The Cretaceous period is the third of three periods of the Mesozoic Era, the "middle era" of complex multicellular life on the Earth. The Cretaceous period extended from the end of the Jurassic period, about 145 million years ago, to approximately 65 million years ago, when the Cretaceous–Tertiary extinction event wiped out most of the dinosaurs. The ancestors of modern-day birds are dinosaurs that survived.
The Cretaceous period was an extremely important period for life on Earth. The diversity and size of dinosaurs was at its height. Insects began to diversify as well. The maniraptora clade evolved, a transition clade between dinosaurs and birds. Pterosaurs continued to rule the sky, specializing in an ecological niche similar to modern-day birds for over 150 million years.
The Cretaceous oceans started off being dominated by plesiosaurs and pliosaurs, as they had for most of the Mesozoic era. But by the mid-Cretaceous period they started to decline, and modern-day sharks, rays, and certain fish species become more abundant. Mosasaurs, something like a cross between a snake and a monitor lizard which could swim, were the leading marine predator by the end of the Cretaceous period. The largest of these grew to 17 m.
At the opening of the Cretaceous period, there was a cool period, a continuation of a trend that was occurring throughout the Jurassic. The temperatures at that point were still hotter than today, but more similar than most other Mesozoic climates. By the middle of the Cretaceous period, volcanic activity picked up, throwing carbon dioxide into the air and increasing the temperature once again. In the oceans, huge ridges widened, being filled by mantle plumes from below, creating vast, shallow seas.
Due to a slow temperature gradient from the equator to the poles, there was less upwelling in the world's oceans, making them substantially warmer and more stagnant than those of today. For geologically short periods, the tropical oceans may have reaches temperatures as high as 42 °C (107 °F), averaging 37 °C (98.6 °F). In the end of the Cretaceous period, sea levels dropped level than any previous time in the Mesozoic period.
The Cretaceous was the golden age of giant sauropods, including Brachiosaurus, Seismosaurus, and Supersaurus. The longest among these were 40 meters (130 ft), in length, weighing 100 tons or so, with a possible species, Bruhathkayosaurus, reaching as much as 240 tons. At such sizes, they would have approached the maximum weight a terrestrial animal can be and still support its own weight.
The Cretaceous period was brought to a close by a giant meteor. This is evidenced by iridium deposits worldwide. Iridium is very rare in the Earth's crust, but plentiful in comets and asteroids.