Given its relative obscurity today--at least compared to more famous dinosaurs like Tyrannosaurus Rex and Triceratops--most people are surprised to learn that Troodon was discovered in the mid-19th century. In fact, the classification and naming of this dinosaur makes for one of the most convoluted stories in all of paleontology, and it has only been in the last couple of decades that Troodon has rested on a reasonably secure footing.
Way back in 1856, the famous American paleontologist Joseph Leidy found himself in receipt of a single tooth that had been discovered in Montana's Judith River formation. Leidy promptly erected the new genus Troodon ("wounding tooth"), but classified it as a lizard rather than a true dinosaur. So Troodon languished for nearly 50 years, until a reexamination of the tooth in 1901 caused another paleontologist to ascribe this genus to a type of dinosaur closely related to Megalosaurus. (This wasn't much of an improvement, since Megalosaurus was, and remains, one of the least understood meat-eating dinosaurs!)
The story took another twist 20 years later, when Charles W. Gilmore examined the tooth yet again and concluded that Troodon was in fact a pachycephalosaur (i.e., bone-headed dinosaur) closely related to Stegoceras (an entirely different dinosaur from Stegosaurus). So convinced was Gilmore, and contemporary paleontologists, about this classification that for a couple of decades pachycephalosaurs were referred to as "troodonts." That all changed in 1945, when the equally distinguished paleontologist Charles M. Sternberg demonstrated conclusively that Troodon was a theropod, not an ornithopod.
A New Complication: The Discovery of Stenonychosaurus
Confused yet? Well, brace yourself for more. Back in 1932, Sternberg had examined some bone fragments from the Canadian province of Alberta and erected the new theropod genus Stenonychosaurus ("narrow-clawed lizard"), which he determined was closely related to Coelurus. Twenty years later, in 1951, Sternberg speculated that the unusual foot structure of Stenonychosaurus went foot-in-mouth (so to speak) with the unusual tooth structure of Troodon. Might these have been the same dinosaur?
Let's jump ahead another 20 years or so, to 1969. A paleontologist named Dale Russell described a new, unusually complete specimen of Stenonychosaurus that had been discovered in Alberta's Dinosaur Provincial Park formation. This individual's unusually large brain-to-body ratio caused Russell to speculate how it might have evolved in the absence of the K/T Extinction that killed all the dinosaurs; technically, Russell's intelligent "dinosauroid" was based on Stenonychosaurus, not Troodon.
Rather than Coelurus, Russell associated Stenonychosaurus most closely with the feathered dinosaur Saurornithoides. It was up to the paleontologist Phil Currie, in 1987, to re-examine the fossil evidence for Stenonychosaurus and two other extremely obscure theropod genera (Polyodontosaurus and Pectinodon) and conclude that they were all "junior synonyms" of Troodon. (At least one paleontologist attempted to assign Saurornithoides to Troodon as well, but as of now, that dinosaur remains in its own genus.)
The End of the Troodon Story? Not Quite
So has the controversy about Troodon been settled once and for all? Not by a long shot. Only a few years after Stenonychosaurus was absorbed into Troodon, paleontologists began to wonder if the Troodon umbrella had become too wide, and some of the specimens referred to it deserved their own genera (or at least their own species). The specimens that had originally been used to diagnose Stenonychosaurus, for instance, are referred to in some quarters as Troodon inequalis, and there was an attempt to describe the Pectinodon fossils as Troodon bakkeri (after paleontologist Robert Bakker). These would join the Troodon species erected by Leidy 100 years before, T. formosus, as well as one other controversial species, T. asiamericanus.
There is one final twist to this story. A 2011 paper by a group of paleontologists, reviewing all the fossil evidence for Troodon and its associated species and genera, proposes that Pectinodon may be a valid genus after all, and that Joseph Leidy's tooth was an insufficient basis for erecting the genus Troodon in the first place. It will probably take years for the dust to settle, but the possibility exists that Troodon will be rendered a nomen dubium, and this big-brained, late Cretaceous theropod may revert to Stenonychosaurus, Pectinodon, or some other name!