As the world learned in the recent documentary March of the Penguins, penguins withstand extreme weather and waddle hundreds of kilometers to mate, all in the name of species survival. Now, fossil and genetic evidence suggest that penguin ancestors living about 65 million years ago survived even more extreme conditions, including the impact that may have led to the demise of the dinosaurs.
Paleontologist Ewan Fordyce holds a wing bone in position against a model of a penguin, which was reconstructed from the earliest known fossil penguin skeleton. Scientists used the fossil to calibrate a genetic clock to find out when penguin ancestors got their start. Image is courtesy of University of Otago.
Most scientists agree that some ancestors of modern bird groups living today, including penguins, coexisted with dinosaurs. Disagreement exists, however, about whether those ancestors were part of a single lineage that survived the extinction event and then quickly radiated into modern groups, or if that branching out took place prior to the extinction event, and more than one lineage managed to survive.
Determining when ancestors of todays birds first branched out has been a contentious issue for a decade or so, says Ewan Fordyce, a vertebrate paleobiologist at University of Otago in New Zealand. Geologists know from the fossil record that modern species belong in groups that had ancient origins, he says, but the fossil record for the origins of those modern birds is pretty patchy and highly debated.
To clear up the issue, Fordyce and a team of researchers led by Kerryn Slack, a molecular biologist at Lund University in Sweden, turned to four penguin skeleton fossils that had gone largely unstudied until recently. The skeletons, discovered in mid-1980s within New Zealands Waipara Greensand Formation, represented two species within a newly recognized genus Waimanu, the larger of which would have stood about 1 meter tall, similar to todays emperor penguins. The team dated the fossils on average to be about 60 million years old, making them the oldest known fossils in the penguin lineage.
Next, the researchers studied the DNA of birds living today, determining that modern penguins are closely related to storks, loons and albatrosses, and that other groups such as falcons, hawks, sparrows and ducks are more remote. Once the researchers established this pattern of relationships, they took the ancient penguin fossil date and used it to calibrate a molecular clock to determine the likely splitting times for modern bird groups, Fordyce says.
The genetic relationships imply that if ancient penguins were established by 60 million years ago, then modern bird lineages have older origins, likely branching out at least 74 million years ago well before the mass extinction event about 65 million years ago. Modern birds branching out at that time may have been filling a niche left open by declining populations of flying pterosaurs, the researchers say.
While Alan Feduccia, a paleobiologist at the University of North Carolina in Chapel Hill, agrees that ancestors of modern birds lived alongside dinosaurs, the radiation of modern birds is without doubt a post-extinction event, he says. He points to tremendous skepticism about molecular studies in general, which have previously placed the divergence of modern birds as far back as 100 million years.
The fossil evidence, Feduccia says, is completely contrary to this view. The fossil record indicates that a single fuse lineage survived the extinction event and then gave way to a big bang radiation of birds within a period of about 10 million years a model that Feduccia says perfectly parallels the evolution of modern mammal orders.
Still, Fordyce says that using fossils to calibrate molecular clocks is valid, calling the clocks one of the most exciting new developments in paleontology in the last 10 or 15 years. Evolutionary biologists have a great interest in looking at more groups that are still living that also have fossil records of their ancestors, Fordyce says. We can use those groups then to get an understanding of the bigger picture of evolution in this case, the bigger picture of bird evolution.
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