What does a tiny Hadrocodium wui fossil tell us about the evolutionary advantages that mammals were developing before the Cretaceous–Paleogene mass extinction event that finished off the dinosaurs 65 million years ago? Let an animated mammal skeleton, a lizard, and a puppy help David Attenborough demonstrate. From Rise of the Mammals, Triumph of the Vertabrates, this is Why Warm Blood is Better Than Cold.

Additional reading from Live Science about a landmark 2007 study:

The study reveals two major spikes in the otherwise steady evolution of modern mammals, both of which appear to be independent of the dino wipe-out.

One occurred around 93 million years ago, when the major divisions of living mammals—placentals, marsupials and monotremes, such as the platypus—began to appear. Most of these mammals—such as Andrewsarchus, an aggressive wolf-like cow—belonged to lineages that are either extinct or have dwindled drastically in numbers.

“It was other groups of mammals, not those we see today, that took advantage of the extinction of the dinosaurs,” said study team member Robin Beck of the University of New South Wales.

The second evolutionary spike in modern mammalian history didn’t occur until about 10 to 15 million years after the dinosaurs’ demise, around the start of the Eocene era (about 55 to 34 million years ago), the researchers say. This was the mammalian Golden Age, when the preponderance of mammals, especially the ancestors of many groups alive today—such as primates, rodents and hoofed animals—really took off, according to the new study.

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