Allosaurus was to late-19th century North America what Megalosaurus was to early 19th-century England: both of these dinosaurs were discovered, and named, multiple times by confused paleontologists, who hadn't yet grasped the finer points of theropod evolution. The difference is that no one in cultured, post-enlightenment Europe ever really raised his voice about the classification of Megalosaurus, while the exact position of Allosaurus on the dinosaur family tree was argued loudly by American fossil-hunters embroiled in the "Bone Wars."
Like all good dinosaur discovery stories, this one has a pre-credit sequence. In 1869, what turned out to be the very first Allosaurus fossil (a partial tail vertebra) wound up in the possession of Ferdinand Vandeveer Hayden, head of the U.S. Geological Survey. Hayden invited the prominent naturalist Joseph Leidy (who had already named a dinosaur, Hadrosaurus, discovered in New Jersey) to examine his find. Leidy wound up assigning the tail bone to the already-existing theropod genus Poekilopleuron, then changed his mind and created the new genus Antrodemus ("cave body").
Allosaurus and the "Bone Wars"
The story of Allosaurus proper begins in 1877, when the famous paleontologist Othniel C. Marsh received a motley assortment of fossils (including fragments of ribs, arm bones, and vertebrae) that had been excavated from the late Jurassic sediments of Colorado's Morrison Formation. After examining these bones, Marsh erected the genus and species Allosaurus fragilis (which roughly translates as "strange fragile lizard"). Apparently, Marsh was either unaware of the existence of Antrodemus, or chose to ignore this inconvenient fact for his own benefit!
The reason Marsh was in such a hurry to name his dinosaur was that he was in the midst of an ongoing feud with rival paleontologist Edward Drinker Cope; both men were digging up the American west as fast as they could, often without heed to the law or the niceties of the scientific method. In the course of these "Bone Wars," Allosaurus wound up being named at least three more times: Creosaurus and Labrosaurus by Marsh, and Epanterias by Cope. So eager were Marsh and Cope to claim priority that they didn't examine these fossils closely enough, erecting new genera when, in fact, they should have known that the remains belonged to the already named Allosaurus (or, if you prefer, the already-already named Antrodemus).
What's ironic is that Marsh and Cope were squabbling over fossil leftovers, when the main course was theirs for the taking--if only they had been patient enough to look for it. After the "type fossil" of Allosaurus was discovered in Colorado, Marsh inexplicably turned his attention to Wyoming; it was up to another paleontologist to find a more complete Colorado Allosaurus specimen in 1883. And in 1879, Cope never even bothered opening a crate sent to him by one of his hired fossil-hunters; it was only in 1903 that it was found to contain a near-complete Allosaurus individual!
The Age of Antrodemus
The mess left by Marsh and Cope didn't go unnoticed by other paleontologists. After the Bone Wars ended, the general feeling was that too many unsupported genera had been constructed on too-flimsy fossil remains. In an influential 1920 paper, Charles W. Gilmore concluded that Marsh's Allosaurus was the same dinosaur as Leidy's Antrodemus--and by the rules of nomenclature, Antrodemus had priority. Creosaurus, Labrosaurus and Epanterias faded into obscurity, and the dinosaur millions of kids know today as Allosaurus was mostly referred to as Antrodemus.
This situation was only rectified in the early 1960's, when paleontologists working the Cleveland-Lloyd quarry in Utah uncovered thousands of tangled Allosaurus bones belonging to anywhere from 40 to 70 separate individuals. (No one knows how all these dinosaurs wound up buried together; the most likely explanation is that they got trapped in the sticky mud of a shrinking watering hole). This wealth of remains led to a reconsideration of the single, obscure, poorly attested vertebra examined by Leidy, and the name Allosaurus regained the ascendance.
Introducing Big Al and Big Al Two
Apart from that crated specimen that was overlooked by Edward Drinker Cope, complete, articulated specimens of Allosaurus were scarce on the ground during most of the 20th century. That all changed in 1991, when a team dispatched by the Museum of the Rockies and the University of Wyoming discovered "Big Al," a 95-percent complete Allosaurus individual measuring 26 feet from head to tail. Five years later, some of the same team members discovered a slightly smaller, but even more complete specimen, which they promptly dubbed "Big Al Two."
Ironically, given its name, Big Al wasn't actually an adult Allosaurus, but a "subadult" (the dinosaur equivalent of a teenager) that was only 87 percent fully grown. Despite his tender age, Big Al didn't have an easy life: this dinosaur had 19 broken bones at the time of his death, as well as signs of a serious bone infection. Crucially, many of the bones in Big Al's right foot were broken or diseased, which probably led directly to this dinosaur's demise--after all, it's hard to run down prey when you have a pronounced limp!