It is well accepted that birds evolved from dinosaurs, but the extent of the transformation still inspires wonder and awe. And new research.
One part of that transformation has just been traced in detail, showing that one of the things that happened in bird evolution was that their skulls pulled a Peter Pan and stopped growing up.
Some subtle genetic change slowed or stopped the development of the skull as a bird embryo grew in the shell and after hatching. The body kept on growing and changing proportions, but the skull changed only in size. It did not change in shape.
As a result, the skulls of birds look like those of baby dinosaurs. This kind of change is at the heart of the current understanding of how evolution proceeds, and although scientists are familiar with it from work on salamanders, fruit flies and other creatures, this new research shows how it occurred in two kinds of animals the public loves — dinosaurs and birds — bringing a complicated scientific idea home to the bird feeder and the museum exhibit hall.
The change from dinosaur to bird is one of those grand evolutionary shifts. The smart, quick theropod dinosaurs that gave rise to birds may have had feathers, but they did not fly and they certainly didn’t resemble birds today. Just think of a pigeon’s acrobatics. Or the legendary dive of the peregrine falcon, which can spot that pigeon from 3,000 feet and drop to earth in a 200-mile-an-hour swoop to strike it.
That’s a long way from running on land, a change in speed and agility requiring not only wings and flight feathers but sharp senses and sophisticated brains for long-distance vision and high-speed action. Sure enough, birds’ skulls have room for relatively huge eyes and for a brain that has expanded in the part devoted to the visual sense. And they are the same shape as the skulls of juvenile dinosaurs, not those of adults.
That similarity is what prompted the research by a group of investigators including Bhart-Anjan S. Bhullar, a graduate student at Harvard; Arkhat Abzhanov, Mr. Bhullar’s adviser; and Mark Norell at the American Museum of Natural History. They trained their attention on the evolution of skull shape in dinosaurs and birds, with the working hypothesis that as birds evolved, their growth patterns changed so they kept a juvenile skull shape their whole life.
“A number of us have speculated on this,” Dr. Norell said, referring to the idea that adult birds resembled young dinosaurs in some ways. “It’s a common thing to argue it, but it’s a difficult thing to show.”
To test the idea, the group used CT scan information on all the known fossils of theropod dinosaurs that show skull growth, as well as skulls of birds and crocodilians. They identified 45 points called landmarks on the skulls, and used a computer analysis to see how the areas defined by those points changed both during an individual life and over evolutionary time.
The analysis showed that in dinosaurs like Coelophysis and in crocodilians, the shape of the skulls changed significantly during an individual animal’s growth. The juveniles had short faces, and large brains and eyes. The adults had a longer snout and less room, relative to skull size, for eyes and brain. In primitive birds, however, there was very little change in skull shape during growth, so the skull retained its juvenile form. The modern birds continued the trend. The research was published online in the journal Nature on Sunday.
David B. Weishampel, a paleontologist at Johns Hopkins who was not part of the study, said it was thorough and convincing, “a nice piece of work.”
Dr. Abzhanov said the skull evolution was apparently a result of a change in the relative pace of two processes, body growth and sexual maturation. Crocodilians and dinosaurs, he said, may reach sexual maturity in 7 to 10 years. But modern birds become sexually mature 15 to 20 times as fast, and their body growth stops when their skulls are still like those of juvenile dinosaurs — and when they are ready, some of them, to drop from the sky at 200 miles an hour.
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