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BIOGEOGRAPHY OF THE CENOZOIC ERA Paleogene period fte Paleogene Period was the first period of the Cenozoic Era, and began about 66 MYA. fte period consists of the Paleocene, Eocene, and Oligocene Epochs. fte Paleocene Epoch lasted until about 56 MYA. fte epoch is bounded by the aforementioned Cretaceous-Paleogene mass extinction and the Paleocene–Eocene ftermal Maximum, a period of dramatic warming. During the Paleocene, the continents continued to break up and move toward their present- day positions. fte mass extinction at the end of the Cretaceous left many available ecological niches, and during the early Paleocene the marine biota was rather impoverished. However, over time, marine biodiversity rebounded, and began to resemble our present day fauna. fte demise of marine reptiles at the end of the Cretaceous opened the way for diversification of the sharks, and tropical conditions at the end of the Paleocene led to abundant and diverse vertebrate and invertebrate life, including diverse coral reef communities. Marine mammals, however, would not appear until the Eocene. Soon after the Cretaceous-Paleogene extinction event, ferns became dominant on land; this may provide further evidence of an asteroid strike, since many ferns are known to colonize fire- disturbed areas quickly. But over time, the plant world began to take on a modern look. Palms appeared, and forest cover was abundant, including widespread rain forests late in the epoch. Angiosperms rebounded from the mass extinction event and achieved dominance, along with their insect pollinators. fte small mammals and other vertebrates that survived the extinction event underwent large-scale adaptive radiations in the Paleocene, growing larger and invading available niches. ftese included many modern groups. Reptiles such as crocodylians, turtles, snakes, and lizards were abundant, and birds continued the dinosaur lineage, including large flightless birds such as the predatory phorusrhacids, or “terror birds.” ftese imposing birds ranged up to 3 m in height, and were top predators in South America during the Cenozoic. Among the mammals, the first monotremes appeared, as well as early marsupials, and later the placentals. fte latter group, which in time would come to dominate the world’s mammalian fauna, was represented by early primate-like and insectivore-like mammals, among others. A separate lineage, the rodent-like multituberculates, also appeared, and would survive for 120 million years, the longest period of any mammal lineage. fte “archaic” Paleocene mammals were still anatomically primitive and had small brains compared to most modern mammals. fte Eocene Epoch began about 56 MYA. ftis epoch was characterized by a widely varying climate, beginning with the very warm period during the Paleocene-Eocene ftermal Maximum (PETM), but then cooling gradually for the remainder of the epoch. During the Eocene, the high levels of CO2 in the atmosphere that contributed to the PETM gradually decreased due to sequestration of CO2 at the bottom of the Arctic Ocean, probably as a result of large-scale organic carbon burial. Landmasses continued toward their present-day positions, with Australia breaking away from Antarctica about 45 MYA and Laurasia beginning to separate as North America, Greenland, and Europe moved apart. India would collide with Asia, beginning the formation of the Himalayas. fte high early Eocene temperatures supported widespread forests, but as the climate cooled later in the epoch, the lush evergreen tropical forests were largely replaced by deciduous forests. In the oceans, fish continued to diversify, and marine mammals made their first appearance, including the cetaceans. On land, mammals continued to increase in importance (Fig. 7.10). fte early ungulates appeared, including representatives of the modern hoofed ungulates, such as Eohippus (“dawn horse”). Strange carnivorous ungulates were present as well. ftese included Mesonyx, with toes that ended in small hooves instead of claws, and the giant Andrewsarchus mongoliensis, known only from a single large skull and some fragments of bone. Proportional to its head, the body of A. mongoliensis was about 3.5 m long, with a height of about 1.8 m at the shoulder. ftis means that A. mongoliensis may rival the South American short-faced bear, Arctotherium, for the title of largest terrestrial carnivorous mammal. Nimravids, close relatives of cats, appeared in the middle Eocene. ftese were predatory animals that are sometimes called “false saber-toothed cats” because they show convergent evolution with Smilodon saber- toothed cats in many respects. fte first primates also appeared, either in the late Paleocene or early Eocene. ftese were represented by the small, rodent-like genus Purgatorius; quite a humble beginning for an order that would eventually give rise to humans. Early forms of most present day mammal orders appeared in the Eocene. Atmospheric oxygen levels were high during much of the Eocene, perhaps contributing to increased brain size, body size, and dominance of mammals, particularly placentals. Large brains require large amounts of oxygen, and high oxygen levels are also advantageous to pregnant females which must provide substantial amounts to the developing fetus. fte end of the Eocene saw a large extinction event, although not at the same level as the larger mass extinctions. ftere appears to be a variety of potential causes for the Eocene-Oligocene extinction, including cooling climate, large-scale volcanic activity, and meteor impacts. Figure 7.10: Depiction of the Eocene fauna of North America. (Wikipedia ‘Eocene;’ Attribution: Jay Matternes) fte Oligocene Epoch, the final epoch of the Paleogene Period, began about 34 MYA. ftis epoch was characterized by a generally cool, dry climate compared to the rest of the Paleogene. South America detached from Antarctica and drifted north, and Antarctica became colder due to the now uninterrupted flow of the Antarctic Circumpolar Current, which isolated the continent from warmer waters. In general, the oceans cooled during the Oligocene, and as the continents continued to move toward present-day positions, ocean circulation patterns began to take on a modern look as well. A land bridge between North America and Europe allowed exchange between these two continents, resulting in increased similarity of the faunas of the two landmasses. Tectonic activity resulted in mountain formation in western North America and Europe. fte more arid climate favored the continued expansion of temperate deciduous forests at the expense of tropical forests; more open grasslands and deserts increased as well. fte expansion of grasslands led to spread of animal groups associated with them, such as horses, camels, and rhinoceroses, as well as the pig-like merycoidodontoids. fte first true felid, Proailurus, appeared in the Oligocene in Asia. ftis was a relatively small, partially arboreal cat, about the size of a bobcat or lynx. fte isolation of South America led to the evolution of a rather unique and often strange fauna, including mastodon and rhinoceros-like species, terrestrial crocodilian relatives, and the aforementioned terror birds. Camels, rhinoceroses, and horses roamed central North America. In Asia, Paraceratherium, an extinct genus of hornless rhinoceros, roamed the shrublands and forests, browsing on leafy plant material. ftese animals were perhaps the largest terrestrial mammals that have ever existed, weighing in at 15 to 20 tons – roughly the size of a Brontosaurus. In the oceans, modern groups of bivalve molluscs and polychaete worms appeared. Carcharhinid sharks appeared and began to diversify as well. ftis family includes roughly 60 extant species, including such well-known species as the tiger shark and oceanic whitetip shark. Cetaceans continued to diversify, and evolved echolocation, a major adaptive feature. fte first pinnipeds appeared in the late Oligocene. At the end of the Oligocene the climate underwent a warming trend, but this was interrupted by a dramatic cooling event as the Earth moved into the Neogene Period and Miocene Epoch. Neogene period fte Neogene Period and Miocene Epoch began about 23 MYA. Rather than being defined by discrete global events, the Miocene boundaries reflect regional transitions between the relatively warm Oligocene and the cooler Pliocene. During the Miocene, the continents continued to bear greater resemblance to today’s configuration. Tectonic activity caused further mountain formation in western North America, Europe, and eastern Asia. South American continued to move north, and the Andes began to form as a result of contact with the western Pacific subduction zone. fte Isthmus of Panama had yet to form though, so South America and North America remained separated. Climatically, the Miocene was still relatively warm, but overall a general cooling trend continued, accompanied by increasing aridity. By the end of the epoch, the Antarctic ice sheet had increased in size and thickness, and glaciers were forming in Greenland. Download free eBooks at bookboon.com 126 Rain forests continued to contract and grasslands expanded. Deeper, richer grassland soils acted as a carbon sink, reducing atmospheric CO2 and contributing to the cooling climate. Late in the Miocene, C4 grasses appeared and increased in importance. Relative to C3 grasses, C4 grasses are at a competitive advantage in arid conditions because they use water more efficiently during photosynthesis. Accompanying the expansion of grasslands were large numbers of diverse grazing ungulates (Fig. 7.11). Horses, deer and camels were abundant. fte predatory nimravids persisted, as well as entelodonts, huge, pig-like animals that first appeared in the Eocene. ftese creatures are sometimes referred to as “terminator pigs,” and reached sizes of over 400 kg and over 2 m at the shoulder. Like modern pigs, these animals were omnivores, but leaned toward carrion and live prey such as large ungulates. Hominoidea (apes) were diverse in the Miocene. Most modern mammal and bird families had appeared by the end of the Miocene. Figure 7.11: Depiction of the Miocene fauna of North America. (Wikipedia ‘Miocene;’ Attribution: Jay Matternes) In the oceans, whales and sharks were abundant and diverse. Kelp forests supported diverse vertebrate and invertebrate assemblages. In South America, freshwater habitats contained a variety of crocodilians, including the 12-meter long Mourasuchus which, despite its large size, probably collected food in a filter-feeding manner. Mourasuchus would sweep its shovel-like jaws through the water, and then use its rows of numerous small teeth to trap the prey as the water was forced out. South American waters were also home to Megapiranha, a now-extinct piranha that reached lengths of almost a meter. A series of extinctions, known as the Middle Miocene disruption, occurred around 14.8 to 14.5 MYA, probably as a result of a dramatic cooling period. fte Miocene Epoch ended, and the Pliocene began, about 5.3 MYA. During the Pliocene, the continents continued to drift to positions very close to their modern day configuration. A major geological event during the Pliocene was the formation of the Isthmus of Panama, which was completed about 3 MYA. ftis event would have dramatic ramifications both climatically, and in terms of the Earth’s biota, particularly that of South America. Lowering ocean levels would also expose the land bridge connection between North America and Asia. fte Mediterranean Sea would form as a result of the joining of Africa and Europe. Climatically, the Pliocene continued to become cooler, drier, and more seasonal, though temperatures were still slightly warmer than today’s. ftere were several possible reasons for this cooling trend. fte formation of the Isthmus of Panama disrupted the flow of warm equatorial ocean currents, leading to the cooling of the now isolated Atlantic Ocean by waters from the Antarctic and Arctic regions. Also, formation of mountain chains such as the Rocky Mountains of western North America may have disrupted jet stream flow, leading to cooler temperatures. Finally, there was a decrease in atmospheric CO2 that may have lessened the greenhouse effect during the Pliocene. Increased glaciation could have led to further cooling by increasing the Earth’s albedo. ftese changing climatic conditions led to further increases in grasslands, savannas, and deserts, increasing deciduous and coniferous forests, and reduction of tropical forests. Marine and terrestrial faunas were increasingly similar to modern-day forms. In Africa, ungulates were abundant, and primates continued to diversify. fte Tribe Hominini arrived on the scene, and this lineage would eventually lead to humans and our close relatives. Snakes continued to evolve, and the first rattlesnakes appeared. fte most dramatic changes in biogeographical patterns occurred as a result of the “Great American Interchange” associated with the formation of the Isthmus of Panama. ftis land bridge allowed two-way dispersal of flora and fauna between continents. Before this connection formed, South America had been isolated for tens of millions of years since its separation from Antarctica. In its isolation South America evolved a biota rivaling that of Australia in its uniqueness. South America was home to huge xenarthrans such as ground sloths and armadillo-like glyptodonts. ftere was also a unique ungulate fauna that included the camel- like Macrauchenia and other members of the Order Litopterna, as well as a diverse group of ungulates known as the notoungulates. Caviomorph rodents flourished as well. ftis group includes the capybara, porcupine, and guinea pig, as well as the largest of all rodents, the extinct Josephoartigasia monesi. ftis giant weighed in at one ton, about 20 times the weight of today’s largest rodent, the capybara. Extinct predators include the aforementioned terror birds, as well as the cougar-like Thylacosmilus. With its oversized canines, this marsupial “cat” showed convergent evolution with the later saber-toothed placental cats (Smilodon) of the Pleistocene. In both cases, the bite was probably relatively weak compared to that of similarly- sized modern-day predatory cats such as the leopard. But the strong cervical vertebrae and powerful neck muscles give away these saber-tooths’ modus operandi – rather than biting, the large canines were used in a powerful stabbing motion. Before the formation of the Panamanian land bridge, South America was home to a rich variety of rodent-, rabbit-, cat-, and bear-like marsupials that shared the continent with their placental cousins for millions of years. But significant change would come in the form of the Great American Interchange made possible by the land bridge. ftis interchange has been the subject of detailed analyses, particularly by paleontologists Larry Marshall and David Webb, who wrote many groundbreaking papers on the subject in the 1970s and 80s. Based on Marshall and Webb’s 1982 paper in the journal Science, 24 North American genera dispersed southward, whereas only 12 South American genera dispersed northward. ftese numbers represented similar proportions of the numbers of genera of the respective continents at that time. Immigrants from North America included small and medium-sized mammals such as rodents, rabbits, shrews, raccoons, and foxes, but also some larger mammals such as bears, horses, llamas, and mastodons (Fig. 7.12). fte short-faced bear, Arctotherium, may have been the world’s largest terrestrial carnivorous mammal, weighing in at about 1.5 tons. In the other direction, opossums, porcupines, armadillos, glyptodonts, the notoungulate Mixotoxodon, and ground sloths were among the dispersers. Figure 7.12: Examples of migrant species in the Americas after the formation of the Isthmus of Panama. Olive green silhouettes denote North American species with South American ancestors; blue silhouettes denote South American species of North American origin. (Wikipedia ‘Pliocene;’ Attribution: Woudloper) By any measure, the southward dispersers from North America were more successful. Dispersing mammals from North America left about 85 descendent genera in South America; this accounts for about 50% of the total present-day mammal genera of South America. In present-day North America, on the other hand, only 29 genera, or 21% of the total, are descended from South American dispersers. ftis result is due to a combination of two interrelated factors – greater adaptive radiation of North American dispersers in South America, and higher extinction rates of native South American genera. In South America, nearly all of the once diverse marsupial fauna went extinct; only the “monito del monte” (a tiny, arboreal, mouse-like marsupial), the order Paucituberculata (several species of shrew- or rat-like marsupials known as “shrew opossums”), and the true opossums (Family Didelphidae) remain. fte latter group is widespread, and the Virginia opossum (Didelphis virginiana) is even found throughout much of North America. But the monito del monte is found only in the highlands of southwestern South America (Chile and Argentina), and the shrew opossums only in the Andes Mountains. Other South American mammals suffered as well in the aftermath of the Great American Interchange. Invasion by North American saber-toothed cats, bears, and various canids led to the demise of many large South American herbivores. All litopterns except Macrauchenia and the closely related Xenorhinotherium died out, as did many notoungulates. Many South American carnivores lost out in competition with the North American invaders as well. Phorusrhacids and sparassodonts were displaced. South American dispersers moving north had much less success, although the Virginia opossum, nine-banded armadillo (Dasypus novemcinctus), and North American porcupine (Erethizon dorsatum) have survived and flourished. Others, such as glyptodonts, giant sloths, and phorusrhacids made it to North America, but subsequently went extinct. ftere have been several explanations proposed for this asymmetrical success rate. One of these explanations rests on the long period of isolation that South America underwent after separation from Antarctica. As we saw when we discussed island biogeography (Chapter 6), it is well known that isolated species on small islands are often at a great disadvantage when faced with newly introduced predators or competitors from the mainland. fte same principle appears to apply, albeit to a lesser extent, to larger isolated landmasses as well. During the Cenozoic, North America was linked to Eurasia via the Bering Land Bridge, and Africa was connected to Eurasia as well. ftis meant that there was a vast, contiguous land area containing a great diversity of species competing with each other, and preying on and evading each other, all in a wide variety of different habitats and climatic conditions. Under these conditions, the Northern Hemisphere’s evolutionary arms race produced swifter, more intelligent, and more competitive species. South America, on the other hand, was a smaller landmass that had been isolated for millions of years. During South America’s isolation, its species interacted with fewer competitors and predators, and there were far fewer individuals and much smaller gene pools to provide the raw material of evolutionary innovation. ftus, during and in the aftermath of the Great American Interchange, South American mammals were at a competitive disadvantage in their newly-established interactions with their North American counterparts. But there were other factors at play as well. To begin with, climate favored the North American dispersers. fte climate of Central America is more similar to that of South America (tropical) than to North America. So dispersers moving south into South America would have of necessity already been well adapted to the tropical climate, whereas dispersers moving north would have encountered very different, drier and cooler conditions in southern and central Mexico. Also, a cooling climate at the time was producing a more arid, savanna environment in the region, which favored the North American species at the expense of the South American species, which were more adapted to tropical closed forest environments. It appears that lower competitiveness or intelligence among the South American groups does not tell the complete story of the Great American Interchange. About 2.6 MYA, the Pliocene ended and the Quaternary Period and Pleistocene Epoch began. Quaternary period fte Quaternary Period is divided into two Epochs: the Pleistocene, from 2.6 MYA to 11,700 years ago, and the Holocene, from 11,700 years ago to the present. During the Pleistocene, the continents were in essentially their present-day positions. ftis epoch was characterized by repeated cycles of glaciation. At least 11 major glacial events occurred in the Pleistocene, punctuated by relatively mild periods, or “interglacials,” which are warm and extended enough for temperate vegetation to exist. A shorter, cooler event, in which conditions might support boreal taxa, is known as an “interstadial.” Several factors drive the process of glaciation. Important among these are the development of ice sheets at the poles and in Greenland. ftese ice sheets have higher albedo, or reflectivity, than the Earth as a whole. ftis means that, when these ice sheets are present, more of the incoming solar radiation is reflected rather than retained, leading to planetary cooling and a potential ice age. With us you can shape the future. Every single day. For more information go to: www.eon-career.com Your energy shapes the future. Download free eBooks at bookboon.com Click on the ad to read more 133 But how can the fairly regular pattern of cold and warmer periods during an ice age be explained? An explanation, now widely accepted, for this pattern was proposed in the 1930s by Milutin Milanković (1879–1958; Fig. 7.13), a Serbian mathematician, climatologist, and geophysicist. Interestingly, Milanković apparently began some of the early work on his theory while he was imprisoned. He was on honeymoon with his new bride in Austro-Hungary near the outbreak of World War I. Tensions were high between Austro-Hungary and Serbia, and, being a Serbian citizen, Milanković was arrested. However, a friend arranged for Milanković to serve his captivity in Budapest so he could continue his work. Figure 7.13: Milutin Milanković, Serbian scientist who explained the cyclical pattern of the Earth’s climate using “Milanković cycles”. (Wikipedia ‘Milutin Milanković;’ Attribution: Source unknown) Milanković’s theory rested on three factors: the Earth’s orbital eccentricity (the elliptical, rather than circular, shape of the Earth’s orbit around the sun); the tilt of the Earth’s axis; and the variation in orientation, or “wobble,” of the Earth’s axis. All of these factors affect the amount of solar energy reaching the Earth and, since these three phenomena all follow predictable patterns, Milanković reasoned that combining them should make it possible to predict climate change patterns. More recent research and evidence have largely verified and improved on Milanković’s theory. A particularly important paper in this area was published by James Hays, John Imbrie, and Nicholas Shackleton in 1976 in the journal Science. ftis paper confirmed that these “Milanković cycles” can be detected in marine sediments, and played a great role in bringing Milanković’s theory to the forefront. fte predominately cold climate, dramatic climate changes, and advancing/receding glaciers had dramatic impacts on Pleistocene flora and fauna. During ice sheet expansion, many regions that are now temperate forests were either occupied by tundra vegetation, or even glaciated. During interglacials, the reverse was true. In each case, relatively small refugia provided environments in which isolated groups of species could survive. For instance, some high altitude locations remain tundra during interglacials, allowing organisms adapted to such conditions to survive in isolated pockets. During the Pleistocene, terrestrial and marine faunas were similar to those of the present day in general, but the epoch did play host to an amazing group of animals known as the Pleistocene megafauna. Many of these were holdovers from the Neogene. fte Pleistocene megafauna (Fig. 7.14) varied somewhat from continent to continent, but included, among others, giant sloths, the American lion (the largest subspecies of lion), mammoths and mastodons, glyptodonts, saber-toothed cats, cave bears, short-faced bears, three now-extinct species of bison, wooly rhinos, and the Irish elk, with its massive antlers. Large birds were present as well, including the now-extinct giant condor, Aiolornis incredibilis. With a 5 m wingspan, this was one of the largest known flight-capable birds. Figure 7.14: Pleistocene fauna of what is now northern Spain. (Wikipedia ‘Pleistocene;’ Attribution: Mauricio Antón, Public Library of Science) Australia was home to a variety of large marsupials, including the “marsupial lion,” Thylacoleo carnifex, and giant 3-meter long wombats (Diprotodon). ftree-meter tall flightless birds in the Family Dromornithidae also graced the Australian landscape; with their powerful crushing beaks, these birds may have filled a predator/scavenger niche similar to that occupied by today’s hyenas. fte Pleistocene fauna of Australia also included giant snakes in the genus Wonambi. ftese snakes reached up to 6 m in length. ftey were probably predators that ambushed such prey as kangaroos and wallabies, and killed them through constriction. ftese snakes achieved mythical status in aboriginal cultures. All of the megafauna species described above are now extinct, probably due to a great extent to the spread of another species that would begin to assert its dominance in the Pleistocene, Homo sapiens. Much of the megafaunal extinctions took place during the transition from the Pleistocene to the Holocene Epoch. fte Holocene epoch will be discussed in the next chapter, which addresses human evolution and biogeography.

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