Anyone who's caught a good portion of "Jurassic Park" knows that Tyrannosaurus rex isn't to be trifled with. Up to 45 feet (14 meters) long (for its usual posture was horizontal), its 4-foot- (1.2-meter-) long jaw could crush your head once it got a hold of you [source: Padian]. Once you were firmly in its jaws, the dinosaur's serrated teeth could gnaw your face off.
But wait a second. Our despotic dinosaur had an adaptation that isn't just a cosmetic embarrassment, but an actual impediment. It's those tiny arms that were so wildly ineffective that poor T. couldn't even use them to grab prey or put said prey in their mouth [source: Barrett]. In fact, Mr. or Ms. Rex couldn't use their little arms to push themself off the ground [source: Barrett]. (It's worth remembering that a lack of arms has never kept some snakes from being formidable predators.)
Even stranger, T. rex only had two fingers on each hand, unlike most dinosaurs with three -- which makes grabbing a lot easier. Apparently, evolution might've slowly been phasing out the arms of the T. rex, instead relying on the strong jaw of the animal to pick up and devour prey. Some even propose that if the dinosaur had stuck around much longer -- a million years or so -- those arms would've adapted right into oblivion [source: Barrett].