Nowadays, pretty much all paleontologists agree that modern birds descended from dinosaurs. However, that wasn’t the case in the 1960's, when John H. Ostrom was the first researcher to propose that dinosaurs had more in common with big, flightless birds like ostriches than with contemporary reptiles (to be fair, the heavyweight paleontologist Othniel C. Marsh had proposed this idea in the late 19th century, but he didn't have enough proof at his disposal to carry the weight of scientific opinion).
Ostrom's theory about the dinosaur-bird evolutionary link was inspired by his 1964 discovery of Deinonychus, a large, bipedal raptor that had some uncannily birdlike characteristics. Today, it's (nearly) an established fact that Deinonychus and its fellow raptors were covered with feathers, not a popular image a generation ago. (In case you were wondering, those "Velociraptors" in Jurassic Park were really Deinonychus, disregarding the fact that they were portrayed with reptilian skin.)
In discovering Deinonychus, Ostrom smashed open the dinosaur equivalent of a hornet's nest. Paleontologists weren't used to dealing with muscular, man-sized, predatory dinosaurs, which prompted speculation about whether an ostensibly cold-blooded reptile could engage in such energetic behavior. In fact, Ostrom's student Robert Bakker was the first paleontologist to forcefully propose that all theropod dinosaurs were warm-blooded, a theory that's currently on slightly shakier ground than the dinosaur-bird connection.
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