Event 23

60 million years ago

Mammalian Adaptive Radiation
Diagram of Adaptive Radiation of Mammalian Orders. Soon after the Cretaceous-Tertiary extinction of the dinosaurs around 65 million years ago, there was an accelerated evolution of the mammalian order with large-scale diversification becoming evident in the fossil record.
Credit: © 2005 Steven M. Carr, after Romer. This diagram is used with permission.

With the extinction of the dinosaurs 65 million years ago, mammals had many new niches available to them. These warm-blooded, hairy forms, with sweat glands modified to feed milk to their young, were particularly able to adapt to the many land and sea niches left open after the dinosaurs died out. Mammals had been around for some time, coexisting with the dominant dinosaurs for millions of years.

These small insect-eating animals fed at night to help escape being eaten themselves! After the dinosaurs were gone, the mammals soon spread and became very diverse, becoming the dominant animals on the earth up to today (thus the name for our era, “The Age of Mammals.”

Most of the modern orders of mammals were to emerge shortly after this time, including:
Monotremata (the primitive egg-laying platypus and echina or spiny anteater)
Artiodactyls (even-toed animals such as deer, antelope, cows, pigs, camels, and giraffe)
Perisodactyls (odd-toed animals such as horses, tapirs, and rhinos)
Probiscideans (elephants and extinct mammoths and mastadons)
Carnivores (dogs, cats, bears, raccoons, hyaenas, mongeese)
Cetaceans (whales, dolphins and porpoises)
Pinnepeds (seals, sea lions, and walruses)
Sirens (dugongs, manatee (sea cows)
Chiropterids (bats)
Insectivores (moles, shrews, hedgehogs)
Tubulidents (aardvarks)
Lagomorphs (hares, rabbits, pikas)
Primates (prosimians, monkeys, apes, humans)
Rodents (mice, rats, squirrels, chipmunks, muskrats, gophers, hamsters, gerbils)
Edentates (armadillos, sloths, anteaters)
Pholydacta (pangolin)
Hyracoids (hyrax and dasies)

Although many early orders of mammals are now extinct, about 5,000 species of mammal have evolved over the past 65 million years and have taken up residence in various places on our planet.


Early mammal skeletons are sometimes found with developing fetuses in their body cavity (womb), indicating that the young would have developed being nourished. Early mammal fossils include Deltatherium fundamini, Ankalagon and Chriacus (primitive hoofed animals), some primitive primate-like animals such as Purgatorius and Plesiadipis, from North America.


Placental animals were the ancestors of the human lineage as well as the ancestors of most of the animals we are intimately involved with such as dogs, cats, cows, horses, donkeys, pigs, goats, and sheep.





This is a kid-friendly website with a surplus of information about mammals and mammalian evolution.

A mammalian genetic tree compiled through various University sources.

This is a discussion of the Age of Mammals.

This is a website that briefly discusses the Cenozoic and mammalian radiation.


Comments are closed.