In the earliest paintings of dinosaurs, from the mid-1800s, they writhe like beached sea serpents or slouch like reptilian potbellied pigs. Now we know better. Dinosaurs stood erect and walked or ran great distances. Many were huge. One species—Argentinosaurus—reached 125 feet long. Now we know dinosaurs had complicated social lives—they raised their young, and they probably lived and hunted together in herds. We even know that the 9,000 species of birds all around us are living, feathered dinosaurs.

We know better because paleontologists are unearthing astounding numbers of fossils. “There are six or seven new species described every year,” says paleontologist Paul Sereno at the University of Chicago. “Even though you’d think it might be slowing down, the pace of discovery has quickened.” Modern technological devices, like CT scanners that illuminate hidden recesses of skulls and molecular probes that detect ancient proteins, offer a new depth of understanding scientists could never have dreamed of only a decade ago.

We know better, but dinosaurs remain highly mysterious. “It’s still humbling how little we know,” says Hans-Dieter Sues of the Smithsonian Institution. “Most of the big questions have yet to be really answered.”


A Discover survey of paleontological studies around the world identifies those questions as well as the tantalizing new clues that could lead to answers.

1.  Why did dinosaurs conquer Earth 210 million years ago?

Dinosaurs were not the first vertebrates to walk on land. If you had visited Earth during the Triassic Period, which began with a mass extinction 250 million years ago (see “The Day Everything Died,” page 64), you would have encountered the ancestors of today’s lizards, turtles, and alligators. The forerunners of humans—reptilelike precursors of mammals—were common. Yet once dinosaurs emerged into this bustling world, they became the dominant herbivores and predators.

Eoraptor, an early dinosaur that first appeared 228 million years ago, grabs lunch. Eoraptor was a primitive three-foot-long theropod and likely feasted on a variety of small prey, including lizards, bugs, and worms. Here it has captured a mammal-like cynodont dubbed Probelesodon. In the background is a herd of the herbivorous dicynodont Ischigualastia. Cynodonts and dicynodonts predate dinosaurs and may be an evolutionary link between reptiles and mammals.

Scientists once theorized that even the earliest dinosaurs must have been intrinsically superior to other animals. “We tended to look for the logic of evolution—there had to be a reason for the success of dinosaurs,” Sereno says. But doubts about this theory grew as researchers discovered new species. Now we know that although the earliest dinosaurs appeared roughly 230 million years ago, they remained relatively rare for the rest of the Triassic. Various reptiles and the forerunners of mammals dominated the landscapes. Sereno asks the obvious question: Why did it take so long for dinosaurs to become the major species? “Why were these other clunky animals the most common thing you stumbled over? It just doesn’t make sense.”

It’s possible that another mass extinction—210 million years ago—killed off the competition dinosaurs faced. That extinction, which involved the demise of more than half of all species on land and sea, is just as mysterious as the mass extinction that began the Triassic 40 million years earlier. Some geologists speculate that massive volcanic eruptions covering areas as large as modern continents triggered the release of methane buried in the ocean floor, causing a greenhouse effect. There is little doubt that the eruptions happened, but there is uncertainty that they occurred before the extinctions. They may have erupted soon afterward.

Paul Olsen, a paleontologist at Columbia University, offers an alternative theory: Yet another large asteroid smacked Earth. He and his colleagues have found layers of rock from the end of the Triassic that are loaded with iridium, a marker rare on Earth but common in asteroids, as well as spores of ferns, which would have quickly sprouted after an asteroid impact wiped out the world’s forests.

Olsen’s team then added dinosaurs to their time line. In the eastern United States and Canada, Olsen has studied over 10,000 footprints of late Triassic dinosaurs and other land animals. He and his colleagues found that right at the end of the Triassic Period, many nondinosaur footprints suddenly disappeared, and dinosaur footprints became much more common. And as the footprints became more common, the dinosaurs also got bigger, in some cases doubling their mass.

While many paleontologists are intrigued by Olsen’s evidence, others have scoffed. “People have a very violent reaction to this,” says Olsen, who readily admits that it’s hard to choose between an asteroid and volcanoes with the scant evidence at hand. “Deep in my heart I am quite ambivalent about whether an asteroid was involved.”

2.  How did those long-necked plant eaters get so big?

Some vertebrates, such as birds and  mammals, grow fast when they’re young and then stop growing altogether when they reach adult size. Alligators and many other reptiles don’t have such a big initial spurt. Instead, they grow slowly for their entire lives. These different growth patterns leave marks on bones. Fast-growing animals expand their bones continually when they’re young, and some of their old tissue gets destroyed as new bone forms. By contrast, the bones of many slow-growing animals look like tree rings because the animals grow in short bursts.

These microscopic clues survive in fossils. For example, researchers have studied the bones from an 80-million-year-old crocodile called Deinosuchus that could grow to 50 feet long. They concluded that it reached its huge size the way crocodiles do today, growing for 50 years or more.

Dinosaurs were different. “They did not grow like typical reptiles,” says Kevin Padian, a paleontologist at the University of California at Berkeley. Tyrannosaurus rex, for example, took only 20 years to reach full size. But the biggest dinosaurs were off the charts. Apatosaurus (also called Brontosaurus), one of the long-necked plant eaters known as sauropods, needed only 15 years to reach 25 tons. “They’re just growing faster than anything on land today,” says Padian.

The physiology behind this feat remains a mystery. Equally puzzling is the fact that dinosaurs managed to reach such big sizes, while the land animals that came before and after them never came close. Some researchers have proposed an environmental explanation: Perhaps levels of carbon dioxide were unusually high, producing lush vegetation that could fuel the dinosaur boom. But for now that is just speculation. “We don’t have a good answer why dinosaurs were such big animals,” says Hans-Dieter Sues.

3. Were dinosaurs warm blooded?

If a bass swims in chilly water, its body  becomes chilly. If it swims in warm water, it becomes warm. Put an otter in the same cold or warm water, however, and its body temperature barely changes. Living animals vary tremendously in their metabolism, and one of the biggest outstanding questions about dinosaurs is where they fall between the bass and the otter.

Reptiles are cold blooded. Or more precisely, they have a low metabolic rate and can’t produce enough heat to keep their bodies warm. When dinosaurs were first discovered, paleontologists considered them overgrown reptiles, which presumably meant they must be cold blooded too. Doubts emerged about the cold-blooded dogma three decades ago, when studies of birds, which are warm blooded, suggested they evolved from dinosaurs. Diehards have argued that, at least for the larger dinosaurs, there would be no advantage in being warm blooded. Their bodies could store plenty of heat from the sun. Meanwhile, other paleontologists have tried to find a clear-cut marker for warm-bloodedness in fossils, but without much luck. At the moment, the most promising clues come from the study of dinosaur growth. According to Kevin Padian, the results fit better with warm-blooded dinosaurs.

 “Look, these things are sustaining rapid growth the way only warm-blooded animals do today,” says Padian. “Unless a physiologist can tell us otherwise, it’s much more reasonable to say dinosaurs had evolved warm-bloodedness.”