Читать книгу Spying on Whales - Nick Pyenson - Страница 12

I walked with my eyes trained on a long, gray road cut through a hillside on a cattle ranch. In the late-afternoon California sun, the waving golden grass gave the hill the look of an enormous, shaggy animal, revealing its sedimentary flank. I followed a small path, parallel to the exposed fossil-rich layer. Walk, stare, walk some more, scrape the exposure, and then walk again; maybe you get lucky.

A few yards away from me, my colleague Jim Parham was doing the exact same thing. I’ve known Jim since graduate school, and we don’t need to say much to each other in the field. Jim’s an expert on turtles and other reptiles. As it is for me with whale bones, Jim has seen enough specimens that even the smallest fragments of fossil shell can help him solve riddles about turtle origins, which stretch back even deeper in geologic time than whales, though we tend to find fossil sea turtles in the same type of rock as fossil whales. Jim and I reliably fall on the same page, by temperament and by rock units.

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, 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 whales—especially Basilosaurus—whose skeletons number in the hundreds, spread across miles of desert expanse edged by cliffs and wind-battered mesas. These skeletons come from bonebeds, just in a different mode from Sharktooth Hill. Over three hundred skeletons of early whales are littered across one hundred square miles, including the first complete Basilosaurus skeletons ever found, with skulls, arms, rib cages and tail vertebrae, and legs—everything down to the four tiny toes intact. What these legs might have done in life, beyond being mere vestiges, remains unclear. (Some scientists have speculated that these small legs might have been used for copulation, especially given the animal’s extreme snakiness.)

Complete skeletons of Basilosaurus give us plenty of clues about its behavior. Like some of its predecessors, Basilosaurus had acoustically isolated inner ears, letting it hear directionally underwater, but it lacked the anatomical space to house any kind of echolocation organ on its face. (Basilosaurus therefore heard only low-frequency sounds—not the ultrasonic ones that echolocating toothed whales use today.) It ate fish, based on fossilized stomach contents. Every tooth in the head of Basilosaurus was capable of crushing bone, and its overall bite force exceeded that of any other mammal, living or extinct, including hyenas. Bite marks on the skulls of another, smaller species of early whale from the Fayum suggest that Basilosaurus ate other whales, the way killer whales do today. One major difference with killer whales: Basilosaurus could crush its food, whereas killer whales rip and tear, oftentimes working together. At the moment, we can’t say whether Basilosaurus moved about in pods—there’s no good fossil correlate for that kind of behavior.

Basilosaurus

The fossil-rich rocks of Wadi Al-Hitan reflect ancient shorelines formed during episodes of periodic sea-level rise and fall at the end of the Eocene, around 40 million to 35 million years ago. Basilosaurus probably inhabited these lagoonal environments (it certainly was buried in them); it lived not unlike many dolphins do today, ranging from coastal shores to open water. By the time Basilosaurus went extinct, at the end of the Eocene, subsequent branches in the whale family tree leading to today’s whales had already evolved. While we don’t have a good fossil record for the very beginning of today’s echolocating and filter-feeding whales, we suspect they looked a fair bit like Basilosaurus—fully aquatic, though less snaky, sized-down shadows of the leviathans that they would later become, tens of millions of years later.

So much for the hows of fossil whales with legs. But why? What led whales to return to the water from land in the first place? That question takes us to the gap between the first and second phases of whale evolution, the gap that remains in the family tree between the branches leading to Maiacetus and Basilosaurus. In about ten million years, whales went from looking like the four-legged Pakicetus to something closer to Basilosaurus. Sometime during that interval (and probably in the last half of it), whales ambled and swam equal amounts, with shorter hind limbs and blowholes migrating backward along their snouts. And then, at some point, a generation of whales never emerged out of the water back onto land, and their descendants begat blue whales, humpbacks, sperm whales, dolphins, and every other living whale species (along with many extinct ones, like Kellogg’s finds from the Miocene).

The search for true causes—especially in the evolutionary sciences—is usually not as conclusive as the search for patterns and their data. Hows are much more forthcoming than whys. For whale origins, multiple explanations for their reentrance abound: they returned to escape predators on land; to take advantage of more prey at sea; to seize new habitats unexploited by any major marine predator since the demise of gigantic marine reptiles at the end of the Cretaceous, about twenty million years prior. Each one of these explanations is plausible but difficult to test. Maybe we’ll one day refine those explanations into a hypothesis with a prediction that we can evaluate, perhaps using the geologic context of these early whales, comparisons of their osteology with those of marine reptiles, or a novel analytical tool. One thing for sure: we will certainly benefit from more fossils—so we should keep looking.

Every scrap of fossilized bone found in the field may be novel, but they’re not all precious. There’s always some decision making about whether any particular fossil should be collected in the first place. It is, really, all about the questions at hand and how any certain fossil find can help answer it. Bonebeds are like caches of evidence: areas rich with clues, either because of the density of remains contained within them, as at Sharktooth Hill, or because of the completeness of the specimens in a given space, as at Wadi Al-Hitan. One fossil find can tell us about that individual, but it also captured a snapshot of a real ecological interaction, lost in geologic time. That’s an important detail from life in the distant geologic past, especially when we want to know the details of not merely the anatomy or evolutionary relationships of extinct organisms, but the food webs and ecosystems in which they lived millions of years ago. Finding these kinds of paleontological caches is thrilling, and it can also be overwhelming, as I would soon find out for myself.