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A bit like a nimbler, scaled-down version of the much later Tyrannosaurus Rex, Allosaurus was one of the most common predatory dinosaurs of the Jurassic period, a fearsome meat-eater with sharp teeth and a well-muscled body (though it's unclear how fast it was able to run). Allosaurus had an especially prominent head, some of the anatomical features of which (such as the distinctive ridges over the eyes) may have been meant to attract the opposite sex and not to intimidate smaller dinosaurs. (See 10 Facts About Allosaurus and a gallery of Allosaurus pictures.)
As a sign of how common Allosaurus was in the plains and woodlands of modern-day North America, paleontologists have discovered dozens of fossils, more or less intact, many of them in a single quarry in Utah. It's likely that these unfortunate Allosaurus individuals were attracted by various herbivorous dinosaurs (most likely small, bipedal ornithopods) mired helplessly in the mud, and became trapped themselves when they stopped for a quick snack. (See an article about the discovery of Allosaurus.)
By the way, in late Jurassic North America, Allosaurus coexisted with another famous dinosaur, the spiked, plated Stegosaurus. While it's unlikely that even a hungry Allosaurus adult would have dared to take on a full-grown Stegosaurus, such encounters must have occasionally transpired. For more, see Allosaurus vs. Stegosaurus - Who Wins?.