A shortened form of Allosaurus, a large carnivorous theropod dinosaur from the Jurassic period.
From 'Allosaurus' (from Greek 'allo-' meaning other/different + 'sauros' meaning lizard), called 'different lizard' because it was different from other reptiles. Used informally by paleontologists and dinosaur enthusiasts.
Allosaurus was basically the Tyrannosaurus rex of its time—it hunted sauropods in the Jurassic period about 100 million years before T. rex existed, and some scientists think allosaurs hunted cooperatively in packs, making them even scarier than the lone wolf T. rex.
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