Dinosaurs fascinate everyone, and Brusatte, professor of paleontology and adviser to the Jurassic World film franchise, has named more than 15 new species. However, mammals are his first love, and this delightful account will convert many readers. According to the popular belief, dinosaurs ruled the Earth until they were wiped out by a meteor strike 65 million years ago, whereupon mammals succeeded them. This is correct except that mammals not only succeeded dinosaurs; they existed alongside them back to their beginning. In fact, both share a common ancestor that appeared perhaps 325 million years ago. This small lizardlike creature evolved into two major lineages, one eventually becoming reptiles (including birds), the other mammals. Readers who remember high school biology know that mammals have warm blood, hair, and mammary glands that produce milk. Such true mammals did not appear for 100 million years, and these features do not fossilize well, but Brusatte excels in explaining how paleontologists figured matters out. Only mammals chew; most have complex teeth. Birds and reptiles swallow food whole; their teeth, when present, look alike. Mammals have three tiny bones in their ears, which allow them to hear better than other vertebrates, which have only one. Ancient mammals and pre-mammals were small. Their surviving bones were fragmentary and their teeth nearly microscopic, so early paleontologists sifted tons of dirt to detect minuscule fossils until the present century, when new sites, especially in China, have revealed spectacularly complete skeletons, often including hair, feathers, and embryos. Many readers consider humans the most interesting mammal, closely followed by extinct behemoths such as mammoths and saber-toothed tigers. Brusatte, however, gives humans “about the same attention as horses and whales and elephants. After all, we are but one of many amazing feats of mammalian evolution.” Throughout, the author employs lucid prose and generous illustrations to describe the explosion of mammal species that followed the disappearance of dinosaurs.