This freshwater, carnivorous reptile is among the very earliest alligatoroids known to science. The first Brachychampsa fossil was discovered in Montana's Hell Creek Formation—an ancient suite of rocks that was laid down near the end of the Cretaceous period. That individual animal lived not so long before the Cretaceous-Tertiary extinction that wiped out many of Earth's species some 65 million years ago.

The first alligator ancestors evolved some 245 million years ago. About 80 million years ago, during the Cretaceous period, crocodilians appeared. This group includes alligatoroids, such as Brachychampsa, as well as their close relatives the crocodiles and caimans.