Jungle
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PORTRAYALS OF A dramatic “extinction” of dinosaurs often lead us to associate them with failure. Not only do we forget that by existing between 230 million and 66 million years ago, they are one of the most successful animal groups ever to have roamed the planet (around 23 times longer than our own hominin clade and 547 times longer than our own species), but we also commonly neglect the fact that they actually still live, or rather fly, around us today. Birds are dinosaurs. They share a huge number of clear, unique anatomical features with the theropod dinosaurs that we now know are their closest ancient relatives. We have the fossils to show that dinosaurs had feathers, or at least feather-like structures, and thus that the ancestors of birds metaphorically flew out of the end-of-Cretaceous doomsday brought on by an extraterrestrial impact. And, as we all know, birds are the most successful vertebrates on Earth, with a staggering 18,000 species distributed over practically the entire planet, according to some estimates. Birds also maintain an incredibly close relationship with seed-producing plants, including within the tropics. Tropical forests are home to a vast variety of birds today, from the fantastic bowerbirds of Australia and Papua New Guinea, which clear themselves a stage within the forest to dance and prance for potential mates, to the harpy eagle of the Amazon Basin, which is one of the largest eagles worldwide and terrorizes monkey communities in the canopy. Across the world, tropical forests, including both gymnosperms and angiosperms, continue to provide green shelters for some of the greatest diversity of living dinosaurs.14
Figure 3.2. Hadrosaurids developed highly complex jaw and teeth morphologies that would have allowed them to consume a diversity of plant matter, including, perhaps, expanding angiosperms. De Agostini via Getty Images
Whether or not Jurassic or Cretaceous dinosaurs coevolved with gymnosperms or angiosperms, birds undoubtedly play some of the most important roles in keeping flowering plant populations alive and kicking around the world today. Plants actively seek to attract the color vision of birds by producing bright fruits so that these wide-ranging fliers will eat them and spread their offspring to new horizons. So important are birds to the spreading of seeds that, in the tropics, there is hardly a forest ecosystem that could remain healthy without them. Again and again, biologists find that birds ensure the reproduction and genetic diversity of tree populations, from the tropical montane forests of the Andes to the lowland evergreen rainforests of the Amazon Basin, from the dry evergreen forests of India to the island tropical forests of the Pacific. Not only that, but birds are some of the most important agents in the recovery of disturbed tropical forests. Modern agriculture, ranching, and infrastructure not only cause dramatic deforestation but, by disturbing resident birds, also threaten any hope of a new beginning. It seems likely that nonbird dinosaurs had significant relationships with plants and forest communities that were dynamically changing on an increasingly fractious tropical “Gondwanan” surface. Their descendants certainly did and still do. However, the dinosaurs, while spectacular, are not the only major type of animal to have both been buffeted by forests and to have gnashed them back. We now turn to another key group of animal life with a deep-rooted, symbiotic relationship with tropical forests—one that is perhaps a little closer to home…15
Chapter 4
“TREE HOUSES” FOR THE FIRST MAMMALS
Mammals are everywhere. They stock our supermarkets with meat and dairy products. They fill our houses with barking, meowing, and squeaking. They carry us trekking over rough landscapes or on a day out at the beach. They are some of the most frequently used symbols for brands, making appearances on logos ranging from the badges of sports teams (the bulls of Chicago) to boxes of cereal (Tony the Tiger). They dominate some of the most iconic ecosystems presented to us in documentaries or recreated for us in zoos. And in the form of the trumpeting African elephant and the hulking blue whale, members of the class Mammalia represent the most giant living animals on both land and sea. They are found in every single land-based environment on Earth. They are also, of course, us, comprising the approximately 7.8 billion of the world’s still-growing human population. In this bustling “Age of Mammals,” we rarely take pause to ask, “How did we get here?” In fact, while we take our current mammalian success for granted, for two-thirds of the existence of mammals on the planet, from c. 210 million to 66 million years ago, they were never larger than a badger, cowering beneath the mighty dinosaurs. Not only that, but it is mind-boggling to think that all modern mammals—from the Etruscan pygmy shrew, which weighs in at two to three grams, to the blue whale, at 190,000 kilograms—are actually related to just a single common and tiny ancestor from a time when the planet was ruled by reptiles.1
Approximately 66 million years ago a giant asteroid crashed into the coast of Yucatán in Mexico, leaving a crater with a diameter of 180 kilometers—the center of which lies directly under the modern-day town of Chicxulub. The impression left on our planet by this vast extraterrestrial object was not just geological. Its impact incinerated much of North America, threw soot and ash into the atmosphere, and likely triggered massive volcanic eruptions in the Deccan Plateau of India that induced a global winter. With sunlight reduced to around half previous levels and lashings of acid rain caused by the released volcanic gases mixing with water in the air, the Earth’s surface and the life it sustained spiraled into disarray. Many species of plants, birds, insects, and marine life and, perhaps most significantly, all of the nonbird dinosaurs went extinct—more than 75 percent of all species on Earth at the time. Although some mammal species were also wiped out in this “end of days” event, traditionally the fire and brimstone at the end of the Cretaceous period has been seen as a window of opportunity for our evolutionary line. With no dinosaurs left to molest us, we could fill landscapes left empty by disaster, take our pick of remaining food sources, and expand and diversify in a march of seemingly inevitable progress until the present day. However, while attractively simple, such a narrative certainly does not tell the whole story.2
Fantastic fossil finds are now beginning to show us that mammals and their closest, but now extinct, “mammaliaform” (literally meaning “mammal-shaped”) relatives were actually already experimenting and expanding before the dinosaurs went extinct. These little but increasingly varied critters did not just scurry below these fearsome reptiles; they also climbed above and even flew around them—first in Jurassic gymnosperm-dominated forests and later in Cretaceous forests that were increasingly invaded by flowering (angiosperm) plants. Furthermore, the asteroid-induced mass extinction at the Cretaceous-Paleogene (K-Pg) boundary was, for mammals, merely the beginning of our brushes with climatic and environmental change. During the following Paleocene (66.0 million to 56.0 million years ago) mammals accompanied the expansion of angiosperm-dominated megathermal forests across an increasingly fragmented planet by the end of the period. A climatic record from the Arctic has shown that mean atmospheric temperatures rose from between 16°C and 18°C to a balmy 25°C at the Paleocene-Eocene boundary (56.0 million years ago). As we saw in Chapter 2, tectonic shifts continued to rearrange the world’s continents toward their current orientation. In particular, in the middle Eocene, Australia and Antarctica split apart, creating a deep-water passage for the flow of cold polar water toward the equator. The new patterns of ocean circulation and their relationship to the atmosphere saw swings between warmer and cooler states in the Eocene (56.0 million to 33.9 million years ago) and Oligocene (33.9 million to 23.04 million years ago). They also resulted in the global expansion of grasslands and retreat of megathermal forests as the planet broadly became cooler, drier, and ever more seasonal heading through the Miocene (23.04 million to 5.33 million years ago). The latest science shows that the geology- and climate-induced ebbs and flows of warm, wet forests over this span of 45 million years played a major role in shaping mammalian distribution, diversity, and evolution, leaving us with the modern ecosystems we gaze in awe at today. Not just that, but without forests growing in the tropics of the Jurassic,
the “Age of Mammals” may never have even begun.3
IN NORTHEASTERN CHINA, the provinces of Liaoning, Inner Mongolia, and Hubei meet in one of the most important regions for paleontologists trying to track the world-changing transition of life on Earth as it swung from the realm of the dinosaurs to the dominion of mammals. Here, fine particles of ash and waterborne mud generated by past volcanic activity and flooding sealed soft body parts and entire skeletons across millions of years into what have become some of the world’s most-celebrated and best-preserved fossil beds. This part of northeastern China has yielded Sinosauropteryx, one of the first dinosaurs found with intact feathers, so complete that paleontologists could even identify cells that give the feathers of birds today their color. While not part of the direct lineage that eventually led to modern birds, Sinosauropteryx nonetheless highlights the biological innovations of some avian dinosaurs in the Cretaceous period that allowed them to hang on beyond the K-Pg boundary. Even more importantly, this region has also preserved one of the most significant fossils in the evolutionary history of the group that was to replace the dinosaurs as the new heavyweights of the animal kingdom. In 2011, Professor Zhe-Xi Luo of the University of Chicago and his colleagues uncovered the remains of an animal they named Juramaia sinensis, which they dated, using radiometric dating methods, to 165 million to 160 million years ago and the Middle-Late Jurassic period, a time when this part of the world would have been located at the boundary of the tropics. These methods compare the amount of naturally abundant radioactive isotopes (in this case Uranium235 or 238) to the elements they are known to decay into (Lead207 and 206) within minerals such as zircon trapped in the fossil layers of interest. As the rate of decay can be calculated, these measurements can be used to estimate how long ago the fossil was buried. Based on its age and the characteristics of the fossil bones, the team gave it a name that denotes it as the “Jurassic mother” of all true placental mammals. “Despite its minute stature, it gave birth to almost all of the mammalian diversity we see around us today,” says Luo.4
The word “mammal” literally means “breast” in Latin, giving immediate insight into the new biological features that separated these animals from their dinosaurian overlords. Although monotremes (like the duck-billed platypus) lay eggs that are later hatched and raised in a pouch, the majority of mammals give birth to live offspring, which they feed with milk. Back in the Jurassic and Cretaceous periods, this would have represented a major evolutionary adaptation, allowing mammals to develop their young in the comfort of safe burrows. Like their extant relatives, these early mammals would also have been warm-blooded and had fur, enabling them to hunt at night when temperatures were low and most predators were asleep. Unsurprisingly, however, spotting the first appearance of these traits in the fossil record can be incredibly difficult. Soft tissues rarely, if ever, preserve over these vast timescales. This has meant that scientists have looked to sturdier, more resistant skulls for clues. Comparison of fossils with modern mammals demonstrates what sets the earliest mammals apart from their reptilian counterparts: the separation of a newly fused jawbone from ear bones that moved backward in the skull. A more stable jaw supported teeth that interlocked like a jigsaw, slicing and dicing through food. While reptiles tend to swallow their food whole, using their teeth simply to lock onto and tear prey, this new adaptation enabled a “preprocessing” of food before it reached the stomach, ultimately releasing more nutrients. Meanwhile, separate ear bones, located away from the gnashing jaw, meant better hearing (essential when active at night) and also provided space for mammalian brains to grow outward and backward. The earliest mammaliaform creatures in which these features have been found are known as morganucodonts. Fossils uncovered from the Late Triassic across Europe, Africa, America, and Asia show this group was starting to scamper around just after the arrival of the first dinosaurs.5
Figure 4.1. Artistic reconstruction of Juramaia sinensis, the “Jurassic mother” of all placental mammals. Carnegie Museum of Natural History / M. A. Klinger
New and varied mammaliaform creatures continued to make an appearance throughout the Jurassic and Cretaceous periods. Paleontologists have been frantically searching for when these mammal-like creatures became the “true” ancestors of most mammals alive today. In Juramaia, Luo thinks we have it. Juramaia had a miniscule body length of between seven and ten centimeters. We can actually see its soft tissues, including fur, thanks to impressions left in its imprisoning rock. Importantly, detailed comparison of Juramaia’s paw bones and teeth with other extinct and living mammals demonstrates that it was alive shortly after the evolutionary division between all of today’s placental mammals (mammals that protect and feed their young in a womb using a placenta until they are fully developed) and marsupial mammals (mammals like kangaroos that release their embryos early to be developed in a pouch). Juramaia, which, like us, is part of the placental line, shows that this split must have occurred approximately 170 million years ago. This is in line with molecular “clock” estimates that compare genetic diversity in modern mammals, as well as assumed rates of mutation, to estimate the age of their last common ancestor, although more recent finds by members of the same team may ultimately push the beginnings of this process back still earlier. More incredible fossils, again from northeastern China, show that the split between placental and marsupial mammals was certainly complete by the Early Cretaceous, with the earliest marsupial, the tiny pointed-nosed Sinodelphys, and, until Juramaia, the earliest placental mammal, Eomaia (“dawn mother”), dating to 125 million years ago. From Juramaia and from northeastern China, we can therefore see the building blocks needed to reconstruct the evolutionary history of the majority of mammals that exist on the planet today.6
Significantly, the rich fossil beds of northeastern China have also provided insights into the early interaction between some of our earliest mammalian ancestors, their closest “mammal-shaped” relatives, and diverse subtropical and tropical forest ecologies. Until recently, scientists had assumed that the earliest mammals were all small, all ground dwelling, and all nocturnal, with variation in mammals occurring only after most dinosaur communities were blasted into oblivion. Nevertheless, studies of plant fossils found alongside Jurassic and Cretaceous mammals, as well as analyses of the shape and size of their teeth and limbs, which provide insights into the stresses and uses they were put to, have revealed that early mammals and mammaliaforms went through two major periods of ecological of “radiation” prior to the K-Pg boundary: one in the Jurassic and one in the Cretaceous. In the Jurassic period (201.4 million to 143.1 million years ago), weighing in at fifteen to seventeen grams (just a little more than a “AAA” battery), Juramaia would not have been particularly conspicuous, and study of its teeth reveal that it mainly ate insects. Nevertheless, its strong forearms suggest that, rather than cowering in fear on forest floors, it ascended trees to live up at eye level with the dinosaurs. Indeed, living in trees significantly benefits mammals in terms of life expectancy and protection from ground-based threats. Moving away from Juramaia, fossil finds of Jurassic mammals reveal they practiced a variety of arboreal, ground-based, burrowing, and even aquatic modes of movement. Another Jurassic fossil from northeastern China has even revealed the existence of “gliding” mammaliaforms in the Jurassic approximately 160 million years ago. Remarkable preservation has enabled scientists to observe the winglike membrane that would have enabled these animals to fly between trees. Not only that, but through comparison with modern gliders, which are all herbivores (with some supplementing their diet with insects), and a detailed analysis of the shape of their teeth, Luo and his team argue that these Jurassic mammaliaforms had developed a new dietary adaptation—one involving consumption of the young leaves, cones, and tender tissues of seed ferns and various lush gymnosperm forest vegetation that dominated Jurassic landscapes. As Luo puts it, “The forests of the Jurassic provided the perfect ‘tree houses’ for the earliest mammals, not just for protection, but also for food as part
of increasingly diverse mammalian ecological niches.”7
Figure 4.2. A reconstruction of Repenomamus snacking on a young Psittacosaurus. Wikipedia / Nobu Tamura
This diversity continued into the Cretaceous period. In the Early Cretaceous, findings of fossils of the badger-like Repenomamus, again in northeastern China and dating to 125 million to 123 million years ago, overturned ideas of what early mammals were capable of. Not only were this mammal’s teeth adapted to carnivory, but it has even been found with a dinosaur preserved in its stomach! The Late Cretaceous saw a further, second major radiation in mammal and mammaliaform diversity. A number of the early mammalian groups went extinct between 125 million and 80 million years ago, with those surviving being primarily small and adapted to eating insects. However, from about 80 million years ago, changes in the teeth, jaws, and body sizes of eutherian ancestors of placental mammals, metatherian marsupial ancestors, and their close relatives indicate the development of varied dietary specializations. So great was this diversity that the numbers of different Late Cretaceous mammaliaform species are similar to the renowned diversity of small-bodied mammal communities that live in tropical forests today, and mammals were thriving at the time of the Chicxulub impact. This mammalian ecological radiation once again seems to have links to the equatorial forests of the time. This time, forests that were being encroached on by flowering angiosperms, with their fleshy fruits, nutrient-rich leaves, seeds, and tubers. Indeed, one of the earliest-dated fossil angiosperms (called Archaeofructaceae, or “ancient fruit”) comes from Early Cretaceous deposits in same part of northeastern China that we have explored throughout this chapter. The teeth and jaws of our placental ancestors, with their dual-purpose crushing and shearing capacity, were well set up to access the new nutrients provided by novel vegetation communities, as well as insect populations that blossomed as the pollen couriers for the new flowers. They were not alone, however, and a now extinct mammalian lineage, the rodent-like “multituberculates” (so named because of the many tubercles, or “cusp bumps,” on their teeth) also developed teeth specifically adapted to the consumption of complex angiosperm vegetation, with sharper, bladelike teeth for slicing at the front and bumpy teeth for crushing at the back. Although debate continues as to the significance of the role of mammals in the initial Cretaceous origins and expansion of these new types of plants, and many of the more specialized mammal groups, like the multituberculates, eventually disappeared, creating an intimate relationship with these innovating plant communities was perhaps the best thing the mammals ever did. Not only did it give them a number of sturdy companions with which to survive one of the worst extinction events ever seen on Earth, but, coming out on the other side, it also guided them into a new era of global domination.8