The Rise and Fall of the Dinosaurs: A New History of a Lost World: The Definitive Dinosaur Encyclopedia with Stunning Illustrations, Embark on a Prehistoric Quest!

by Steve Brusatte

oceans—a domain dinosaurs were never able to conquer. The warm waters of the Cretaceous, as during the Jurassic and Triassic beforehand, were the hunting grounds of various types of giant reptiles: plesiosaurs with long noodle-shaped necks, pliosaurs with enormous heads and paddlelike flippers, streamlined and finned creatures called ichthyosaurs that looked like reptilian versions of dolphins, and many others. They dined on each other and on fish and sharks (most of which were much smaller than today’s species), which in turn fed on tiny shelled plankton that choked the ocean currents. None of these reptiles were dinosaurs—even though they are often mistaken for dinosaurs in popular books and movies, they were merely distant reptilian cousins. For whatever reason—and we don’t yet know the answer—no dinosaurs were able to do what whales did: start on the land, change their bodies into swimming machines, and make a living in the water.

Occasionally I catch one of them dive-bombing an unsuspecting tourist, spearing a french fry or two with its beak before launching back into the sky. When I observe this type of behavior—the cunning, the agility, the nastiness—it’s easy to see the inner Velociraptor in an otherwise forgettable seagull.

They’re small, they have feathers, they can fly—we shouldn’t call them dinosaurs. On the face of it, that may seem like a reasonable argument. But I always have a quick retort up my sleeve. Bats look and behave a whole lot differently than mice or foxes or elephants, but nobody would argue that they’re not mammals. No, bats are just a weird type of mammal that evolved wings and developed the ability to fly. Birds are just a weird group of dinosaurs that did the same thing.

pterosaurs. Often referred to as pterodactyls, these were reptiles that glided and soared through the air on long, skinny wings anchored by a stretched fourth finger (the ring finger). Most were about the size of average birds today, but some had wingspans wider than small airplanes. They originated around the same time as dinosaurs in the Pangean days of the Triassic, and died out with most dinosaurs at the end of the Cretaceous, but they were not dinosaurs, and they were not birds. Instead, they were close cousins of dinosaurs. Pterosaurs were the first group of vertebrates (animals with backbones) to evolve wings and fly. Dinosaurs—in the guise of birds—were the second.

Huxley agreed that Archaeopteryx was a transitional fossil, linking reptiles and birds, but he went one step further. He noticed that it bore an uncanny resemblance to another fossil discovered in the same lithographic limestone beds in Bavaria, a small flesh-eating dinosaur called Compsognathus. So he proposed his own radical new idea: birds descended from dinosaurs.

There are no other groups of animals—living or extinct—that share these things with birds or theropods: this must mean that birds came from theropods. Any other conclusion requires a whole lot of special pleading.

But tiny amounts of them fall at a more or less constant rate from the deep reaches of outer space as cosmic dust. The Alvarezes reasoned that if the clay layer had only a tiny peppering of iridium, then it had formed very quickly; if it had a larger amount, then it must have formed over a much longer time period. New instruments now allowed scientists to measure even very small concentrations of iridium, including one in a lab at Berkeley run by one of Luis Alvarez’s colleagues. They weren’t prepared for what they found. They found iridium all right—lots of it. Too much of it. There was so much iridium that it would have taken many tens of millions of years—maybe even hundreds of millions of years—of steady cosmic dusting to deliver it all. Which was impossible, because the limestones above and below the clay were dated well enough that the Alvarezes knew that the clay layer could have been deposited over only a few million years at most. Something was amiss.

Dinosaurs had none of these advantages. Most of them were big, and they couldn’t easily scamper into burrows to wait out the firestorm. They couldn’t hide underwater, either. They were parts of food chains with big plant-eating species at the base, so when the sun was blocked and photosynthesis shut down and plants started to die, they felt the domino effects. Plus, most dinosaurs had fairly specialized diets—they ate meat or particular types of plants, without the flexibility that came with the more adventurous palates of the surviving mammals. And they had other handicaps as well. Many of them were probably warm-blooded or at least had a high metabolism, so they required a lot of food. They couldn’t hunker down for months without a meal, like some amphibians and reptiles. They laid eggs, which took between three and six months to hatch, about double the time for birds’ eggs. Then, after the eggs hatched, it took dinosaur youngsters many years to grow into adults, a long and tortured adolescence that would have made them particularly vulnerable to environmental changes.