The Cretaceous Period was a time of peak diversity and size for dinosaurs and the evolution of maniraptoran clade. Plesiosaurs and pliosaurs dominated the oceans, but were later replaced by modern sharks, rays, and fish species. The period saw a cool phase followed by volcanic activity and rising temperatures. Tropical oceans reached temperatures of up to 42°C. The period ended with a meteor impact.
The Cretaceous Period is the third of three periods of the Mesozoic Era, the “middle age” of complex multicellular life on Earth. The Cretaceous Period extended from the end of the Jurassic about 145 million years ago to about 65 million years ago when the Cretaceous-Tertiary extinction wiped out most dinosaurs. The ancestors of modern birds are surviving dinosaurs.
The Cretaceous was an extremely important period for life on Earth. The diversity and size of dinosaurs was at its peak. Insects also began to diversify. The maniraptoran clade evolved, a transitional clade between dinosaurs and birds. Pterosaurs have continued to dominate the sky, specializing in an ecological niche similar to modern birds for more than 150 million years.
The Cretaceous oceans began to be dominated by plesiosaurs and pliosaurs, as they had for most of the Mesozoic Era. But by the mid-Cretaceous they began to decline and modern sharks, rays and some fish species became more abundant. Mosasaurs, something like a cross between a snake and a monitor lizard that could swim, were major marine predators by the end of the Cretaceous period. The largest of these grew to 17 m.
At the beginning of the Cretaceous there was a cool period, a continuation of a trend that was occurring throughout the Jurassic. Temperatures at that point were still warmer than today, but more similar than most other Mesozoic climates. By the middle of the Cretaceous period, volcanic activity increased, emitting carbon dioxide into the air and raising temperatures once more. In the oceans, huge ridges have spread out, filled with 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 they are today. For geologically short periods, tropical oceans may have reached temperatures of up to 42 °C (107 °F), averaging 37 °C (98.6 °F). At the end of the Cretaceous period, sea levels fell compared to any other time in the Mesozoic period.
The Cretaceous was the golden age of giant sauropods, including Brachiosaurus, Seismosaurus and Supersaurus. The longest among these was 40 meters (130 ft), long, weighing about 100 tons, with one possible species, Bruhathkayosaurus, reaching up to 240 tons. At that size, they would have approached the maximum weight a land animal can support and still support its own weight.
The Cretaceous period was brought to an end by a giant meteor. This is evidenced by iridium deposits around the world. Iridium is very rare in the earth’s crust, but abundant in comets and asteroids.
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