Pyenson describes another protowhale that appears to have stalled mid-phase between a prodigious crocodile and a leopard seal: Basilosaurus had a bite force that, pound for pound, exceeded that of any other known creature. It retained the diminutive hind limbs its forebears had deployed to kick off from their terrestrial habitat and rummage in shallow reefs, though Basilosaurus occupied open waters, where it is believed to have hunted other prehistoric whales. (A few years ago, a Basilosaurus skeleton with a second whale inside it was exhumed from the floor of an Egyptian valley, a kind of ossified cetacean turducken.) Basilosaurus bones can still be found in the southern United States. (Basilosaurus is the official state fossil of Alabama.) Paleontologists have sometimes discovered their vertebrae, not lodged in sedimentary rock or tumbling from eroded riverbanks but repurposed as andirons in fireplaces, foundation stones in buildings, or parts of furniture. Basilosaurus is a sea monster we’ve unknowingly domesticated.

Forty million years ago lived whales that looked rather like today’s iguanas, albeit larger. Others appeared more fishy. Some resembled an elongated hippo whose body tapered into the snickering head of an oversize ferret. By the time of Odobenocetops, the walrus-faced cetacean of the Miocene epoch, the course of evolution had streamlined whales’ bodies and dispensed with the back legs. Odobenocetops had two asymmetrical tusks protruding downward from its squashy muzzle. The right tusk grew twice as long as the left for reasons unknown (perhaps it had to do with its diet of mollusks, or with courtship displays the males performed). To the 21st-century viewer, these tusks give Odobenocetops the lopsided charm of an oracular character in a Hayao Miyazaki film.

As far as researchers are aware, more than 80 species of cetacean inhabit modern-day oceans and estuaries. But the seas are deep and resist surveillance. It is possible that yet more whales swim below, awaiting discovery. Genetic analysis is recategorizing misidentified remains: Hitherto unknown cetaceans are being discovered in bone and tissue samples. The geologic record that Pyenson sets out to explore documents some 600 prehistoric whale species that no longer exist, and reveals evidence of bygone eras during which whale ancestors occupied a wide range of ecological niches worldwide. Many of the bones that prove pivotal to verifying cetacean evolution are as small as tokens from a board game.

Beneath the lower spine of some modern rorqual whales remain the vestigial knobbles of two back legs, folded up like an airplane’s landing gear. These defunct hindquarters are fleetingly visible on the exterior of the animal in the womb (when, for a time, a whale fetus looks a lot like a huddled piglet). This is the magic of whales: They contain forms both familiar and stupefying. Dissecting the innards of a minke in Iceland, Pyenson discovers at least one organ whose specific function remains mysterious—a gelatinous sphere the size of a volleyball, capsuled in the tip of the whale’s chin. Cellular bundles in the organ turn out to be pressure sensors. Figuring out what this large structure might indicate about how whales perceive their ocean habitat, and how their senses developed over eons, is the ongoing labor of Pyenson and his peers. However far into the flesh of whales they plunge, they find only more questions to wonder at.