by Brian Switek
This general lack of other transitional forms stirred debate about the relationship of Basilosaurus to other mammals (the biologist D’Arcy Thompson, for example, thought it was more closely related to seals than to whales), but the anatomist William Henry Flower was certain it was relevant to whale evolution. In an 1883 lecture reviewing all that was known of whales, collectively called cetaceans, Flower reviewed the common suggestion that Basilosaurus had been preceded by a seal-like stage in evolution, and this as-yet-undiscovered type would exist in the gap between Basilosaurus and the land-dwelling carnivores authorities like Huxley thought gave rise to whales.
But this hypothetical sequence was not without its problems. Flower noted that seals and sea lions used their limbs to propel themselves through the water, while whales lost their hind limbs and swam by oscillations of their tail. He could not imagine that early cetaceans used their limbs to swim and then switched to tail-only propulsion at some later point. The semi-aquatic otters and beavers were better alternative models for the earliest terrestrial ancestors of whales. If the early ancestors of whales had large, broad tails it could explain why they evolved such a unique mode of swimming.
Flower could not unequivocally support living mammalian carnivores as the best models for early whale ancestors, either. Ungulates, or hoofed mammals, shared some intriguing skeletal similarities with whales. To Flower the skull of Basilosaurus had more in common with ancient “pig-like Ungulates” than seals, thus giving the common name for the porpoise, “sea-hog,” a ring of truth. If ancient omnivorous ungulates could eventually be found, Flower reasoned, it would be likely that at least some would be good candidates for early whale ancestors. Flower hastened to add that his ideas were still speculative, but he ended his lecture by envisioning a hypothetical cetacean ancestor easing itself into the shallows.
We may conclude by picturing to ourselves some primitive generalized, marshhaunting animals with scanty covering of hair like the modern hippopotamus, but with broad, swimming tails and short limbs, omnivorous in their mode of feeding, probably combining water plants with mussels, worms, and freshwater crustaceans, gradually becoming more and more adapted to fill the void place ready for them on the aquatic side of the borderland on which they dwelt, and so by degree being modified into dolphin-like creatures inhabiting lakes and rivers, and ultimately finding their way into the ocean.
The fossil remains of such a creature remained elusive. By the turn of the twentieth century the oldest fossil whales were still represented by Basilosaurus and similar forms like Dorudon and Protocetus, all of which were fully aquatic. As E. D. Cope admitted in an 1890 review of whales, “The order Cetacea is one of those of whose origin we have no definite knowledge.” This state of affairs continued for decades.
FIGURE 55 - A reconstruction of the mesonychid Pachyaena. It was once thought that whales evolved from these carnivorous mammals.
While revising the relationships of ancient meat-eating mammals in 1966, however, the evolutionary biologist Leigh van Valen was struck by the similarities between an extinct group of carnivores called mesonychids and the earliest known whales. Often called “wolves with hooves,” mesonychids were medium- to large-sized predators with long, toothy snouts and toes tipped with hooves rather than sharp claws. They were major predators in the northern hemisphere from shortly after the demise of the dinosaurs until about thirty million years ago, and the shape of their teeth resembled those of whales like Protocetus.
The known mesonychid menagerie did not illustrate a finely graded transition from the land to the water, but they were the best candidates available for the long-lost ancestors of whales. Van Valen hypothesized that some of them may have been marsh dwellers that “were mollusk eaters that caught an occasional fish, the broadened phalanges [finger and toe bones] aiding them on damp surfaces.” A population of mesonychids who lived in a marshy habitat, then, might have been enticed into the water by seafood. Once they had begun swimming for their supper, succeeding generations of the population would become more and more aquatically adapted until something “as monstrous as a whale” evolved.
Van Valen’s mesonychid hypothesis sparked new interest in the origins of cetaceans, but there was still little to work with. Even the evolution of modern whales remained mysterious. The two living whale groups, the baleen whales (mysticetes, such as the humpback, blue, and right whales) and the toothed whales (odontocetes, such as dolphins, porpoises, and the orca) were so different that some researchers could not imagine how they could have evolved from a common ancestor. Perhaps each group had their own distinct terrestrial ancestors, with Basilosaurus being a third offshoot of whale evolution.
As if on cue, the discovery of a fossil mysticete with teeth, Aetiocetus , sunk the independent origins hypothesis. It confirmed that both odontocetes and mysticetes had evolved from toothed ancestors. The common ancestry of toothed and baleen whales was stabilized, but year after year rolled by without any sign of creatures recording the transition from land to the water. Protocetus was still the closest whale approximation to the presumed ancestral mesonychid type (often represented by the fifty-six-million-year-old Sinonyx from China).
This lack of early transitional forms led some scientists to doubt that transitional whales would ever be discovered. In a 1976 paper, Jere Lipps and Edward Mitchell noted that archaeocetes (early whales that are not odontocetes or mysticetes) appeared abruptly in the fossil record. There was no explanation for why this should be, and whale evolution did not seem to fit the traditional “slow and steady” model. This had long been a bugbear of evolutionary paleontology, all the way back to when Darwin was formulating his evolutionary theory. While the authors recognized that the lack of transitional forms might mean that paleontologists had not been looking in the right places, Lipps and Mitchell thought it more likely that whale evolution happened so fast that the fossil record had preserved little sign of it.
A startling discovery made in the arid sands of Pakistan, announced by University of Michigan paleontologists Philip Gingerich and Donald Russell in 1981, finally delivered the transitional form scientists had been hoping for. In freshwater sediments dating to fifty-three million years ago, the researchers recovered the fossils of an animal they called Pakicetus inachus . Little more than the back of the animal’s skull had been recovered, yet it possessed a feature that would unmistakably connect it to cetaceans.
It is easy to identify modern whales by their tail flukes, blowholes, and blubbery hide, but these are all later adaptations to life in the sea that not all early whales possessed. The features that link all whales are less obvious, and one of the most important is found on the skull. Cetaceans, like many other mammals, have ear bones enclosed in a dome of bone on the underside of their skulls called the auditory bulla. Where whales differ is that the margin of the dome closest to the midline of their skulls, called the involucrum, is extremely thick, dense, and highly mineralized. This condition is called pachyosteosclerosis, and whales are the only mammals known to have such a heavily thickened involucrum. The skull of Pakicetus exhibited just this condition.
Even better, the teeth of Pakicetus were very similar to those of mesonychids. It appeared that van Valen had been right, and Pakicetus was just the sort of marsh-dwelling creature he had envisioned. The fact that it was found in freshwater deposits and did not have specializations of the inner ear for underwater hearing showed that it was still very early in the aquatic transition. Gingerich and Russell wrote, “[Pakicetus is] an amphibious intermediate stage in the transition of whales from land to sea. Postcranial remains will provide the best test of this hypothesis.” The scientists had every reason to be cautious, but the fact that a transitional whale had been found was so stupendous that full-body reconstructions of Pakicetus appeared in books, magazines, and on television. It was presented as a stumpy-legged, seal-like creature, an animal caught between worlds that hinted that other such creatures might yet be found from the Eocene rock of Pakistan.53
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sp; There were still things to learn about Basilosaurus, too. In 1896, the paleontologist Charles Schuchert discovered a pair of hips and part of a femur associated with a Basilosaurus skeleton. It was difficult to tell whether the hips were attached to functioning limbs or were suspended in the flesh of the body wall, but since some living whales have cartilaginous vestiges of hips and hind limbs inside their body it seemed fair to assume that Basilosaurus, too, had lost its external hind limbs. There would be no reason for the whale to have them if it was indeed fully aquatic, but in 1990 Gingerich and several colleagues described numerous Basilosaurus hind limb fragments found in Egypt. No single leg was entirely preserved, but the scientists had found enough bones to almost fully reconstruct the small, three-toed hind limbs that would have stuck out on the underside of Basilosaurus. Similar discoveries revealed that other ancient whales like Takracetus and Gaviocetus had external hind limbs, too. Perhaps these archaeocetes were not so far removed from their terrestrial ancestors as had been thought.
Throughout the 1990s the skeletons of other more-or-less aquatically adapted archaeocetes were discovered at a dizzying pace, but a more complete skeleton of Pakicetus proved elusive. The stubby, seal-like form depicted in so many places began to make less and less sense as more archaeocetes became known. It was not until 2001 that J. G. M. Thewissen and colleagues described the sought-after skeleton.
The skeleton of Pakicetus attocki, reconstructed next to that of its smaller relative Ichthyolestes, was that of a wolflike animal. It was not the slick, seal-like animal that had originally been envisioned. Together with other recently discovered genera like Himalayacetus, Ambulocetus, Remingtonocetus, Kutchicetus, Rodhocetus, and Maiacetus however, it fits snugly within the graded series of archaeocetes that exquisitely document early whale evolution. Just as Huxley could not call Basilosaurus a direct ancestor of living whales, though, the panoply of known archaeo-cetes cannot be arranged in single evolutionary trajectory. It is not possible to draw a direct line of descent from Pakicetus through Ambulocetus, Rodhocetus, Georgiacetus, and Basilosaurus to modern whales. Instead, each genus represents a particular stage of whale evolution and together they illustrate how the entire transition took place.
FIGURE 56 - The reconstructed skeletons of the fossil whales Dorudon (top) and Maiacetus (bottom). Together they represent two grades of whale evolution, with Maiacetus still being capable of moving around on land and Dorudon being fully aquatic.
The earliest known archaeocetes were creatures like the fifty-three-million-year-old Pakicetus. They looked as if they would have been more at home on land than in the water, and they probably got around lakes and rivers by doing the doggie paddle. A million years later there lived other archaeocetes like Ambulocetus, an early whale with a crocodilelike skull and large webbed feet. The long-snouted and otterlike remingtonocetids appeared next, with small forms like the forty-six-million-year-old Kutchicetus exemplifying the greater diversity archaeocetes were attaining. These early whales lived throughout nearshore environments, from saltwater marshes to the shallow sea nearby; not all were becoming adapted to life in the ocean.
FIGURE 57 - The reconstructed skeletons of Pakicetus and its close relative, Ichthyolestes, based on more complete fossil material.
Living at about the same time as the remingtonocetids was another group of even more aquatically adapted whales, the protocetids. These forms, like Rodhocetus, were nearly entirely aquatic. In fact some later protocetids, like Protocetus and Georgiacetus, almost certainly lived their entire lives in the sea and dispersed to the shores of other continents. This shift allowed the fully aquatic whales to expand their ranges and diversify, and the sleeker basilosaurids like Dorudon, Basilosaurus, and Zygorhiza populated the warm seas of the late Eocene. These forms eventually died out, but not before giving rise to the early representatives of the two groups of whales alive today, the odontocetes and mysticetes. The early representatives of these groups appeared about thirty-three million years ago near the boundary between the Eocene and the Oligocene and ultimately gave rise to forms as diverse as the gigantic blue whale and the Yangtze River dolphin.54
The discovery of so many early fossil whales finally allowed researchers to reassess van Valen’s mesonychid hypothesis, which primarily rested on dental evidence. The teeth of early archaeocetes and mesonychids were extremely similar, so much so that initially the teeth of the early whales Gandakasia and Ichthyolestes were thought to have belonged to mesonychids. This strengthened the mesonychid-archaeocete link, for if the fragmentary remains of one group could so easily be thought to belong to those of another it was probable they shared a close relationship.
Studies coming out of the field of molecular biology conflicted with the conclusions of the paleontologists, however. When the genes and amino acid sequences of living whales were compared to those of other mammals the results often said that whales were most closely related to artiodactyls, even-toed ungulates like antelope, pigs, and deer. Even more surprising was that comparisons of the proteins used to determine evolutionary relationships often placed whales within the Artiodactyla as the closest living relatives to hippos.
FIGURE 58 - The astragali (ankle bones) of several mammals. The astragali of the fossil ungulates Phenacodus and Pachyaena (a mesonychid) resemble each other in having only one “pulley.” The ankle of Pakicetus and other early whales, however, share the specialized “double pulley” shape with the early artiodactyls Diacodexis and a living artiodactyl, the pig.
This conflict between the paleontological and molecular hypotheses seemed intractable. Mesonychids could not be studied by molecular biologists because they were extinct, and no skeletal features had been found to conclusively link the archaeocetes to ancient artiodactyls. Which were more reliable, teeth or genes? The conflict was not without hope of resolution. Many of the skeletons of the earliest archaeocetes were extremely fragmentary, and they were often missing the bones of the ankle and foot. One particular ankle bone, the astragalus, had the potential to settle the debate. In artiodactyls this bone has an immediately recognizable “double pulley” shape, a characteristic mesonychids did not share. If the astragalus of an early archaeocete could be found it would provide an important test for both hypotheses.
It was not until 2001 that archaeocetes possessing this bone were described, but the results were unmistakable. Archaeocetes had a “double pulley” astragalus, confirming that cetaceans had evolved from artiodactyls. Mesonychids could not longer be considered the ancestors of whales.
The group of artiodactyls that gave rise to whales, however, was still unknown. Hippos were the closest living relatives to whales but they and their extinct relatives (the anthracotheres) were already too specialized to represent the kind of animal whales evolved from. A better candidate was presented in 2007 when J. G. M. Thewissen and other collaborators announced that Indohyus, a small deerlike mammal belonging to a group of extinct artiodactyls called raoellids, was the closest relative to whales.
FIGURE 59 - The reconstructed skeleton of Indohyus. A close relative of early whales, it may represent the kind of animals from which the first cetaceans evolved.
FIGURE 60 - An evolutionary tree depicting the relationships of hoofed mammals. Whales, contained within the Cetaceamorpha box near the top of the diagram, group closely with hippos and are nested within the larger family of hoofed mammals called Artiodactyls. (Mesonychids, once thought to be ancestors of whales, group most closely with other carnivorous mammals near the bottom of the diagram.)
It was another accident that established this relationship. While preparing the underside of the skull of Indohyus, a student in Thewissen’s lab broke off the section covering the inner ear. It was thick and highly mineralized, just like the bone in whale ears. Study of the rest of the skeleton also revealed that Indohyus had bones marked by a similar kind of thickening, an adaptation shared by mammals that spend a lot of time in the water. While it was too late to be a direct whale ancestor, Indohyus preserved just th
e sort of traits that the archaeocete ancestor would have possessed. When the fossil data was combined with genetic data by Jonathan Geisler and Jessica Theodor in 2009, a new whale family tree came to light. Raoellids like Indohyus were the closest relatives to whales, with hippos being the next closest relatives to both groups combined (as well as the closest living relatives to whales given the extinction of the raoellids). At last, whales could be firmly rooted in the mammal evolutionary tree.
Establishing the outline of cetacean evolution and their relationships to other mammals has been an important task, but the “archaeocete revolution” extends beyond taxonomic classification. The fossil remains of archaeocetes also document how the transition from land to water was actually effected. Whales were not the first vertebrates to become secondarily adapted to life in the sea, however, and in order to fully understand the unique origin of whales it is profitable to look at a much older group of animals. Two hundred and forty-five million years ago, while the ancient ancestors of artiodactyls inhabited the corners of a world ruled by dinosaurs, a particular group of reptiles became adapted to life in the water.