Section: Science

181) Excerpt from http://www.nationalgeographic.com/science/prehistoric-world/cretaceous/

Continents were on the move in the Cretaceous, busy remodeling the shape and tone of life on Earth. At the start of the period, dinosaurs ruled the loosening remnants of the supercontinent Pangaea as rodents scurried at their feet through forests of ferns, cycads, and conifers. At the end of the period, about 80 million years later, oceans filled yawning gaps between isolated continents shaped much as they are today. Flowering plants were spreading across the landscape. And mammals sat poised to fill the void that soon would be left by the vanished dinosaurs. A giant crater smoldered on what would become known as the Yucatán Peninsula.

EXTINCT SPECIES
Whether or not the asteroid or comet that carved the Chicxulub crater caused the extinction of more than half the planet's species at the end of the Cretaceous remains a matter of scientific debate. But the shifted continents, expanded coasts, and widened oceans had cooled and moistened the planet's climate and set in motion dramatic changes to the flora and fauna. An extraterrestrial impact or a bout of volcanism from within was perhaps too much for many of Earth's species to handle.

Long before the carnage began, the Cretaceous picked up where the Jurassic left off: Gigantic sauropods led parades of dinosaurs through the forests, over the plains, and along the coasts; long-necked and toothy marine reptiles terrorized fish, ammonites, and mollusks in the seas; pterosaurs and hairy-feathered birds filled the skies. But as the continents spread, the ocean currents churned with ever more vigor. After a temperature spike in the mid-Cretaceous, the climate began to cool, and the tenor changed.

NEW DINOSAURS
Though dinosaurs ruled throughout the Cretaceous, the dominant groups shifted and many new types evolved. Sauropods dominated the southern continents but became rare in the north. Herd-dwelling ornithischians like Iguanodon spread everywhere but Antarctica. Toward the close of the Cretaceous, vast herds of horned beasts such as Triceratops munched cycads and other low-lying plants on the northern continents. The carnivore Tyrannosaurus rex dominated the late Cretaceous in the north while monstrous meat-eaters like Spinosaurus, which had a huge sail-like fin on its back, thrived in the south. Smaller carnivores likely battled for the scraps.

Other creatures, such as frogs, salamanders, turtles, crocodiles, and snakes, proliferated on the expanded coasts. Shrewlike mammals scurried about the forests. The largest pterosaur known soared overhead though the group as a whole faced ever stiffening competition from fast diversifying birds: Ancestors to modern grebes, cormorants, pelicans, and sandpipers all show up in the Cretaceous.

In the warm, shallow seas that spilled onto the continents, the long-necked plesiosaurs gave way to the giant, snakelike mosasaurs. Rays and modern sharks became common. Sea urchins and sea stars (starfish) thrived; coral reefs continued to grow. Diatoms, a type of shelled plankton, made their first radiation into the ocean.

But it was the rapid dispersal of flowering plants that stole the show—a spread enhanced with the help of insects from bees and wasps to ants and beetles. Magnolia, ficus, and sassafras quickly outnumbered ferns, conifers, gingkoes, and cycads.

Much of this rich life—including all dinosaurs, pterosaurs, pliosaurs, and ammonites—perished in the extinction event at the end of the period 65 million years ago. In fact, the land, seas, and skies would never be the same in the new era that dawned after the close of the Mesozoic era.

According to the passage, what caused the end of the dinosaurs, pterosaurs, pliosaurs, and ammonites?