by Brian Switek
While many of the early proboscidean lineages became extinct, one gave rise to more elephantlike forms such as Palaeomastodon and Phiomia. These and all their later relatives belonged to a group called the elephantiformes, marked by the possession of a nasal opening high up on the skull for the attachment of a trunk; enlarged, forward-facing tusks; and the loss of a few teeth, meaning that their premolars and molars carried the full duties of chewing.
This masticatory transition can be seen in another recently described fossil from eastern Africa named Eritreum. At twenty-seven million years old, Eritreum was a relatively small, four-tusked creature that shared traits with both earlier elephantiformes like Palaeo- mastodon and the later group containing mastodons, mammoths, and living elephants called elephantimorphs.67 In its lower jaw it only possessed a pair of tusks (enlarged second incisors) and a set of molars, yet a tooth still embedded in the lower jaw of the first described specimen shows that it had a trait similar to what is seen in modern elephants.
FIGURE 73 - A simplified elephant family tree (highlighting the relationship of Eritreum to other proboscideans). Living elephants are contained within the Elephantoidea, the diverse gomphotheres within the Gomphotheroidea, and mammoths within the Mammutidae.
FIGURE 74 - The restored lower jaw of Eritreum (with photos of the actual specimens) as seen from the side and from above. The third molar had only just begun to erupt and, as seen in the top photo, the scientists had to dig into the lower jaw to fully expose it. Artwork by Gary H. Marchant.
African and Asian elephants are among the longest-lived of all mammals. It takes a lot of time and energy to reach such prodigious size, and this requires that elephants consume huge quantities of relatively low-quality foods such as leaves and grass. Their teeth dare worn down severely by all that chewing and do not regenerate.68
Elephants have a very interesting adaptation that allows them to avoid wearing down their teeth too fast. Rather than popping out of the gums all at once, their teeth erupt sequentially with only one grinding molar in use at a time. As subsequent teeth develop in the jaw they eventually push out the previous, worn-down tooth, ensuring that elephants have fresh teeth for much of their lives (though some exceptionally old individuals have been observed grind down their very last set).
An early incarnation of this adaptation can be seen in Eritreum. While its molars were intermediate in size between proboscideans like Palaeomastodon and the first true mastodons, it did not get all of its adult teeth at once. In the specimen examined by scientists, the third molar was still developing inside the jaw but there was no room for it in the lower jaw. As it emerged, it would have pushed the second molar forward, and the first molar, by then worn down considerably, would have been pushed out of the jaw. As far as is presently known this adaptation was not present in early elephantiformes, so Eritreum appears to have been very close to the common ancestor of the profusion of mastodons that would spread all over the world.
While the name “mastodon” was first applied to what was once known as the American Incognitum (or Mammut americanum to today’s paleontologists), today the term is used for a diverse array of elephants with coarse ridges on their molars. Despite this shared tooth form, however, mastodons came in a variety of shapes and sizes. Some, such as Gomphotherium, had low skulls with two upper and two lower tusks that stretched forward to give it a long profile. The shovel-tuskers Platybelodon and Amebelodon were a modification of this form, as were later, short-faced mastodons such as Stegomastodon.
With the exception of the long-lived genus Deinotherium, it was primarily the mastodons that left Africa during the Miocene and began to disperse through Eurasia to North and South America. They were so prolific, in fact, that in many places several species and even genera lived in the same habitats at the same time, each feeding on different types of plants. Some, like the wide-ranging Gomphotherium, were mixed feeders while other proboscideans that shared the same landscape were more exclusively browsers or grazers.
Proboscideans remained widespread through the following Pliocene and Pleistocene epochs, many of them being adapted to a grazing diet as the world cooled and grasslands spread across the globe. New forms continued to evolve alongside more archaic lineages, including the one containing the American mastodon. Though it survived until about 10,000 years ago, the American mastodon was a remnant of a lineage that had split off many millions of years earlier. By the Pliocene, however, some parts of the earlier mastodon radiation had been lost, and a new diversification event in Africa would give rise to the most famous representatives of near prehistory, the mammoths.
FIGURE 75 - A restoration of Eritreum (foreground) with the much larger Gomphotherium (background).
Though the genus Mammuthus first evolved in Africa around four million years ago, much of mammoth evolution took place in Eurasia and, much later, North America. The dispersion of mammoths started when a mammoth adapted to warm climates, Mammuthus rumanus, moved out of Africa around three million years ago. It spread out through Eurasia (perhaps as far as China). One population of this species might have been ancestral to the species M. meridionalis.
M. meridionalis has long been considered the progenitor of the much larger steppe mammoth, M. trogontherii, which is thought to be ancestral to both the smaller woolly mammoth (M. primigenius) and the Columbian mammoth in North America (M. columbi). For a time this evolutionary pattern seemed quite simple, with the mammoths developing shorter, high-domed heads and an increased number of folds to their grinding molars over time, but recent reexaminations have shown that mammoth evolution could not be so easily fitted into a single-file pattern.
M. meridionalis was a long-lived and widespread species, but some of the later members of this species look very similar to early steppe mammoths. This suggests that a population of M. meridionalis may have been ancestral to the steppe mammoth (or at least been modified into an as-yet-unknown species that in turn was ancestral to M. trogontherii ). While the two species overlapped in time for about a half million years, by about 600,000 years ago the steppe mammoth exclusively occupied the habitats once inhabited by its ancestral species.
The steppe mammoth wandered all over the northern hemisphere, even crossing the Bering Strait into North America. When they reached the New World about 1.5 million years ago they moved south into warmer grassland habitats where a population was adapted into a new species, the Columbian mammoth, which ranged southward into the heart of Central America. Once again, the ancestral species became extinct while the descendant spread out.
The most famous descendant of the steppe mammoth, however, was the smaller woolly mammoth. The transition occurred between about 200,000 and 150,000 years ago, but not in the slow-and-steady pattern that might be expected. By looking at the molars of the mammoths paleontologists have been able to see that the change was relatively abrupt. The steppe mammoth is represented by a particular molar type, but the woolly mammoth molar type evolved quickly and stabilized. The tooth forms seem to be static on either side of the transition, and this points to a particular evolutionary pattern called punctuated equilibrium.
Punctuated equilibrium, known as “punk eek” for short, was first proposed by paleontologists Niles Eldredge and Stephen Jay Gould in 1972. Both scientists were invertebrate specialists, and as such they were able to look at patterns in the fossil record with somewhat more detail than vertebrate paleontologists. Many prehistoric invertebrates were prolific, covered in hard external parts that allowed them to be fossilized relatively easily, and represented by so many individuals that many minute comparisons could be made.
Eldredge’s work on trilobites, a diverse group of marine arthropods that looked like a cross between a horseshoe crab and a pillbug, tipped off the duo that species did not always evolve at a uniform rate over time. Eldredge saw a general condition of stasis in which there was almost no change at all, followed by the abrupt appearance of new species alongside their ancestral forms. This suggested that populations of organisms were becoming i
solated and undergoing rapid change into new species that would overlap in time with the species from which they had evolved.
This idea was controversial from the start, especially among evolutionary theorists who specialized in genetics, like Richard Dawkins and John Maynard Smith, but as paleontologists reviewed their data they found similar patterns. For years scientists had ignored stasis because it simply was not interesting, but Gould and Eldredge’s paper insisted that stasis was relevant to evolution. The pattern of mammoth evolution supports this. The evolution of the northern hemisphere mammoths over the past four million years shows a pattern of mammoths spreading out, populations evolving into a new species, and that new species spreading out to eventually occupy the land of their ancestors after a period of overlap.
In the case of the woolly mammoth, the species evolved somewhere in eastern Eurasia and then spread both westward toward Europe and eastward into North America, replacing the larger steppe mammoth. At the height of their distribution during the Pleistocene, woolly mammoth populations formed a continuous belt from southern Spain eastward into the heart of the United States, though this range fluctuated as habitats changed. The woolly mammoth was a grazer, dependent on grassland habitats to survive. Where the plains spread, so did the mammoths.
The continuously shifting range of the Pleistocene mammoths was also dictated by the waxing and waning of the world’s glaciers. The world has seen many periods marked by the advance of ice sheets and glaciers, but the most recent cycle began about 2.5 million years ago, when ice sheets began to creep over the northern hemisphere. Since that time, they have advanced and retreated according to a 40,000- to 100,000-year cycle, with this pattern constantly opening and closing pathways for mammoths. When the glaciers built up the sea level dropped, opening coastal routes to otherwise inaccessible feeding grounds; when the ice melted sea levels rose and flooded those routes. Not only did this cycle influence where mammoths roamed, but it also led to the evolution of pygmy mammoths.
Around 47,000 years ago, glacial advance dropped sea levels enough to reduce the distance between the Channel Islands off California and the mainland to a distance of about four miles, a stretch that the mammoths apparently swam across just as Asian elephants can swim for miles out in the sea today. The mammoths that took the brisk dip eventually made it to the island of Santa Rosae, but as the glaciers melted sea level rose and increased the distance between the island and the California coast, making it too far for the mammoths to swim back. Rather than die out, however, the mammoths became adapted to their new island home by becoming dwarfed. How this occurred is still being debated by scientists, but the island mammoths eventually stabilized at no more than seven feet high at the shoulder, half the height of the ancestral species. They were distinct enough to receive their own species name Mammuthus exilis, and they persisted on that island until about 11,000 years ago.
Other populations of mammoths and elephants underwent similar changes. There was a collection of different species of dwarf elephants on islands presently submerged in the Mediterranean, islands containing miniscule representatives of the genus Stegodon in Indonesia, and miniaturized woolly mammoths on St. Paul Island off Alaska and just across the Bering Strait on Wrangel Island, just north of Siberia. The causes that led to the creation of the island mammoth off California probably led to the origin of these species as well, and some of them survived for a long time on their island refuges. The St. Paul Island mammoths survived until about 8,000 years ago and the Wrangel Island mammoths until about 4,000 years ago, the latter date being around the time that the Egyptian empire was flourishing and the alphabet was being developed.
As mammoths spread across the northern hemisphere, other elephants spread to new continents as well. Around the time that Mammuthus was beginning to colonize Eurasia, the “splendid isolation” of the South American continent was ended when it was joined to North America via the isthmus of Panama. As continental drift steadily pushed the continents closer together, various species island hopped across the narrowing strip of water between them, but the establishment of the land connection about four million years ago opened a thoroughfare for creatures to strike out for new territories. From the south came creatures like giant ground sloths and their tanklike glyptodont cousins, and the North American mastodons Stegomastodon and Cuvieronius (along with numerous other creatures) pushed south.
With the exception of Australia and Antarctica, during the Pleistocene there were numerous genera and species of elephants on every continent. Some of these, such as the American mastodon, were the last stalwarts of much older lineages, while other elephants, especially the mammoths, rapidly speciated and spread far and wide. But by about 10,000 years ago, most of these creatures were almost, if not entirely, extinguished, including the American mastodon, the woolly mammoth, Stegomastodon and Cuvieronius in South America, and many of the dwarfed island elephants. Only the ancestors of the modern African and Indian elephants, the “tattered remnants” of elephant diversity, remained.
Nor were the mammoths and mastodons the only species to disappear. Many of the world’s largest animals, from saber-toothed cats to giant ground sloths, all went extinct at around the same time. Even the large mammals that survived, such as lions and horses, had their ranges severely restricted. What could have killed so many unique Pleistocene animals and spared the creatures we are familiar with today?
FIGURE 76 - Restorations of a Columbian mammoth (left) and an American mastodon (right), two of the elephants which lived in North America until about 10,000 years ago.
As with any murder mystery, identifying the victims and their time of death is important, and each continent that was home to the lost “megafauna” of the Pleistocene tells a different story. In North America about thirty-four large mammals, in addition to a handful of smaller ones, disappeared during an extinction spike between about 16,000 and 10,000 years ago. Though the timing has so far been difficult to pin down for South America, it appears that many of their large mammal species were gone by 10,000 years ago, as well.
The pattern in Eurasia was more complex. Between about 49,000 and 23,000 years ago many of the large mammals adapted to warm climates—such as the straight-tusked elephant Elephas antiquus and hippos—became extinct. As glaciers advanced so did cold, dry grasslands that were home to grazers like mammoths and horses. Yet some of these large steppe mammals themselves went into sharp decline as of 14,000 years ago, eventually dying out.
Australia, while it was never home to any elephants, also lost species. The island continent was home to many large marsupials unlike anything seen anywhere else in the world, like the wombat relative Diprotodon, the size of a small car, and a leopard-sized carnivorous marsupial named Thylacoleo, which had shearing teeth shaped like meat cleavers. The extinction event that claimed them kicked off earlier, about 40,000 years ago, with different groups tapering subsequently into nonexistence.
The only exception to the general pattern appears to be Africa. The continent has lost some large mammals within the last 100,000 years and yet it still is home to many of the large mammal groups that where decimated elsewhere.
Numerous non-mutually-exclusive explanations for the great dying during the late Pleistocene have been proposed over the years. Despite recent headlines about how a swarm of comets that struck North America about 12,900 years ago may have been the extinction trigger, most of debate has centered over the influence of two major known killers: climate change and humans.69
Climate change has been the traditional suspect. Around 20,000 years ago the world was marked by the Last Glacial Maximum, or the further reach of the ice sheets before they began to recede again. After this time, the warming climate peeled back the ice from the continents, causing sea levels to rise, and forests grew where cold grasslands once dominated. This rapid climate change established the present interglacial period, generating the relatively warm and wet world we presently inhabit. As we have recently become aware due to the effects of huma
n-caused climate change, rapid swings in climate can put large animals adapted to cold habitats at risk of extinction. Large mammals in the past would likewise have been affected by rapid change.
In the case of the mammoths, their success was largely tied to the spread of grasslands, and after 20,000 years ago that grassland habitat was shrinking. Their preferred haunts became more restricted to areas like northern Siberia, and fragmentation of their habitat caused the flow of genes between populations to be cut off. The same pressures would affect other grazers that shared the same steppe habitats, and the predators of the large megamammals would find their food sources restricted as the herbivores dwindled.
Global climate change would seem to be a nearly made-to-order cause for extinctions that would affect a large number of animals all over the world, yet it is not without some significant problems as an extinction trigger. One snag is that many of the mammals that became extinct had survived similar fluctuations before. It is difficult to explain why they would have survived previous disruptions but perished during the most recent. Nor was it just the cold- adapted grazers that became extinct. The American mastodon, a browser who lived in more forested environments, its South American cousins, and the Columbian mammoth, which had evolved to live in more temperate climates—all became extinct, too. How climate change would have bent their habitats to the breaking point is unknown, and at present our understanding of the impact of climate change does not entirely fit the global pattern of extinction.
In recognizing the habit of our species to alter the environment in ways detrimental to large mammals, though, some researchers have preferred a scenario in which humans took a direct role in eliminating the world’s megafauna. Though Homo erectus, Neanderthals, and other prehistoric humans inhabited Eurasia over the last two million years, it was Homo sapiens that became a true world traveler, leaving Africa about 100,000 years ago and spreading all over the world.
