In the plains and woodlands of late Jurassic North America, circa 150 million years ago, two dinosaurs stood out for their size and majesty: the gentle, small-brained, impressively plated Stegosaurus, and the agile, three-fingered and perpetually hungry Allosaurus. Before these dinosaurs meet in to-the-death combat, let's look at their specs. (Don't be afraid to express your opinion about how the battle will turn out, and see more dinosaur death duels.)
In the Near Corner - Stegosaurus, the Spiked, Plated Dinosaur
About 30 feet long from head to tail and weighing in the neighborhood of two to three tons, Stegosaurus was built like a Jurassic tank. Not only did this plant-eater have two rows of roughly triangular bony plates lining its back and neck, but its skin was extremely tough (and probably much harder to bite through than the epidermis of an elephant). Its name, "roofed lizard," was bestowed before paleontologists properly understood the orientation of its famous "scutes," or bony plates.
Advantages. In close combat, Stegosaurus could rely on its spiked tail--also known as a "thagomizer"--to deter hungry theropods. We don't know how fast the average Stegosaurus could swing this deadly weapon, but even a glancing blow might well have taken out a theropod's eyes, or inflicted some other nasty wound that would convince it to go after easier prey. Stegosaurus' squat build, and low center of gravity, also made it difficult to dislodge from an advantageous position.
Disadvantages. Stegosaurus is the genus everyone has in mind when they talk about how spectacularly dumb dinosaurs were. This hippopotamus-size herbivore only possessed a brain the size of a walnut, so there's now way it could outsmart a theropod like Allosaurus (or even a giant fern, for that matter). Stegosaurus was also considerably slower than Allosaurus, thanks to its low-to-the-ground build and much shorter legs.
In the Far Corner - Allosaurus, the Jurassic Killing Machine
Pound for pound, a full-grown Allosaurus would have been an even match for an adult Stegosaurus. The largest specimens of this two-legged killing machine measured about 40 feet from head to tail and weighed about three tons. Like Stegosaurus, Allosaurus has a slightly deceptive name--it means "different lizard," which didn't tell early paleontologists much except that it was an entirely different dinosaur from Megalosaurus.
Advantages. The deadliest weapon in Allosaurus' armory was its teeth. This theropod's plentiful choppers grew to to lengths of three or four inches, and were continually growing, and being shed, during its lifetime--meaning they were more likely than not to be razor-sharp and ready for the kill. We don't know quite how fast Allosaurus was able to run, but it's a sure bet that it was speedier than the plodding, walnut-brained Stegosaurus.
Disadvantages. As fearsome as it was, there's no evidence that Allosaurus ever got the hang of hunting in packs, which would have been a considerable advantage when attempting to take down a creature the size of a Sherman tank. It's also unlikely that Allosaurus could do much with its relatively puny arms, which were still, however, much deadlier than the near-vestigial appendages of the much later Tyrannosaurus Rex.
Our full-grown Allosaurus happens upon the Stegosaurus while it's busy feeding on low, tasty shrubs. He lowers his neck, builds up a head of steam, and butts the Stegosaurus in the flank with his big, bony head. Startled, but not toppled, the Stegosaurus lashes out with the thagomizer on its tail, inflicting only superficial wounds on Allosaurus' hind legs; at the same time, it crouches closer to the ground, so as not to expose its soft underbelly. Undeterred, the Allosaurus charges again, and this time succeeds in flipping the Stegosaurus onto its side.
And the Winner Is...
Allosaurus! Once dislodged from its defensive position, the slow-witted Stegosaurus is nearly as helpless as a flipped turtle. A modern tiger would mercifully bite its prey in the neck and end its misery, but Allosaurus digs into Stegosaurus' belly and eats its entrails while he's still alive. Other hungry theropods cluster around the scene, eager for a taste but sensible enough to let the much bigger Allosaurus eat its fill first.