The Age of Mammals
Ironically, the same characteristics that helped mammals maintain a low profile during the Mesozoic Era also allowed them to survive the K/T Extinction event that doomed the dinosaurs. As the theory goes, that giant meteor impact 65 million years ago produced a kind of "nuclear winter," killing most of the vegetation that sustained the herbivorous dinosaurs, which themselves sustained the carnivorous dinosaurs. Because of their tiny size, the early mammals could survive on much less food, and their fur coats helped keep them warm in the plunging temperatures.
With the dinosaurs out of the way, the Cenozoic Era was an object lesson in convergent evolution: the surviving mammals were free to radiate into open ecological niches, in many cases taking on the general "shape" of their dinosaur predecessors (giraffes, as you may have noticed, are eerily similar in body plan to ancient sauropods like Brachiosaurus). Most importantly, from our perspective, early primates like Purgatorius were free to multiply, populating the branch of the evolutionary tree that led eventually to modern humans.
Here's a list of the most notable early mammals of the Mesozoic Era; just click on the links for more information.
Alphadon An early marsupial of the late Cretaceous.
Castorocauda This early mammal is known as the "Jurassic beaver."
Cimexomys Did this tiny mammal feast on Troodon eggs?
Cimolestes The tiny ancestor of all modern-day carnivores.
Crusafontia This early mammal had some squirrel-like characteristics.
Docodon Perhaps not a true mammal, but a "mammal-like reptile."
Eomaia This early mammal is known from a single, spectacular fossil.
Eozostrodon This Triassic critter may (or may not) have been the first true mammal.
Fruitafossor The earliest digging mammal yet discovered.
Hadrocodium This tiny mammal had an unusually large brain.
Megazostrodon A transitional form between therapsids and true mammals.
Morganucodon Another early mammal of the late Triassic period.
Oligokyphus An extremely mammal-like reptile of the early Jurassic.
Purgatorius Could this have been the direct ancestor of human beings?
Repenomamus The only early mammal known to have hunted dinosaurs.
Sinoconodon Another intermediate form between reptiles and mammals.
Sinodelphys One of the earliest marsupials yet discovered.
Teinolophos An early ancestor of the modern platypus.
Triconodon A common mammal of the late Jurassic.
Zalambdalestes This tiny mammal may be the ancestor of modern rodents.
