One theory to explain the large South American animals involves locality. At the beginning of the Mesozoic era, all the land on Earth was merged into the continent of Pangea. During the Jurassic period, the supercontinent broke into the continent of Laurasia (which would eventually become North America and Asia) and Gondwanaland (eventually Africa, Antarctica, Australia, India, and South America). And not long afterward, South America and Africa began to split apart.

Most scientists theorize these splits were the pivotal points in these dinosaurs’ evolution. For a short time, the dinosaurs crossed a land bridge from North to South America; geologic activity eventually destroyed the bridge, cutting off access and allowing creatures like the migrated Megaraptor to evolve separately in the south. The North American animals, such as the Tyrannosaurus rex, developed a specialized skull, forelimbs, and pelvis; the South American dinosaurs, such as the Giganotosaurus, maintained most of the general features of their ancestors and became much larger.

Other scientists believe that southern Megaraptors and their northern counterparts originally evolved separately from common ancestors. They suggest that the reason why the two carnivorous giants like the Tyrannosaurus rex and Giganotosaurus resembled each other was possibly due to similar environmental conditions (this is called parallelism); when the landmasses began to break up, the animals continued to evolve separately.