After the extinction of dinosaurs, small reptiles and rodent-like mammals became the primary land animals. These mammals diversified rapidly and became the dominant Cenozoic animals. Early Cenozoic animals adapted to nocturnal hunting and differentiated into various lineages, including the clade Ferae, which gave rise to modern carnivores, ungulates, and cetaceans. Large herbivorous animals radiated from the condylarts, and mammals adapted to live in both air and oceans. Many early Cenozoic animals are now extinct.
Cenozoic animals are those that have existed for the past 65.5 million years, since the annihilation of the dinosaurs by a massive asteroid strike called the KT extinction event. After the KT extinction, the primary land animals remaining were small reptiles and rodent-like mammals. These rodent-like mammals diversified rapidly and came to occupy all the major niches left empty by the departure of the dinosaurs. With fast metabolisms, careful care for their young, and relatively large brains, mammals were well positioned to take over terrestrial ecosystems and become the dominant Cenozoic animals.
The only Cenozoic animals living immediately after the KT extinction were small insectivores with excellent hearing and smell but poor eyesight. Early Cenozoic animals adapted primarily to nocturnal hunting, when dinosaurs were probably less active due to their slower metabolisms. These insectivores differentiated into lagomorphs (rabbits, hares, and pikas), bats, early primates, true rodents, and tree shrews in the late Paleocene, about 55 million years ago, while another lineage developed in the ancestors of most large mammals today – members of the clade Ferae.
The clade Ferae was differentiated into the ancestors of modern carnivores (dogs, cats, and bears), ungulates (hoofed animals), and cetaceans (whales, dolphins, etc.). The dominant carnivores for much of the early Cenozoic were creodonts, first cousins of carnivores; the mesonychids, which were related to modern ungulates but were carnivores; and entelodonts, large pig-like animals with bony protuberances on the sides of their cheeks that they used to kill their prey.
The large herbivorous animals of the Cenozoic era mainly radiated from the condylarts, an extinct group of animals that are the common ancestor of all today’s ungulates, including cows, pigs, horses, deer, hippos, rhinos, camels, elephants, etc. The most significant evolutionary radiation event occurred when condylars shifted their diet to include plant matter, a change from their previous diet which was exclusively insectivorous. There is disagreement about where and when exactly these evolutionary events took place.
During the Cenozoic, mammals adapted to live in both the air (bats) and the oceans (whales), in both cases evolving from exclusively terrestrial ancestors. The blue whale, a marine mammal, has become arguably the largest animal of all time, even larger than the largest confirmed dinosaurs. Many Cenozoic animals that thrived early in the period are now extinct, leaving only their descendants.
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