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Did Modern Birds Coexist with Dinosaurs?

Evolutionary Biology Program director Irby Lovette explains the importance of a newly discovered ducklike fossil

Sixty-five million years ago, something cataclysmic happened to the earth--a global mass extinction that now defines the boundary between the geological periods of the Cretaceous and Tertiary. Most paleontologists believe that a major contributor to this mass extinction--perhaps even its primary cause--was the asteroid strike that left a huge crater underneath what is now the Yucatán Peninsula in Mexico and the adjacent Caribbean Sea. In addition to generating an immediate and massive tsunami, an impact of this magnitude must have transformed the global climate. The particles, soot, and acidic sulfur dioxide released into the upper atmosphere would have blocked sunlight for years, resulting in rapid and severe cooling.

The best-known outcome of the Cretaceous-Tertiary mass extinction was the demise of the typical dinosaurs. Fossils of dinosaurs are abundant in Cretaceous deposits but absent from Tertiary rocks. One of the longest-standing controversies in avian evolution also centers on the Cretaceous-Tertiary extinction: did some or all of the major groups of existing birds originate in the Cretaceous (or earlier) and survive the mass extinction event, or did the diversification of modern avian orders occur later in the Tertiary after the extinction of non-avian competitors? Until recently, the fossil evidence has been ambiguous enough that it could be interpreted to support either side of this argument. However, in a recent issue of Nature (January 20, 2005), a team led by researcher Julia Clarke published a new description of a Cretaceous fossil from Antarctica that pre-dates the mass extinction, and has the potential to tip the debate in favor of the mass survival scenario.

The revolutionary aspect of this fossil is that many attributes of the preserved skeleton suggest that it belonged to a bird that resembled a modern duck. Why is the presence of a ducklike bird in the Cretaceous an exciting discovery? We know from many other lines of evidence that the evolutionary split between ducks and other waterfowl happened more recently than the separation of waterfowl from the "chicken" group, which in turn happened more recently than the split between the combined waterfowl and chickens and the "ratite" group (which includes the ostriches, emus, and tinamous). Finding of a duck in the Cretaceous therefore indicates that all of these groups started to diversify while the dinosaurs were still around, a finding that is also supported by genetic evidence of divergence among modern orders of birds.

This fossil will almost certainly get scrutinized by advocates of both camps, and the mass extinction/mass survival controversy may continue if these specialists cannot agree on whether this specimen truly represents an early duck. The finding is also certain to galvanize the search for additional examples of modern birds in the Cretaceous. We will have to wait to see what the next discoveries reveal, but one or two additional examples of undisputed modern birds from the Cretaceous would suffice to end the debate for good.


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