Spying on Whales. Nick Pyenson

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.

      

      Imagine floating above the great tapered tail of South America from space, seeing it stripped of clouds, ice, soil, and water so that the geologic world beneath is made visible. The familiar outline of the continent rises in stark, jagged relief. The high spine of the Andes is draped in red and gray bands to the east, toward Argentina, and ochers and sands cover Chile to the west. From this vantage, the cone of South America is locked in by a jigsaw puzzle of oceanic plates, and a surprisingly deep, dark cut mars its western boundary.

      This incision marks the border between the Nazca and South American tectonic plates, where the lip of the former inexorably and slowly rolls under the edge of the latter. This action uplifts what was once seafloor, carrying ancient organisms buried in it—extinct whales included—slowly to dry land on the western edge of South America. This tectonic motion, called subduction, eventually yields mountain chains like the Andes over geologic time. But at the scale of human lifetimes, subduction can cause megathrust earthquakes that convulse entire cities, maroon fishing boats, and kill thousands in a span of seconds.

      In 1835 a young Charles Darwin observed the outcome of this very process along the coast of Chile, near Concepción. Three years into its round-the-world voyage, the HMS Beagle had rounded the horn of Tierra del Fuego and made its way up the west coast of South America. Darwin was ashore when the earthquake started. A crescendo over the course of hours allowed most of the residents of Concepción to flee and limited the scope of fatalities to a few dozen people. Aftershocks rattled terrified locals for several days thereafter. Darwin later surveyed the devastation in Concepción firsthand, noting that most of the city was flattened, burned, or flooded by an accompanying tsunami—and that the entire shoreline of the harbor had risen several feet, stranding limpets and starfish. Darwin surmised that these catastrophic effects were connected with the volcanic eruptions he had observed during earlier forays hundreds of miles south near Chiloé. Darwin suspected that volcanic eruptions, the sudden uplift of coastlines, earthquakes, and tsunamis were linked by a common underlying mechanism. His intuitive guess was more right than he could have known; they are all consequences of subduction—the jerky slippage of great masses of tectonic plates against one another—and the central process that underpins the idea of plate tectonics.

      Plate tectonics is a very young idea about how the Earth works. Until the late twentieth century, geology textbooks did not have a clear answer for why South America’s eastern edge fit so nicely with the west coast of Africa, which is a bit like launching moon-bound rockets without knowing Newton’s physics. Eventually scientists discovered that convection currents from deep inside the Earth drive the fragmented crust of the Earth’s rocky surface into constant motion, over geologic time. Every continent, and the ocean plates between them, floats on a vast, molten, and churning globe. The idea of plate tectonics also neatly explains a variety of patterns in the fossil record, including why so many plants and extinct animals across the southern continents look so similar—namely because