The going theory about the Tyrannosaurus rex, in case you didn’t know, had been until very recently that this famous mega-dinosaur sported stumpy little arms as an evolutionary adaptation related to its jumbo body size. Then a fossil of Raptorex kriegsteini—perhaps 1/100th the size of T. rex, its descendant—came along and upended that notion with its own disproportionately puny forelimbs.