At the beginning of the Dinosaur era, all of the continents were
stuck together in a huge super-continent called Pangea ("all-Earth").
Pangea slowly broke up into several pieces, each piece carrying
a group of dinosaurs with it. These groups of dinosaurs were originally
the same, but as time passed, they evolved in different ways because
of different environmental conditions. So we had T-Rex's in North
America and the Giganotosauruses in South America--similar in looks,
size, and life style, but not the same species. The groups remained
separated for tens of millions of years. Images
courtesy Dr. Ron Blakey at Northern Arizona University.
Then, over a
period of a few million years at the end of the Cretaceous, the
climate grew colder and the global sea level fell. The shallow seas
and coastal plains slowly dried up, and dry "land bridges"
between the continents appeared. Sea-going mosasaurs and ichthyosaurs
would have disappeared as the shallow seas dried. On land, dinosaur
habitats broke into ever-shrinking patches, forcing dinosaurs to
move about looking for places to live. Some dinosaurs may have crossed
the land bridges between the continents to mix with cousins they
had been separated from for millions of years. The mixing would
have brought increased competition between similar species for food.
Species efficient at getting food would have survived; the others
would have died out.