describe this creature's physical features
It was a typical large theropod, having a massive skull on a short neck, a long, slightly sloping tail and reduced forelimbs.
Allosaurus
"Allosaurus" was a typical large theropod, having a massive skull on a short neck, a long, slightly sloping tail and reduced forelimbs. 
how many years ago did the allosaurus exist? 155 to 145 million years ago which period was that? The late Jurassic period
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Allosaurus [SEP] Description
"Allosaurus" was a typical large theropod, having a massive skull on a short neck, a long, slightly sloping tail and reduced forelimbs. "Allosaurus fragilis", the best-known species, had an average length of , with the largest definitive "Allosaurus" specimen (AMNH 680) estimated at long, and an estimated weight of . In his 1976 monograph on "Allosaurus", James H. Madsen mentioned a range of bone sizes which he interpreted to show a maximum length of . As with dinosaurs in general, weight estimates are debatable, and since 1980 have ranged between , , and for modal adult weight (not maximum). John Foster, a specialist on the Morrison Formation, suggests that is reasonable for large adults of "A.