Allosaurus lived in the Late Jurassic period some 150 million years ago.  The dominate predator of the river floodplains and forests of what is today's western United States, Allosaurus had to contend with the likes of Apatosaurus (popularly known as Brontosaurus) and the truly massive Brachiosaurus.  It may have been a solitary hunter or hunted in packs, but probably preyed on the young, the weak, and those that strayed from the packs.

Aurora #736, Allosaurus