6. Why did Triceratops and its relatives look so strange?
Triceratops had three horns on its face along with a huge bony frill that extended from the back of its head. It’s easy to picture the animal using the frill to block a predator’s lunge for its neck, or goring its attacker with one of its horns. But this picture is probably fiction. “Pretty much everybody rules out defense for the horns and frills,” Norell says. “They just weren’t sturdy enough, and we don’t find a lot of broken ones.”
Paleontologists have had the same counterintuitive experience with Pachycephalosaurus, a bipedal plant eater with a dome of thick bone on its skull that was studded with knob-shaped outgrowths. Generations of paleontologists favored the idea that males rammed their heads together the way bighorn sheep do today. But on closer inspection it turns out that the dome was covered with tissue rich with blood vessels. One good head butt would leave Pachycephalosaurus badly wounded.
Kevin Padian and Jack Horner think they can explain these frills and horns and domes, as well as the strange ornaments on other dinosaurs. “We’re proposing that all these crests and frills and horns and plates and stuff have nothing to do with mechanical things,” says Padian. “Rather, they are for species recognition.”
Many living animals carry identity badges—horns, bright colors, and such—that allow them to be recognized as members of a particular species. These identity badges may help animals avoid interbreeding with closely related species that share their range. Antelope horns, for example, vary a lot from species to species, not because these variations let them fight off different predators but because they need to recognize their own kind.
Padian and Horner theorize that identity badges were also common among dinosaurs. Triceratops, for example, belongs to a big group of species called ceratopsians, many of which had frills and horns in a dizzying variety of shapes and sizes. You would expect that when a lot of ceratopsian species lived side by side, they would evolve a bigger variation of ornaments than more solitary species. “We think they passed the test,” says Padian.
7. How did some dinosaurs evolve feathers?
The first fossil feathers were discovered in 1861 in a limestone quarry in Germany, where workers unearthed a 145-million-year-old bird, Archaeopteryx. It still retained signs of its reptilian past, such as the teeth in its beak, the claws on its winged fingers, and its long tail. But which reptiles were its ancestors was the subject of fierce debate for more than a century.
In the 1970s a growing number of paleontologists theorized that birds are living dinosaurs. They pointed out a number of features in bird skeletons that could also be found in one group of midsize bipedal meat eaters known as dromaeosaurs. Feathers, they suggested, might have first evolved in dromaeosaurs, but it would be hard to test this hypothesis since feathers rarely fossilize. “If you had asked me 15 years ago if we would ever find traces of feathers on a dinosaur, I would have said, ‘Yeah, but not in my lifetime,’ ” says Sereno.
Sereno is still alive and well, and more than a dozen species of wingless feathered dinosaurs have emerged in northeastern China. The most surprising thing about these fossils is that they include a forerunner of T. rex as well as predators related to dromaeosaurs. That leads to one obvious conclusion. “Feathers did not evolve for flight,” says Padian.
Fossils are not the only source of clues to the evolution of feathers; bird embryos are as well. A feather begins as a hollow tube. It then produces barbs that hook together loosely. This design can lead to the downy feathers that keep birds warm. In other cases, a feather will produce more hooks that lock the barbs into straight rows, creating the flat surface that birds use to fly.
In 1999 Richard Prum of Yale University and Alan Brush of the University of Connecticut proposed that these growth patterns reflect how feathers evolved. Paleontologists have now found enough feathered dinosaurs to test their theory, and it is holding up well. The feathered dinosaurs most distantly related to birds had hollow tubes sprouting from their skin. More closely related species had a few barbs branching from a central axis. And the feathered dinosaurs that were the closest relatives of birds had the most complex feathers.
What’s not clear is what purpose the protofeathers of dinosaurs served. In the early stages of their evolution, feathers may have served in mating displays or species recognition. If T. rex and other theropods evolved warm-bloodedness, feathers could have helped them insulate their bodies to stop heat loss. For now, no one knows for sure.
8. How did some feathered dinosaurs begin to fly?
Dinosaurs needed more than just feathers to fly. They had to evolve the special arrangement of bones and muscles that makes it possible for birds to flap their wings. Traditionally, paleontologists have put forward alternative scenarios in which flight evolved either from the trees down or the ground up.
In the trees-down scenario, tree-climbing dinosaurs jumped to the ground or to other trees, and feathers on their outstretched forelimbs helped extend their fall. Eventually their descendants became gliders, and then finally they began to flap their wings. In the ground-up scenario, small theropods somehow evolved a flight stroke even while they were adapted for running on the ground—perhaps by grabbing prey with their hands.
“This was a false dichotomy,” says Hans-Dieter Sues. In recent years, for example, scientists have discovered some very small feathered theropods that may have been able to climb trees. Meanwhile, Ken Dial of the University of Montana has found that partridges can run up vertical surfaces. As the birds run, they flap their wings so that they act like spoilers on a race car, pushing the bird against the surface.
“We’ve got animals able to climb trees if they want to, able to take off from the ground if they have to,” says Padian.
These discoveries have prompted paleontologists to give feathered dinosaurs a fresh look. It’s possible that some of them may have already been able to fly, if only a little. “The only way this will really be answered is to get some better fossils, build really good models, and do experiments in wind tunnels,” says Norell.
9. What finally killed off the dinosaurs?
Birds may be living dinosaurs, but they’re peculiar ones. They are the only dinosaurs that managed to survive beyond the Cretaceous Period, which ended 65 million years ago. All the dinosaurs without wings became extinct, along with roughly 75 percent of all other species—a pulse of mass extinctions that struck land and sea alike.
What happened? For one thing, a six-mile-wide asteroid crashed into the Gulf of Mexico at the very end of the Cretaceous Period. It left behind many signs of its impact, from bits of fractured quartz showered thousands of miles away and vast arcs of tidal-wave-driven debris to the impact crater itself, buried deep underground off the Yucatán Peninsula.
A number of researchers have argued that cascading environmental effects from the impact wiped out the dinosaurs in a geologic blink. As debris from the impact rained down around the world, it triggered vast wildfires. Dust may have blocked sunlight for months, and acid rain may have killed off plants and marine organisms long afterward. Cut off from their food supply, most dinosaurs became extinct. Birds survived because a few of them happened to be sheltered in burrows or rock piles.
When this hypothesis emerged in the 1980s, it was hugely popular. “Fifteen or 20 years ago, people would have said, ‘Ah, it’s just the asteroid,’ ” says Norell. But things have changed. “Now everybody agrees that an asteroid hit, but its role in the extinctions is being questioned,” he says.
Recent studies suggest that the impact may not have been quite as fierce as scientists once thought. A team of Canadian and British researchers surveyed six sites in North America and found no sign of soot 65 million years ago, leading them to doubt that the impact led to a global firestorm. Gerta Keller of Princeton University has also found reason to doubt in a core she drilled from the impact crater itself. She claims that marine species became extinct 300,000 years after the impact.
“I think more people are looking at it more like a perfect storm,” says Norell. The asteroid was one of several major blows that life suffered at the end of the Cretaceous. The climate was cooling, sea levels were retreating, and huge volcanoes in India were belching out lava and lots of atmosphere-altering gases. “Things weren’t going that great for the ecosystems in general when the asteroid hit,” says Norell.
Whether an asteroid or a perfect storm turns out to be the primary culprit, says Sues, paleontologists still have a long way to go to explain why the resulting extinction killed off such a peculiar range of species. “There doesn’t seem to be any rhyme or reason to it. It picks off things left, right, and center,” says Sues. “Why did small dinosaurs disappear but crocodilians and other large reptiles did not? That’s one of the enduring mysteries that makes this extinction fascinating.”