From Natural History, July 2008, copyright © Natural History Magazine, Inc., 2008. (Republished with permission).
During the past 40 million years, three great lineages arose in the dog family.
Two are now extinct, but diversity thrives in the array of living species.
Original story by Xiaoming Wang and Richard H. Tedford ~ Illustrations by Mauricio Antón (www.mauricioanton.com)
This article was adapted from Dogs: Their Fossil Relatives and Evolutionary History, by Xiaoming Wang and Richard H. Tedford and illustrated by Mauricio Antón. Copyright © 2008 Columbia University Press. Used by arrangement with Columbia University Press. All rights reserved.
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THE DOWNFALL of the dinosaurs 66 million years ago gave mammals an incredible opening, and they ran for it, rapidly becoming the dominant land vertebrates. Among those to emerge were the earliest carnivorans (members of the order Carnivora), whose living representatives include the cats and closely allied families, such as hyenas and mongooses, as well as dogs and closely allied families, such as bears, weasels, and seals. As their name implies, most carnivorans eat meat, and even those that aren’t carnivorous – such as the giant panda – can be recognized by the last upper premolar and first lower molar on each side of the mouth. Those teeth are specially adapted for shearing, and are known as carnassials. Only in some species, such as seals and sea lions, have the carnassials evolved into simpler forms.
Back when mammals got their big break – during the Paleocene epoch, which lasted ten million years – conditions around the globe were warm and humid. And the epoch that followed, the Eocene, was marked by a warming trend so great that even the polar regions were quite hospitable to life. Surging into prominence, flowering plants diversified and created lush forests all over the Earth. In North America, where tree canopies sheltered a growing number of primates and other forest-dwelling mammals, the earliest carnivorans arose. From there they spread to Eurasia, over land bridges that then existed to Europe or near the present-day Bering Strait. Mostly the size of small foxes, or smaller, the carnivorans were adapted to life in and around trees, probably preying on invertebrates and small vertebrates. They lived in the shadow of the generally much larger hyaenodonts, a group of mammalian predators that had come on the scene earlier but which later became extinct.
When did the carnivorans split into their catlike and doglike divisions? No one knows exactly; it may have been 50 million years ago or even earlier. By 40 million years ago, however, the first clearly identifiable member of the dog family itself, the Canidae, had arisen in what is now southwestern Texas. Named Prohesperocyon wilsoni, the fossil species bears a combination of features that together mark it as a canid. Fittingly enough, these include features of the teeth – including the loss of the upper third molars, part of a general trend toward a more shearing bite – along with a characteristically enlarged bony bulla, the rounded covering over the middle ear. Based on what we know about its descendants, Prohesperocyon likely had slightly more elongated limbs than its predecessors, along with toes that were parallel and closely touching, rather than splayed, as in bears.
The dog family thrived on such limb adaptations, which helped support a cursorial, or running, lifestyle in response to a changing environment. And none too soon, for the subsequent epoch, the Oligocene, between 34 million and 23 million years ago, started a long trend of climatic deterioration. Ice sheets appeared on the Antarctic continent for the first time, while in mid-latitude North America, conditions became progressively dryer and seasonal variations more pronounced. The lush, moist forests of the late Eocene gave way to dry woodlands and then to wooded grasslands, with large areas of open grassland developing by 30 million years ago. Mammalian herbivores began to evolve teeth adapted to eating grass (so-called high-crowned teeth, which continue to erupt as the chewing surfaces are worn down). For both predators and prey, the ability to run and survive in an open, exposed landscape became crucial. To a large extent, the history of the dog family is a story of how a group of cursorial predators evolved, through speed and intelligence, to catch changing prey in a changing landscape.
The canids are one of three modern families of carnivorans notable for including top predators, species capable of hunting down prey several times their own size. The other two are the cat family (the felids) and the hyena family (the hyaenids). On land, at least, there appears to be a body-size threshold of around forty-five pounds beyond which a mammalian predator must begin to tackle larger prey in order to get enough energy. Chris Carbone, a senior research fellow in biodiversity and macroecology, and colleagues at the Institute of Zoology, the research division of the Zoological Society of London, have suggested that small predators can sustain themselves on invertebrates and small vertebrates because of their low absolute energy requirements.
In 1871, pioneer vertebrate paleontologist Edward Drinker Cope formulated the principle that in animals, small body sizes tend to evolve into large body sizes. With the help of our colleagues Blaire Van Valkenburgh, a functional morphologist at the University of California at Los Angeles, and John Damuth, a biostatistician at the University of California, Santa Barbara, we have examined the canid fossil record with that idea in mind. We have concluded that, indeed, larger and larger species have repeatedly evolved in many lineages. Consequently, many species have independently passed the threshold where they needed to take down large prey. Features of their jaws and teeth show that the larger canid species have also tended to become hypercarnivorous, that is, more purely meat-eating.
The cat family and the hyena family similarly evolved hypercarnivorous top predators. (One might think the bear family, the ursids, should be added to this list, but only the polar bear is hypercarnivorous, and it is a rather atypical member of the family. Most bears are omnivores.) It’s only a slight oversimplification to say that felids almost invariably approach their prey in stealth and try to pounce on it in surprise attacks. Modern canids, by contrast, have a decidedly different tactic, one suited to their ancestors’ lifestyle on the open plains. In that setting, surprise attack is seldom achieved; it is less important to subdue the prey in the shortest possible time than to outrun and exhaust the quarry. Lacking retractable claws, a powerful weapon for most felids, canids rely more on social hunting when confronting large prey – using sheer numbers and coordinated hunting strategies rather than sophisticated weaponry to overwhelm them.
Hyaenids are more closely related to cats, yet they more strongly resemble canids, both behaviorally and anatomically. They kill their prey by consuming them alive, rather than by delivering a killing bite on the neck as felids do. They too are persistent pursuers rather than stalkers that ambush prey, and they tend to be highly social hunters. The similarities are a good example of convergent evolution, an understandable outcome when one realizes that for much of their evolutionary history, the two groups were not direct competitors but were facing similarly open environments. Canids were at first confined to North America, whereas hyaenids arose in Eurasia.
When did canids become so diverse? From Hesperocyon, a descendant of Prohesperocyon, the family experienced its initial radiation in the early Oligocene, about 34 million years ago, splitting into three major subfamilies: the Hesperocyoninae and the Borophaginae (both extinct lineages known only from fossils), and the Caninae, whose descendants survive today. But it is at first only among the hesperocyonines that we see some really dominant dogs, capable of hunting prey larger than themselves. They were the size of small wolves and equipped with teeth specialized for ripping into raw meat, comparable to those of modern African hunting dogs. The early borophagines, on the other hand, were all smaller and tended toward less predatory lifestyles. And biding its time was the Caninae subfamily, comprising only a few inconspicuous species (we’ll avoid calling them “canines,” a term that is usually used in a narrower sense).
Altogether, by about 30 million to 28 million years ago, twenty-five species of canids roamed western North America, a peak of diversity within a continent unequaled before or since by any single family of carnivoran. The dog family was making its mark. Meanwhile, the hyaenodonts and other archaic predators had begun to decline, and they were eventually overtaken by the successful carnivorans.
North American herbivores, the potential prey for canids, steadily diversified during the first half of the following epoch, the Miocene, which lasted from 23 million to 5 million years ago. That was thanks not only to evolution but also to immigration of Eurasian native species via land bridges. The herbivores reached an all-time peak of diversity around 15 million years ago, and perhaps not coincidentally, canids experienced a second peak of diversity (some twenty species) at the same time. But now mostly the borophagines were the ones to flourish. The hesperocyonines were on the verge of extinction, while the Caninae continued to keep a low profile.
Among the factors driving canid evolution was the increasing speed of the grazing herbivores, which in turn was a response to being preyed upon in open habitats. The well-known illustration of this process is how members of the horse family essentially came to run on the tips of their toes, evolving longer toe bones and eventually losing their lateral digits. Even though canids were getting faster, they also had to adjust to competition from new carnivoran immigrants, including members of the cat family; false saber-tooth cats (which were catlike but not true felids); large mustelids; and giant bear dogs (family Amphicyonidae). Bone-cracking became a specialty of the new borophagine species that arose at the time, suggesting that they regularly scavenged carcasses – a kind of resource that is easier to locate in a more open environment. The ability to consume bones may have arisen as a byproduct of group feeding among social predators, in which individuals, trying to consume as much food as quickly as possible, ate bone (or swallowed meat plus bones indiscriminately).
The Caninae lineage, present from the early Oligocene, finally made its big move during the late Miocene, as the open grasslands continued to expand. One distinctive feature of the subfamily, which had slender, elongated limbs, is that the front and hind big toes became progressively smaller, and ceased to be functional. This cursorial feature, not found in the other two canid subfamilies, became an advantage when the landscape opened up. By the late Miocene, early precursors of the modern “true” foxes (tribe Vulpini) had emerged, as well as a genus, Eucyon, that was ancestral to the tribe Canini. The latter group comprises the “canines” in the narrow sense of the term, and includes dogs, wolves, coyotes, jackals, certain foxes, and other species.
A key development in Caninae history was the spread of the subfamily out of North America, beginning about 7 million years ago, when some groups crossed the Bering land bridge into Asia. With the exception of a single species in the middle Miocene of China, hesperocyonines never escaped the dog family cradle, nor did any borophagines. Records of the Caninae appeared in Europe first, and almost immediately thereafter in Asia and Africa. The first member of the genus Canis – to which the gray wolf, coyotes, jackals, and the domesticated dog belong – loped onto the scene about 6 million years ago.
During the subsequent epoch, the Pliocene (5.3 million to 1.8 million years ago), a further opportunity opened up for the Caninae. About 3 million years ago, the Panamanian Isthmus formed, linking North and South America. Carnivorans that arrived in South America generally trumped the native predators, and the Caninae were part of that success story, radiating explosively out of a few lineages in Central America and southern North America. Members of the subfamily constitute the largest group of carnivoran predators in South America today. Indeed, with eleven species, South America is home to almost one-third of the entire canid diversity on the planet.
Just as the intercontinental flux led to a new peak of diversity among the canids – one that continues through the Pleistocene epoch and down to the present time – so, too, did it influence the array of prey. Ancestral horse species, which had lost their two outer digits but retained three, were eclipsed in North America by single-digit horses. By Pliocene to early Pleistocene times, the modern horse genus, Equus, had spread to Eurasia and South America, along with members of the camel family (mostly llamas and their extinct relatives), which, like canids, had been confined to North America during much of their existence. While the Caninae subfamily thrived, however, borophagines dwindled to one or two species of highly specialized bone-cracking dogs, which became extinct by the end of the Pliocene.
The third canid expansion brought dogs into contact with hyaenids, which, with one brief exception during the Pliocene, had never expanded into North America. By the Pliocene, however, the competitive landscape had changed significantly for both families, and their members weren’t fighting for the same fare. The foxes and jackal-like dogs that arrived in the Old World were much smaller than most hyaenids, which by now were all large, bone-cracking animals.
If we look around today at the major terrestrial carnivoran families – canids, felids, ursids, mustelids, and others – we see that each has a balanced spectrum of small and large species, but not the hyaenids. Apart from the aardwolf, which is a highly specialized termite-eater, there are only three living species of hyenas, all large carnivores. In the major carnivoran families, if the large-size species become extinct in the future, smaller forms could evolve to replace them. But if the large hyenas one day become extinct, their great evolutionary lineage will end.
Climate change kicked into high gear during the Pleistocene epoch, whose alternating cold, dry ice ages and warm, humid intervals was a tumultuous time for all animal and plant evolution. Many mammal species on the northern continents (North America and Eurasia), particularly herbivores, attained giant sizes as an adaptation to extreme cold. Large body size helped not only to conserve heat, but also to store more fat to cope with winter weather. Woolly mammoths, giant deer, and woolly rhinos roamed Eurasia, and the woolly mammoth, mastodon, giant ground sloth, large saber-tooth cat, and dire wolf reigned supreme in North America. Most such megafauna became extinct at the end of the Pleistocene, about 10,000 years ago. But the gray wolf, Canis lupus, is one of the few exceptions, and remains one of the most successful large canids in the world.
From about the beginning of the Pleistocene the genus Canis has had a continuous presence in Eurasia, along with various species of fox and raccoon dog. Gray wolves are present beginning about 1 million years ago. Early humans – Homo erectus, H. neanderthalensis, and H. sapiens – must have competed with some larger species of canids, because they shared a broadly similar hunting (and scavenging) lifestyle. By the end of the Pleistocene, the inevitable close encounters between modern humans and wolves – in the Middle East or Europe, or possibly China – resulted in the first domestication of a canid. If one counts the domestic dog as a highly specialized adaptation for cohabiting with humans, then canids have achieved ultimate success in occupying nearly every corner of the world – in all sizes, shapes, and speeds.