February 16, 2010
The chicken on the table, the pigeon on the street, the parrot in the zoo: all of them are living descendants of dinosaurs. Over the past ten years a flood of fossil evidence, from evidence of bird-like breathing apparatus to remnants of pigments in preserved feathers, has confirmed beyond a reasonable doubt that birds are dinosaurs. Tyrannosaurus and a turkey have more in common with each other than either does with a crocodile or lizard.
But some scientists are not pleased with this consensus. Way back in the 1920s it was thought that birds and dinosaurs were independent offshoots of a more ancient common stock. This hypothesis eventually was tossed out, but some researchers still believe it is true. This week in the journal PNAS, for example, scientist John Ruben says not only that birds evolved independently of dinosaurs, but that some creatures we now call dinosaurs were actually descendants of early birds.
While Ruben’s article has been much ballyhooed by media outlets, it is actually only a commentary, or the equivalent of an opinion piece. In it Ruben states that the discovery of the feathered Deinonychus-relative Microraptor refutes the idea that birds evolved from feathered dinosaurs, as Microraptor appears to have been adapted to jumping out of trees to glide. Other dinosaur specialists have previously hypothesized that flight evolved in dinosaurs that ran and jumped off the ground. If creatures such as Microraptor represent how flight evolved, then, dinosaurs that lived on the ground would either become irrelevant to understanding bird origins or, as Ruben argues, would have to be considered birds that lost their ability to fly.
Despite the credulous repetition of this story, however, Ruben’s argument is cut down by several flaws. The first problem is that we cannot be sure that Microraptor is a good example of how flight evolved. By the time it lived, 120 million years ago, there had been birds for millions of years, and it lived at the same time as early birds like Confuciusornis. Combined with what we know about its close relatives, it appears that Microraptor was a unique kind of specialized raptor that independently evolved the ability to glide, and perhaps even fly. Whether its mode of gliding can inform us about how birds evolved flight will depend upon which group of feathered dinosaurs turns out to be most closely related to the first birds (which may be strange forms like Epidexipteryx).
Secondly, the “trees down” versus “ground up” debate about the origin of flight is no longer useful in addressing the evolution of birds. So many feathered dinosaurs have been found, and continue to be discovered, that paleontologists are continually having to reassess ideas about how the first birds evolved. Perhaps some of the old hypotheses will turn out to be correct, or perhaps flight evolved in a way we did not expect, but framing things in terms of two mutually-exclusive hypotheses hinders discussion over avian origins rather than helps it.
Furthermore, there is no compelling reason to regard dinosaurs such as Velociraptor as flightless birds. This proposal has often been made by critics of the “dinosaur-bird” connection in order to made sense of the many feathered dinosaurs that have been found. It is a sort of taxonomic reshuffling that removes anything bird-like away from dinosaurs despite all the characteristics these animals have in common with other dinosaurs.
Simply put, Ruben’s hypothesis does not stand up to scrutiny, but what I find even more frustrating is the repetition of such fantastic claims by news outlets. In this increasingly fragmented media landscape, knowledgeable science writers who recognize a fishy story when they see one are getting outnumbered. More often, websites and newspapers simply reprint press releases issued by universities and museums (science writers call this “churnalism”), and this policy sometimes lets questionable science slip through the cracks.
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