Spying on Whales. Nick Pyenson

and ask it a question: How did you get here? Where in the skeleton do you belong? What happened to your owner? There is an undeniable thrill in this chase, whether it’s in the field or in a museum collection, and fortunately you carry your mental library everywhere you go. Anyone can participate too—amateur sleuths sometimes crack cold cases.

      There is, however, a catch: you hardly ever get all the answers. As with other vertebrates, the fossilized skeletons of whales tend to be massively incomplete because the organismal glue that keeps skeletons together—stuff like ligaments, fibers, cartilage, and muscle—decays rapidly and is dispersed by waves, scavengers, and time. Our knowledge of most fossil whale species is based on little more than a battered skull, lacking all but the most diagnostic and unique features. For some time periods, and in some parts of the world, we can put all of what we know about the whale fossil record on one table. These fragments of bones—skulls, teeth, vertebrae, limb bones—can look like a jumbled puzzle waiting for someone to bring the missing pieces. Or, preferably, the cover of the box.

      This situation is what we tend to find for most fossil whales from the first phase of their evolutionary history, the part that took place at least partially on land. We don’t have complete skeletons for Pakicetus, Ambulocetus, Remingtonocetus, or most of the close relatives of Maiacetus. Being aquatic clearly helps in getting preserved intact. Perhaps the size increase from Maiacetus to much larger early whales such as Basilosaurus is part of why we tend to find more complete skeletons belonging to fully aquatic whales (while the bones individually get larger, they are fewer in number when you reduce and eliminate paired leg and foot bones). The fact is we don’t have a good understanding of the intermediary steps between hind-limb-propelled whales to tail-propelled ones. For all the anatomical transformations that happened in the earliest whales, there is a gap in the fossil record and our understanding between the last semiaquatic and the first fully aquatic whales. To fill out the picture we need more fieldwork in the right places with rocks of the right age, and a lot of luck.

      A good paleontologist can go far on scraps alone, and sometimes we’re lucky. There are places—or times, because paleontologists think in both space and time—where the fossil record yields parts of hundreds and even thousands of individuals. These fossil-rich areas are called bonebeds. My mental library comes in handy when I encounter one, helping me distinguish a scrap as the bone of a whale versus that of any other animal. At their densest, fossil whales in bonebeds get jumbled with scraps of other extinct marine mammals, seabirds, sea turtles, and sharks into layers only a few inches thick. At the other end of the spectrum, complete whale skeletons can be distributed over a broad area that can even reach square miles. The definition of a bonebed has mostly to do with the fact that skeletal parts are concentrated within a single layer of rock. What paleontologists and geologists want to know, once they’ve found a bonebed, is how much geologic time has compressed the evidence, which can represent as much as a million years or maybe just a day’s worth of a flood.

      In the 1920s one of my predecessors at the Smithsonian, Remington Kellogg, recognized that the Sharktooth Hill bonebed in the foothills of the Central Valley contained a richness of fossil whales, mostly identified on the basis of broken skulls and individual ear bones. The bones that form the outer, middle, and inner ears of whales are among the most heavily mineralized bones for any mammal. All the better for hearing underwater—and for preservation in the geologic record. The acoustically isolated ear bones discussed previously in bottlenose dolphins can also be found back to the time of the Sharktooth Hill bonebed and beyond, to the age of Pakicetus.

      Kellogg described and named twelve previously unknown fossil whale species from the Sharktooth Hill bonebed, encompassing a range of extinct baleen whales, early sperm whales, oceanic dolphins, and distant relatives of river dolphins. At this point in whale evolution, the world was full of filter-feeding and echolocating whales—all land-dwelling ones were long extinct—but they lived alongside sea cows, strange, hippolike herbivores called desmostylians, early seals, and early walruses.

      The material record of that past world comes from the Sharktooth Hill bonebed, which is an orange and brown layer only a few inches thick, chock-full of bone bits spread over a dozen or so square miles northeast of Bakersfield. Jim first introduced me to the bonebed in my early years of graduate school—he was more interested in its fossil sea turtles. Eventually I focused on figuring out the precise age of the bonebed and how this kind of dense rock unit, full of bone nuggets and occasional skeletal parts, came to be. Context is everything, and without it, answers to the bigger ecological questions about the past are undecipherable.

      The bonebed was essentially an exposed seafloor for several hundred thousand years, collecting the hard-part remnants of Miocene whales, sea turtles, sharks, and other animals that fell to the seafloor while lighter sediment swept past. Those few inches of bonebed today thus capture a condensed interval of time, between sixteen and fifteen million years ago—not much for a geologist but a span much longer than the duration of our own species. It’s also a span of time probably long enough to sample the full range of extinct whales and other backboned animals that lived in the vicinity of this part of California when the Central Valley was an embayment open to the Pacific. Knowing how many fossil whale species were around, giving them all scientific names, and understanding their evolutionary relationships is all ongoing work because there’s so little skeletal material to use as a basis for a species. (Kellogg used ear bones for most of his species, a puzzling move given how limited they are for species-specific identification.) Such work is time-consuming and exacting, measuring and comparing scraps of bone to one another. Many times we’re simply left saying, “This is something new, and it deserves a name, but we can’t say more until someone finds a good skull.”

      Kellogg wrapped up his Sharktooth Hill work after finishing his dissertation and secured an appointment at the Smithsonian in Washington, D.C., where he turned his attention to fossil whales that were more complete, from an earlier time in whale evolutionary history. By the 1930s, the Smithsonian possessed the world’s most extensive collection of early whales, but they weren’t the land-dwelling ones, such as Pakicetus, whose skeleton still wouldn’t be found for another half century. Instead, there were drawers and drawers of Basilosaurus and other species of the first of the fully aquatic whales, more than enough to mount full skeletons for exhibit halls, and sufficient to know something about what these extinct whales were like.

      Basilosaurus hardly seems like a whale—saying it’s almost like a whale would be charitable. It had a toothy, snout-dominated head, looking something like a gigantic leopard seal, except its nostrils were located not at the tip of its snout but about halfway farther back. It had a visible neck, unlike most of today’s whales. While its fingers and hands were probably encased in flesh, forming a paddle, it could bend its arms at the elbow, as no living whale can. The most remarkable thing about it was its long, eel-like body—most of its length came from its tail. Basilosaurus probably had a tail fluke, but it also had cartoonishly small hind limbs. These hind limbs were vestiges from its land-dwelling predecessors; as mentioned previously, they could not have held up Basilosaurus’s enormous weight (about six tons) on land. In other words, Basilosaurus was fully aquatic, living its entire life underwater.

      Kellogg knew only a little about those tiny hind limbs—the collections at the Smithsonian, for all their depth, still comprised only a pelvis and a single femur. Did Basilosaurus have feet? Toes? The answers to those questions eventually came from Basilosaurus skeletons found in Egypt, a world away from the coastal plain of the United States, and many years after Kellogg died.

      Since the nineteenth century, paleontologists have known that the same strata used to build the pyramids of ancient Egypt harbor marine fossils, including fossil whales. These fossil-rich rocks crop out for nearly a hundred miles to the southwest of Cairo, exposed at their grandest scale in a place called Wadi Al-Hitan, loosely translated as “Valley of the Whales,” in the Fayum depression. Toward the end of the twentieth century, more detailed work in the area produced a species list including ten different early whales, along with early sea cows, primates, and the earliest elephant relatives. Wadi Al-Hitan, however, earned its name because of the fossil