THE AMAZING FOSSIL finds from the Jurassic and Cretaceous show that diverse mammalian pioneers were flying around the heads of dinosaurs, scurrying between their feet, and swimming in their rivers; some were even beginning to turn the traditional predator-prey relationship on its head. This growing roster of mammals and their relatives seems, quite literally, to have been energized by gymnosperm-dominated Jurassic forests as well as Cretaceous forests with an increasing representation of angiosperm communities. Nevertheless, while our earliest mammalian forerunners were certainly more vibrant than is often appreciated, their main break still clearly came with the fall of the dinosaurs. To watch how mammalian understudies became the stars of the show, we must leave China and travel to North America. Here, at the Corral Bluffs in Colorado’s Denver Basin, we can find one of just a handful of localities in the world with a fossil record that brackets the disastrous K-Pg impact. The Corral Bluffs are a single, continuous, exposed outcrop that runs for 27 km2 on either side of Highway 94. Thousands of motorists fly past this part of the windswept plains of western North America without ever realizing its significance. Yet approximately 66 million years ago, this location was in the direct firing line of the pulse of heat emanating from the massive Chicxulub impact. Frozen in time, fossils found within the Bluffs span ecosystems that were heading obliviously into disaster during the last 100,000 years of the Cretaceous and, crucially, those that emerged from the rubble in the first 1 million years of a new, green Paleocene era.
“The fossil bonanza that has emerged from the Bluffs makes it one of the best sites, if not the best site, for studying the ‘comeback’ tour of life after the lethal asteroid collision,” says Dr. Tyler Lyson, a paleontologist at the Denver Museum of Nature and Science. Although ancient-fossil detectives are usually on the lookout for skeletons poking out of sediments, this time the treasure could only be found by looking inside inconspicuous “concretions.” To the untrained eye, these might look, for all intents and purposes, like solid rocks. However, as the team began to split these concretions open back in the lab, they discovered the biological secrets that lay within. Specifically, they found the incredibly well-preserved remains of turtles, crocodiles, plants, and, most significantly, a rapid (at least in the context of geological time) succession of mammal forms that show the way in which these animals strutted to the top of the Paleocene. Mammals were undoubtedly also massively impacted by the K-Pg extinction event, and the largest of this lineage that scrambled through this apocalypse weighed no more than half a kilogram. Nevertheless, just 100,000 years down the line, their descendants weighed six kilograms; 200,000 years later, mammals had reached twenty kilograms—far heavier than any pre-K-Pg mammals. By 700,000 years after the disaster, mammals weighed over forty-five kilograms—more than an adult hyaena or chimpanzee. Traditionally, the unrelenting arrival of ever larger mammals so soon after the global apocalypse at the end of the Cretaceous has been put down to a simple absence of dinosaurs. Basically, without the “terrible lizards,” our ancestors would no longer have had any reason to hide in the shadows. However, to settle for such a simple explanation would be to do the dynamic early mammals and their vibrant leafy companions a disservice.9
The asteroid impact and the reduction in access to the Sun’s energy that followed undoubtedly caused ecological disruption and localized loss of vegetation diversity. However, many plant families found as Cretaceous fossils exist today and seem to have had a special capacity to adapt to rapid, global environmental change. The angiosperms were here to stay and were not about to stop expanding. Fossil pollen from the Corral Bluffs record shows us that the increase in mammal body size occurred at the same time as plants were getting more diverse and more nutritious in a warming world where tropical climatic conditions prevailed across much of the planet. While ferns were the first to emerge out of the North American ashes, the angiosperms, growing taller and more nutritious than ever before under the warming Paleocene conditions, soon dominated the landscape. Three hundred thousand years after impact, the largest mammal around was Carsioptychus, a distant ancestor of all modern hoofed mammals, or “ungulates.” This animal had large, flat premolar teeth, with bizarre fold-like shapes, that were ideally adapted to the munching of hard objects, like the nuts of trees from the walnut family, which appears in the fossil plant record at the same time. After another 400,000 years, the largest mammal in the Corral Bluffs record occurs alongside the sweeping entrance of beans into the fossil plant records of North and South America. Their energy-filled leaves and protein-rich seed pods provided ideal sustenance for growing mammal bodies. The close relationship between increasingly large mammals and their tropical angiosperm companions can be seen in the Corral Bluffs fossil record through the ever more popular trend toward vegetarianism. The close bond between mammals and tropical plants seen in Colorado can be found across the global fossil record of the warm, dense forests of the Paleocene.10
The only way was up for mammals and their angiosperm partners as the Eocene (literally, “new dawn”) broke 56.0 million years ago. After a cooler blip in the late Paleocene, the planet warmed rapidly, spurred on by tectonic plate movements, and megathermal forests expanded all the way toward the polar regions. More forests meant more vegetation and more opportunities for mammals. The modern ungulates, which include the majority of large land mammals still around today, expanded. One group, the perissodactyls (which bear their weight on one of their five toes), including the ancestors of horses, tapirs, and rhinoceroses, were particularly successful, with their special gut fermentation system allowing them to process the fibrous leaves of dense forests. Another group, the artiodactyls (which bear their weight equally on two of their five toes), also emerged, including the ancestors of pigs, goats, hippos, and cattle in Africa and the ancestors of the camel in North America. This group remained at the fringes of the largely forested landscapes until the middle-late Eocene. Some even took their chances on the high seas, and evidence from a hippo-like artiodactyl found in Pakistan, dating to approximately 50 million years ago, has been used to show that modern whales and dolphins derive from early land-dwelling artiodactyls that gradually went on to adapt to ocean life through the Eocene. In South America, marsupials, armadillos, and now extinct ungulate forms diversified. Early forms of carnivores with paws, rodents with claws, elephant-related “proboscideans,” bats, and arboreal lemur-like primates had also all appeared on Earth by the middle Eocene, completing the sweep of most of the modern mammal groups across ecosystems that would have seemed remarkably similar to those of the modern world. In fact, the main difference to today would have been the size of these mammals, and many of these early forms would have looked like miniaturized models of their twenty-first-century relatives, with small sizes serving as a coping mechanism against the heat and dense forests that prevailed up to the middle Eocene.11
The Messel Pit, a disused quarry previously mined for oily shale, just southeast of Frankfurt am Main in Germany, provides a remarkably vivid picture of these flourishing, cosmopolitan middle Eocene mammal ecosystems. Here, slap bang in the middle of modern Europe, a fair distance from the tropics, paleontologists have discovered a wet, warm forest ecosystem where early hyaena relatives menaced early rodents, hedgehogs, and even marsupials, where a semiaquatic otter-like mammal took dips in swampy wetlands, and where early relatives of horses and an ancestor of rhinoceroses made increasingly larger strides. In the trees themselves, early primates consumed fruits, and the small catlike carnivore, Paroodectes, stalked along branches of tropical trees like Canarium, cashew, and nutmeg that covered central Germany at this time. Taking leave of the ground, early bats flew around the heads of these other flourishing mammals. Study of the small mammals at Messel show that a “rich” community existed that would not be out of place in the canopy of a multilayered tropical rainforest today, sustained by angiosperm fruits and seeds. Undoubtedly, the spread of angiosperms to almost every corner of the globe encouraged and supported this cornucopia of early mamma
ls found across the major continental landmasses at the time. But this isn’t simply a story of take, take, take on the part of the new and increasingly diverse mammals: it may be that their green companions also benefited.12
Indeed, not just mammals, but also the angiosperm plants themselves, underwent major changes in shape and diversity between the Cretaceous and middle Eocene. First and foremost among these changes was the development of the fruits that many modern plants use to attract primates, bats, and birds so that they will disperse their seeds, carrying their potential offspring to new parts of the landscape often many kilometers away. This strategy is particularly important in tropical forests, where around 70 to 94 percent of woody species produce fleshy fruits. Fruits have been preserved in the angiosperm fossil record back to the Late Cretaceous, but, significantly, angiosperm creativity in terms of seed sizes and fruit types gradually increased, peaking in the early Eocene 56.0 million to 48.1 million years ago, just at a time when most modern mammalian orders were arriving on the scene. Some trees, like the walnut family mentioned at Corral Bluffs, even shifted from winged seeds that were good for flying to more robust seeds that provided better packaging for being couriered by animals. Despite dispute about the extent of the earliest mammals’ involvement in tropical angiosperm evolution and dispersal, it remains highly likely that at least some of the earliest mammals and their relatives, like the multituberculates, had a symbiotic relationship with the plants that were sustaining ever-increasing mammalian dominance.13
THE PERIOD FROM the late Eocene (37.7 million to 33.9 million years ago) through the Oligocene (33.9 million to 23.04 million years ago) to the Miocene (23.04 million to 5.33 million years ago) saw the beginning of significant global swings between cooling and drying and warming and wetting as the continents and increasingly active flows of water between the world’s oceans and hemispheres began to assume their present-day positions. The late Eocene heralded a dawning of seasonality in planetary climate conditions and the gradual retreat of warm, wet forests back toward their current tropical positions. To some extent, this perhaps foreshadowed a decline in the role of these environments in mammalian evolution beyond the tropics. However, these broad vegetation changes forced mammals who had, to this point, been flourishing in megathermal forests to adapt. This process left us with some of the most iconic mammals around us today in both tropical and temperate regions. To take the first example, horses are important not just for transport, or for their cultural significance ranging from western films to a day at the races, but also because their evolution provides one of the clearest perspectives on how the role of megathermal forests has changed in mammalian evolution over the last 35 million years. As paleoecologist Professor Gina Semprebon, of Bay Path University in Massachusetts, an expert in early horse evolution, states, “We might not recognise it today, but our hooved friends, from their appearance in the North American Eocene, through to the present, have actually undergone radical changes in their body shape, size, and preferred diet that track global shifts in environments across much of the Earth.”14
The teeth of animals, including our own, have much to say about their lifestyle and environment. For herbivores, specialized grazers can have high, ridged, “hypsodont” teeth, which extend past the gums and provide greater resistance to gritty diets of coarse, fiber-filled material like grass. Meanwhile, browsers, such as wild goats and deer, tend to have flatter, lower-crowned, and shorter teeth for crushing and grinding. Not only that but the types of food eaten leave characteristic scratches, or “microwear,” on the tooth surface that can be observed through a microscope. Gina is a specialist in using these approaches to study how the ecology of the horse has changed over time. The first horses, represented by the short-faced Hyracotherium and, later, Eohippus (or “dawn horse”), were no bigger than a relatively large dog, such as a Labrador—a far cry from the long-legged, galloping beasts we admire today. Horses are also generally thought of as grazers. However, the teeth of these early horses show only the faintest traces of ridges that were to become so important later on in their evolution. Otherwise, they were generally low and flat with a rounded chewing surface—well suited for eating soft, lush leaves, fruits, nuts, and plant shoots. You would not know it today, but the first horses were right at home in the dense megathermal forests of the early-middle Eocene. Small bodied, and focused on picking off bits of nutritious angiosperm vegetation, they made the most of expanding forest floors, from North America to Europe.15
In the late Eocene and the early stages of the Oligocene (33.9 million to 23.04 million years ago), things got drier, and sandy grasslands began to expand more widely. The new types of horse, or “equids,” changed in response. About 40 million years ago, the so-called Mesohippus (or “middle horse”) appeared in North America. Now that forests could not always be relied on as places to hide away from growing predator threats, the longer and leggier Mesohippus could run faster and more widely in the expanding open areas. Still relatively small (only sixty centimeters at shoulder height), this new horse form had grinding cheek teeth with sharp crests that allowed it to break down some of the earliest gritty grasses that were emerging at this time, something also visible in Gina’s microwear work. The stage was now set for the rise of modern horses, and from the Miocene to the Pleistocene appearance of the modern genus Equus, horses got bigger. Their feet became increasingly focused on the single toe formation used by horses today, and their teeth increasingly tended toward high, ridged, “hypsodont” cheek teeth ideal for dealing with the expanding C4 grasslands of this period. While horses were clearly able to switch between both leaves and grasses right up to the Pleistocene, and indeed still do on the Central Asian steppes, the overall trend toward sprinting, grassland-adapted animals is evident in the mouths of these animals. Continental land bridges played a major role in this “Great Transformation,” enabling new horse forms to repeatedly pass between America and Europe, and later Asia, to make the most of expanding grasslands.16
However, these changes did not just influence the mammal communities left outside the tropics. Many of us, all over the world, flock to zoos to see the peculiarly shaped giraffe. Appearing in the Miocene, a number of giraffids that roamed Africa and Eurasia, including the diminutive Giraffokeryx in India, show a pretty speedy tendency toward longer-neck evolution by 14 million to 10 million years ago. Land bridges between continents provided an ideal means to range widely as well as a safety net to move to new areas should food run scarce in one region. The modern genus Giraffa seems to have evolved somewhere in East and South Asia before entering Africa approximately 7 million years ago. Although ongoing climate change then caused it to be extirpated in Asia, Giraffa found its long-term home in the subtropics and tropics of Africa where the modern-day giraffe eventually emerged 1 million years ago. The main evolutionary driver behind the unique giraffe features we know and love today is thought to be another retreat of megathermal forests rapidly following the expansion of arid-adapted C4 grasslands in India and Africa from 12 million years ago. However, unlike other previously forest-adapted animals, such as horses, that shifted to grasses, the giraffes stubbornly persisted in browsing whatever islands of trees were left. They developed long necks, like the dinosaurian sauropods, to access nutritious evergreen leaves in dry, deciduous woodland and grassland ecosystems in the tropics, as well as teeth and digestive systems that could cope with the tough foliage and toxins produced by some of these trees. One of the most key members of sweeping eastern African tropical savannah ecosystems known today formed among the chaos of the retreat of warm, wet forests to their current tropical realm.17
Bats provide another example. Today bats make up one-fifth of all living mammal species and are one of the most diverse of the mammal orders. The earliest bats of the Paleocene and Eocene evolved powered flight and their amazing echolocation senses in order to hunt the blooming insects in a warming, flower-dominated world. Yet only between the late Eocene and Miocene did some bats, including the group of
“megabats” (Pteropodidae) that still fill the skies and flock around tasty fruit trees in tropical Asia, start along an evolutionary path toward specialized fruit consumption. Changes to the skull, teeth, and jaw occurred among different diversifying lineages of increasingly specialized bats that fed on first soft and then harder and more fiber-rich fruits. This shift occurred at a time when megathermal forests were contracting, leaving behind woodland and forest patches that were becoming less reliable for larger fruit-eating mammals. Flying creatures, such as birds and bats, however, were ideally placed to link up these patches. The angiosperm plants themselves seem to have recognized this, and the development of smaller seeds and fruits among many tree families during the Oligocene may be associated with the attraction of these smaller-bodied but more mobile consumers. Today, these adaptations have left fruit bats, giant or otherwise, as crucial players in seed dispersal in many tropical forests.18
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