Climate and Geography 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. Conditions were as hot and muggy as in the Jurassic, with the added twist of rising sea levels and the spread of endless swamps--yet another ecological niche in which dinosaurs (and other prehistoric life) could prosper.
Terrestrial Life It was during the Cretaceous period that dinosaurs really came into their own. Thousands of genera roamed the slowly separating continents, including raptors and tyrannosaurs. There were also 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. 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 left standing by the time of the K/T Extinction were the plant-eating ankylosaurs, notably Ankylosaurus and Euoplocephalus, and the pachycephalosaurs ("thick-headed lizards"). The early mammals still kept to themselves; they would only emerge from the shadows after the dinosaurs had gone extinct.
Marine Life Shortly after the beginning of the Cretaceous period, the ichthyosaurs ("fish lizards") vacated the scene, to be replaced by mosasaurs, gigantic pliosaurs like Kronosaurus, and slightly smaller plesiosaurs like Elasmosaurus. A new breed of bony prehistoric fish, known as teleosts, roamed the seas in enormous schools, and there were the usual assortment of prehistoric sharks; both fish and sharks would benefit immensely from the extinction of the marine reptiles.
Avian Life By the end of the Cretaceous period, the pterosaurs (flying reptiles) had finally attained the enormous sizes of their cousins on land and in the sea, Quetzalcoatlus being the most spectacular example. This was the pterosaurs' last gasp, though, as they were gradually crowded out of the sky by the first true prehistoric birds (which evolved from land-dwelling feathered dinosaurs, not pterosaurs).
Plant Life The main innovation of the Cretaceous period was the evolution of flowering plants, which spread across the separating continents, along with thick forests and other kinds of dense, matted vegetation; this is also when grass first started to appear on the scene. 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. The herbivorous dinosaurs that fed on the plants died, as did the carnivorous dinosaurs that fed on the herbivorous dinosaurs. The way was now clear for the evolution of mammals.