At the dawn of the Paleogene—the beginning of the Cenozoic era—dinosaurs, pterosaurs, and giant marine reptiles were conspicuously absent from the face of the Earth. Rodent-size (and perhaps larger) mammals emerged, suddenly free to fill the void. Over the next 42 million years, they grew in size, number, and diversity. As the period came to a close, life-forms still common today filled the seas, dominated the land, and had taken to the air.
During the Paleogene the continents drifted farther apart, heading toward their modern positions. Oceans widened the gaps, Europe severed its last ties with North America, and Australia and Antarctica finally parted ways. As the climate significantly cooled and dried, sea levels continued to drop from late Cretaceous levels, draining most interior seaways.
The cooling and drying trend began in earnest following a sudden temperature spike about 55 million years ago. Sea surface temperatures rose between 9 and 14 degrees Fahrenheit (5 and 8 degrees Celsius) over a period of a few thousand years, killing off numerous single-celled marine organisms called foraminifera, along with some other invertebrates. This event also profoundly affected northern forests, previously full of deciduous hardwoods with sequoias and pines. The new, more humid subtropical conditions nurtured abundant palms and guavas. Land mammals responded in kind, radiating and diversifying into many new forms.
As the climate cooled and dried following the warming, forests gave way to open woodlands and grasslands in the northern hemisphere and started to support thundering herds of grazing mammals.
Fish filled in the oceans, food to fuel sharks, which were fast ruling the waters in the absence of the giant mosasaurs and plesiosaurs of the Cretaceous. Squid and other soft-bodied cephalopods replaced their shelled relatives, which once filled the middle rung on the food chain. Sea snails and bivalves that were similar to modern forms lurked on the ocean bottom. New types of foraminifera and sea urchins replaced those that had died off in earlier mass extinctions.
Meanwhile, smaller reptiles that survived the Cretaceous, such as turtles, snakes, crocodiles, and lizards, basked in the tropical warmth along the coasts. Birds, the holdouts of the dinosaur age, diversified and flourished in the skies. But the rapidly evolving mammals stole the show. Starting from a fairly humble position 65 million years ago, primates, horses, bats, pigs, cats, and dogs had all evolved by the close of the period, 23 million years ago.