The Cretaceous period is when dinosaurs attained their maximum diversity, as ornithischian and saurischian families branched off into a bewildering array of armored, raptor-clawed, thick-skulled, and/or long-toothed and long-tailed meat- and plant-eaters. The longest period of the Mesozoic Era, it was also during the Cretaceous that the earth began to assume something resembling its modern form, though life (of course) was dominated not by mammals but by terrestrial, marine and avian reptiles.
Geography and climate. During the early Cretaceous period, the inexorable breakup of the Pangaean supercontinent continued, with the first outlines of modern North and South America, Europe, Asia and Africa taking shape. North America was bisected by the Western Interior Sea (which has yielded countless fossils of marine reptiles), and India was a giant, floating island in the Tethys Ocean. Conditions were generally as hot and muggy as in the preceding Jurassic period, albeit with intervals of cooling and the added twist of rising sea levels and the spread of endless swamps--yet another ecological niche in which dinosaurs (and other prehistoric animals) could prosper.
Terrestrial Life During the Cretaceous Period
Dinosaurs really came into their own during the Cretaceous Period. Over the course of 80 million years, thousands of meat-eating genera roamed the slowly separating continents, including raptors, tyrannosaurs and other varieties of theropods, including the fleet-footed ornithomimids ("bird mimics"), the strange, feathered therizinosaurs, and an uncountable profusion of small, feathered dinosaurs, including the uncommonly intelligent Troodon.
As for the herbivorous dinosaurs of the Cretaceous period, the classic sauropods of the Jurassic period had pretty much died out, but their descendants, the lightly armored titanosaurs, spread to every continent on earth and attained even more massive sizes. Ceratopsians (horned, frilled dinosaurs) like Styracosaurus and Triceratops became abundant, as did hadrosaurs (duck-billed dinosaurs), which were especially common at this time, roaming the plains of North America and Eurasia in vast herds. Among the last dinosaurs standing by the time of the K/T Extinction were the plant-eating ankylosaurs and pachycephalosaurs ("thick-headed lizards").
Mammals. During most of the Mesozoic Era, including the Cretaceous period, mammals were sufficiently intimidated by their dinosaur cousins that they spent most of their time high up in trees or huddling together in underground burrows. Even still, some mammals had enough breathing room, ecologically speaking, to allow them to evolve to respectable sizes; witness the 20-pound Repenomamus, which actually ate baby dinosaurs!
Marine Life During the Cretaceous Period
Shortly after the beginning of the Cretaceous period, the ichthyosaurs ("fish lizards") vacated the scene, to be replaced by vicious mosasaurs, gigantic pliosaurs like Kronosaurus, and slightly smaller plesiosaurs like Elasmosaurus. A new breed of bony fish, known as teleosts, roamed the seas in enormous schools, and there were the usual assortment of ancestral sharks; both fish and sharks would benefit immensely from the extinction of their marine reptile antagonists.
Avian Life During the Cretaceous Period
By the end of the Cretaceous period, pterosaurs (flying reptiles) had finally attained the enormous sizes of their cousins on land and in the sea, the 35-foot-wingspan Quetzalcoatlus being the most spectacular example. This was the pterosaurs' last gasp, though, as they were gradually crowded out of the skies by the first true prehistoric birds (which evolved from land-dwelling feathered dinosaurs, not pterosaurs, and were better adapted for changing climatic conditions).
Plant Life During the Cretaceous Period
As far as plants are concerned, the main innovation of the Cretaceous period was the rapid diversification of flowering plants, which spread across the separating continents, along with thick forests and other varieties of dense, matted vegetation. All of this greenery not only sustained the dinosaurs, but it also allowed the co-evolution of a wide variety of insects, especially beetles.
The Cretaceous-Tertiary Extinction Event
At the end of the Cretaceous period, 65 million years ago, a meteor impact on the Yucatan Peninsula raised huge clouds of dust, blotting out the sun and causing most of this vegetation to die out. (Conditions may have been aggravated by the collision of India and Asia, which fueled an immense amount of volcanic activity in the "Deccan Traps.") The herbivorous dinosaurs that fed on these plants died, as did the carnivorous dinosaurs that fed on the herbivorous dinosaurs. The way was now clear for the evolution and adaptation of the dinosaurs' successors, the mammals, during the ensuing Tertiary period.
Next: The Paleocene Epoch