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Albertosaurus - This Late Cretaceous Tyrannosaur Likely Hunted in Packs
An Opportunistic Killer
Albertosaurus lived in North America about 75 million years ago, during the late Cretaceous period. Over 30 individuals have been found, but none outside of the Canadian province Alberta. A single dig site yielded 26 individual Albertosaurus skeletons. This find strongly suggests that Albertosaurus hunted in groups, as many smaller theropods are likely to have done.
Hunting in a group would have allowed Albertosaurus and other theropods lacking the size of their larger cousin Tyrannosaurus Rex to circle and surround prey, and attack from multiple angles. This would have helped smaller (and in the case of Albertosaurus medium sized) theropods to bring down very large prey as a team, or to bring down heavily armored prey like Triceratops; who was also a late Cretaceous inhabitant of North America. Another advantage of working as a group to bring down larger prey animals would have been that there would have been enough meat to go around following a kill.
Albertosaurus hunted both herbivores and carnivores and may well have been a good enough swimmer to include fish and other aquatic creatures in its diet. Quite simply, it is fairly certain that an Albertosaurus would have killed and devoured just about everything in its path; even if it needed the assistance of some hunting partners to bring a large or particularly well defended animal down.
An Intimidating Presence
While Albertosaurus only weighed about 2 tons and was roughly half the size of T-Rex, several interesting finds have suggested Albertosaurus hunted in packs; including the discovery of 26 individual skeletons in a single dig site, and a find of dinosaur footprints in which three Albertosaurs appear to have been traveling together.
This group hunting behavior may have made a group of Albertosaurs an even more threatening presence than T-Rex. The two killers may even have been in direct competition since they existed at roughly the same time in roughly the same areas. There is very strong evidence that both Albertosaurus and T-Rex considered other theropods to be on the menu, and T-Rex skeletons have been found with deep cuts in the bones made by T-Rex teeth; so it is very likely that T-Rex was cannibalistic, and it's hard to believe he would pass up the opportunity to kill and eat a lone Albertosaur. Since this evidence of T-Rex cannibalism (or at least the tendency to kill off members of its own species) contradicts the idea of cooperation among Tyrannosaurus Rex individuals, and since Albertosaurus was known to have killed and presumably eaten other theropods, it is intriguing to imagine whether a group of Albertosaurs would have considered the loner T-Rex to be a viable food source. If this were the case, it seems unlikely that a lone T-Rex would have fared well against a pack of smaller, faster and more agile versions of itself.
Like the T-Rex, Albertosaurus had small arms with two fingers each, and a powerful tail used for balance and speed. Albertosaurus' tail was so impressive in fact, that many paleontologists believe Albertosaurus would have made a good swimmer in spite of its lack of arm strength. While T-Rex was likely too heavy and relatively cumbersome to be an accomplished swimmer, Albertosaurus may not have had a problem, which means that large herbivores who were accustomed to escaping from large theropods by fleeing into the water may have been in for a surprise when a pack of Albertosaurs didn't hesitate to follow the terrified animal and in all likelihood swim faster than their victim could.