Tyrannosaurus rex arose during the Cretaceous period about 85 million years ago, and thrived as a top land predator until the dinosaurs went extinct 20 million years later. This skeleton, on display in Chicago's Field Museum, is a cast of perhaps the world's most famous fossil: "Sue," a 67-million-year-old T. rex discovered in 1990 in South Dakota by field paleontologist Sue Hendrickson. It is the most complete, best preserved, and, at 42 feet (12.8 meters), the largest T. rex specimen ever found.
During this period, oceans formed as land shifted and broke out of one big supercontinent into smaller ones.
Continents were on the move in the Cretaceous, busy remodeling the shape and tone of life on Earth. At the start of the period, dinosaurs ruled the loosening remnants of the supercontinent Pangaea as rodents scurried at their feet through forests of ferns, cycads, and conifers. At the end of the period, about 80 million years later, oceans filled yawning gaps between isolated continents shaped much as they are today. Flowering plants were spreading across the landscape. And mammals sat poised to fill the void that soon would be left by the vanished dinosaurs. A giant crater smoldered on what would become known as the Yucatán Peninsula.
Whether or not the asteroid or comet that carved the Chicxulub crater