City of Mesa, AZ

Arizona Through Time

Cretaceous Seas



100 million years ago a warm seaway teeming with marine life separated eastern and western North America.  Some terrestrial reptiles returned to the sea in the earlier Triassic Period (252-201 million years ago), some much later.  The three marine reptiles shown here, mosasaur, pliosaur and sea turtle, represent three separate re-invasions of the sea.  Maps courtesy Ron Blakey, Colorado Plateau Geosystems,


This dramatic diorama represents the Black Mesa area of what is now Arizona during the Cretaceous Period.  Marine reptiles congregate in a feeding frenzy near the shore on a warm night.  The long-necked pliosaur has the toothy Encholus in its jaws, unaware that the mosasaur will deliver to it the same fate.


Mosasaurs are not closely related to other marine reptiles but are in actuality gigantic lizards that have returned to the sea from the land.  These voracious predators developed an extra jaw hinge that allowed them to open their mouths widely and clamp down on their prey.  Mosasaurs are most closely related to today's monitor lizards and Gila monsters.  Fragmentary remains have been found in the deposits of Cretaceous seas covering northeastern Arizona 100 million years ago.


Pliosaurs are a type of plesiosaur, a marine reptile with a large bulbous body, long neck, small head and four large flippers.  Pliosaurs had a shorter neck and longer head.


The sea turtle Desmatochelys belongs to the same lineage as modern sea turtles.  Turtles are the only group of marine reptiles from the Cretaceous Period to survive today.


Xiphactinus audax was the largest bony fish of the Cretaceous, reaching a length of 20 feet or perhaps larger.  Xiphactinus swallowed its prey whole, and several specimens have been found with large, undigested prey inside.  It is likely that these Xiphactinus died from trying to swallow the large, struggling prey.  Xiphactinus fossils have been found worldwide, including the middle Cretaceous Black Mesa sediments of Arizona, about 100 million years old.

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