We had visited many other outcrops in the foothills of the Sierra Nevada together. We worked side by side, scanning in silence. “Hey,” Jim said abruptly, reaching down. He raised a palm-sized shark tooth to the sky, its serrated edges cutting the orange light. I looked down and immediately started to see other shark teeth and whale bone fragments recently eroded out of the hillside, gems in the rough. “Oh, check this out,” I said, retrieving a segment of dolphin rib from the newly formed sediment piles. As I flipped it over between my fingers, I noticed something unusual—a set of a dozen parallel lines gouging a path across the bone’s surface. A shark bite. This site was, after all, the Sharktooth Hill bonebed.
The fact that the rib bone belonged to a small species of extinct toothed whale (Odontoceti indeterminate, if you want to be technical) was probably the least interesting thing about it. That kind of identification is merely born out of the same patient study—hours with museum collections—that gives you eyes for spotting bones in the first place. Far more interesting was the fact that it told us part of a story: a little more than fifteen million years ago, in the middle of the Miocene, an ancient shark chomped down on an extinct dolphin’s rib cage.
Whether this particular set of bones represented a fatal encounter or mere scavenging on a carcass we couldn’t know. There was also no real way of knowing whether the shark tooth in Jim’s hand and the marked rib in mine were causally related. We held the two side by side, checking the serrations on the tooth with the gouges on the rib—close, but not a precise match. Even if they were, it would be a stretch to tie the two pieces of evidence, a gumshoe’s leap in causality at a suspected murder scene. Whale bones do tell us stories, but they’re not always satisfying or predictable.
I lodge finds like these on the shelves of whale bones that I keep in my head. I can’t quite tell you how this mental library is organized, but I slip into it every time I see a shard or glint of whale bone, whether out in the field or in a museum drawer. The more fragmentary, the more fun. I pick it up carefully, thumb its creases, divots, and twists, and then scrutinize its topography by eye. My thoughts immediately race through a chain of mental flash cards to arrive at the best possible identification of its former owner: Right or left side? Symmetrical, from the main axis of the skeleton? Cranial or something below the neck? Scavenging marks? Pathologies? These flash cards are marked with names for every bump and hole on a bone’s surface. It has taken me years to build up this cerebral collection, long hours spent with many skeletons within arm’s reach, flipping each piece over and over again, tracing each surface for memory. It’s also good to keep a stack of real literature on hand as a guide, because you certainly aren’t the first one to pick up a whale bone 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,