How many dinosaur species roam the Earth today? "About 10,000 ... in the form of birds," says Oxford paleontologist Robert Benson, one of the authors of a study published in Plos Biology that clarifies dinosaurs' evolutionary path. As Astrobiology explains (and as you may be thinking), the belief that birds are descendants of non-bird dinosaurs is already decades-old. What's new here is that the evolution has been tracked by body mass. Per Astrobiology, the study "showed for the first time that when the Mesozoic chapter of Earth’s history came to an abrupt end 65 million years ago, large body mass was not conducive to survival, whereas small—very small—was." In effect, dinosaurs survived by shrinking.
Science 2.0 reports the team estimated the body mass of 426 dinosaur species using the thickness of the creatures' leg bones, and found that the rapid evolution of dinosaurs' body size slowed "shortly after their origins"—with the exception of the evolutionary line that gave rise to birds. Their body size continued to evolve, and the fossil record indicates that the dinosaurs/birds that survived weighed as little as 15 grams, while the smallest non-bird dinosaurs of the time weighed about 600 grams (that's about 1.3 pounds). The former's advantage: food. They just didn't require as much when an "enormous impact" obliterated food supplies. And, says Benson, "the rates of [bird] evolution never slowed down. They continued to find and exploit new niches throughout their ecologic history." As the authors of an accompanying article point out, the study explains birds' "extraordinary ecological diversity, ranging from large, flightless birds, to small migratory species that fly thousands of kilometers every year."