Mesozoic Era: Cretaceous Period 

145–66 Ma

The Cretaceous: extraterrestrial crash

Compared to the mass-extinction-punctuated period boundaries that preceded, the Cretaceous was ushered-in almost peacefully and continuously from the Jurassic, though it certainly didn’t end that way. Where we ended talking about the “bird-hipped” dinosaurs in the Jurassic, we resume our story first talking here about the saurischian, or “lizard-hipped”, dinosaurs. Two large, long-necked sauropods, like the well-known giant herbivore Brontosaurus or lesser known genera such as Argentinosaurus and Patagotitan, are shown ambling out from a wooded area. These organisms were the largest and longest terrestrial animals in the history of life on Earth, comparable in weight and length only to marine mammals today like the blue whale. For contrast, the sauropods were about 20 times heavier than the average African bush elephant, and more than 3 times the height. These massive animals reached their peak diversity in the late Jurassic and went extinct in the Cretaceous. On the right is everyone’s favorite prehistoric predator and probably needs no introduction, Tyrannosaurus rex. A theropod dinosaur also within the saurischians, T. rex approached 12.5 meters (40 feet) in length from head to tail and probably weighed nearly 9000 kilograms (or close to 20,000 pounds). Like Dunkleosteus in the Devonian before, T. rex probably had one of the largest bite forces in the history of animals, certainly one of the largest amongst terrestrial animals. One look at the skeleton of T. rex and it is no wonder that paleontologists presume it was an apex predator, or top of the Cretaceous food chain, hunting prey organisms like hadrosaurs and ceratopsians, such as our Triceratops, pushed across the edge to the previous panel. And just who was the reason we needed the space in the Cretaceous? It was our duck-billed herbivore, the prominent hadrosaur in the very foreground—Hypsibema missouriensis—the state dinosaur of Missouri. We certainly couldn’t have a life through time mural in the state of Missouri without her! She’s seen here approaching the water’s edge near another new plant, Magnolia, an angiosperm that also first appeared in the Cretaceous and is still around today. In the far background, an ominous asteroid approaches the lower Paleogene boundary that was soon to set life on a different course.