Everybody knows that Tyrannosaurus had small arms tipped in only two fingers. The relatively small arms of the Late Cretaceous predator are part of its charm. When paleontologist Henry Fairfield Osborn described Tyrannosaurus in 1905, however, the fingers and forearm of the dinosaur were missing. Exactly what the giant theropod's arms looked like was a matter of conjecture.

At the time Tyrannosaurus was discovered, the tyrant dinosaurs were poorly known. The skulls and partial skeletons were all generally perceived as belonging to giant dinosaurs, and a lack of overlap between some of the bones caused some species—like Tyrannosaurus rex itself—to carry more than one name. Several specimens discovered by fossil hunter Barnum Brown between 1900 and 1907 filled in the general picture, but even the famous specimen that was reconstructed in the American Museum of Natural History lacked fingers. Given that the Jurassic predator Allosaurus had three fingers, and Tyrannosaurus seemed to be its Cretaceous successor, it seemed probably that the tyrant kind had three fingers, too.

But Osborn and his team at the AMNH were not the only paleontologists working on tyrannosaurs. In 1914 the Canadian paleontologist Lawrence Lambe published the paper "On the Fore-Limb of a Carnivorous Dinosaur." The specimen was a nearly complete skeleton that had been found along Alberta's Red Deer River by Charles Sternberg, Jr. while he was out collecting with his father and brothers. The fossil was "an unusually perfect skeleton" of a tyrant that Lambe would name Gorgosaurus in another publication, but the anatomy of the dinosaur's arm was called out for special attention in an initial notice.

The Gorgosaurus skeleton had only two fingers, Lambe reported, and there was no reason to believe that any fingers were missing. The bone that would have supported the third finger—one of the metacarpal bones of the hand—was nothing more than a vestigial splint, ruling out the possibility that one of the fingers had been lost during fossilization. At last, paleontologists knew what the complete forelimbs of such dinosaurs looked like.

Lambe was puzzled by why such a large animal—he estimated it to be about 26 feet long—had such puny, two-fingered arms, and the discovery caused other paleontologists to revise what they thought about the hands of Tyrannosaurus. Citing his colleague Charles W. Gilmore, in 1916 Osborn noted that it was "probable that Tyrannosaurus will prove to be functionally didactyl," too. The discovery of other two-fingered tyrant dinosaur skeletons supported this conclusion. The extra finger present in early restorations and reconstructions was eventually lost.

Curiously, though, the first complete forelimb of Tyrannosaurus rex was not found until 1989. No one was surprised by the fact that there were only two fingers, but this confirmation is a small lesson in the way paleontology works. Comparative anatomy is one of the cornerstones of the science, and paleontologists are constantly comparing the bones of different creatures to gain insights into the anatomy of organisms that are incompletely known. If our knowledge of a fossil species is incomplete but we know a good deal more about closely related forms, then the anatomy of those relatives can help us fill in the gaps. Though Tyrannosaurus has been the most celebrated of the tyrant dinosaurs, reconstructions and restorations of the enormous carnivore have historically owed debts to more complete skeletons of its cousins Albertosaurus and Gorgosaurus.