Extinct triconodonts and multituberculates were among the first mammals to evolve 220 mya (million years ago) during the Mesozoic Era. Dinosaurs, which had appeared 10 mya earlier, and other carnivorous reptiles limited mammal evolutionary varieties and curtailed mammal population increases throughout the entire Mesozoic.

The environmental catastrophe that wiped out more than 90 percent of Mesozoic cold-blooded land and sea species 65 mya, opened up new habitats for warm-blooded survivors to take over during the Cenozoic Era (65-0 mya). Insulating fur enabled mammals to endure the extreme climate changes that occurred 65 mya, and their differentiated teeth and synapsid openings in the skull permitted efficient chewing of a widespread and varied diet. After the dinosaur extinction, mammals underwent explosive evolution in the early Cenozoic. Chance mutations, that would not have survived to create new species during the Mesozoic because well-adapted competing animals were occupying the most favorable habitats, were now able to survive and take over the areas vacated by the extinct Mesozoic cold-blooded forms.

The first placentals (advanced mammals whose fetal embryos develop entirely within the womb) were ancestral armadillos, sloths, and anteaters. The former included extinct giant glyptodonts, with fused bone-plate armor as on giant tortoises. The earliest sloths lived in trees (where they still survive) and had evolved long curved claws used to grasp branches while moving about. When forest areas began to shrink due to climate changes, many sloths were forced to live on the ground where their feet and claws were ill-suited to locomotion. They walked ponderously on the outer edges of their inwardly curved feet, and eventually succumbed to predators except for gigantic mutant ground sloths whose bulk protected them. These survived until the Ice Age, when the most proficient predators of all times, humans, dispatched them and many others.

A stirrup-shaped stapes ear bone gave all subsequent placentals a hearing advantage not shared with the earlier ones. Sharp hearing as well as keen sight and fast running gave predators advantages over slower prey; and gave healthy adult prey ability to escape all but the prime healthy predators.

Rodents, which constitute 40 percent of all living mammals, appeared 55 mya. Some lived in trees; others on the ground. Some, extinct giant capyberas, were as large as cows. Ancient beavers lived in burrows, even corkscrew ones that enabled them to elude predators. Some extinct rodents even had horns on their nose.

Sirenians, such as Atlantic manatees and Pacific dugongs, are marine mammals with flippers and with dense bones to offset their fat-bodied buoyancy. They evolved from four-legged ancestors which later became their living distant cousins, the elephants. All marine mammals, including seals, walruses, and whales, are air-breathers who returned to the sea, are all examples of Convergent Evolution. They evolved from different four-legged Cenozoic land mammals, probably for similar reasons that impelled many Mesozoic reptiles to adopt an aquatic environment: mutations, such as replacement of feet with flippers, that would have been harmful and would have died out on land, were actually beneficial to coastal animals who found the sea a better source of food and a better refuge from fearsome land predators.

Carnivores, including the cat and dog families, have special shearing teeth (carnassial molars) for slicing off chunks of meat that could be quickly swallowed. Cats are normally faster than dogs because they run on their toes, giving them a longer stride. Cats can also retract their claws while running, which keeps them sharp for combat. Saber-tooth "tigers" were dominant predators during the last Ice Age (until wiped out by humans), but upper canine "saber" teeth had evolved eight other times in older mammals as examples of convergent evolution, including one marsupial predator, Thylacosmilus.

Part 2 of Advanced Mammal Evolution will follow.