nests of many creatures in the hope of getting an easy meal.


photo of Alphadon with T. rex

The first mammals in North America are found in the early Jurassic Kayenta Formation of Arizona. By the late Cretaceous, mammals with more modern characteristics appeared, including Alphadon, a common opossum-like marsupial. Alphadon likely had an appearance and habits like the modern opossum. This little Alphadon bides his time in a recess near the great Tyrannosaurus.

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