27-10-2014, 02:48 PM
The Age of Mammals
World map in early-Eocene (c. 50 mya) (Source: Wikimedia Commons) (map with names is here)
![[Image: eocene.jpg]](http://www.ahealedplanet.net/eocene.jpg)
World map in early-Miocene (c. 20 mya) (Source: Wikimedia Commons) (map with names is here)
![[Image: miocene.jpg]](http://www.ahealedplanet.net/miocene.jpg)
Chapter summary:
Compared to the recovery from the mass extinctions that ended the Devonian, Permian, and Triassic periods, the recovery from the end-Cretaceous extinction was relatively swift. The seafloor ecosystem was fully reestablished within two million years.[361] But the story on land was spectacularly different. By the Paleocene's end, ten million years after the end-Cretaceous event, all mammalian orders had appeared in what I will call the "Mammalian Explosion." While the fossil record for Paleocene mammals is relatively thin, the Mammalian Explosion is one of the most spectacular evolutionary radiations on record.[362] Because of its younger age, the Cenozoic Era's fossil record is generally more complete than those of previous eras.
So far in this essay, mammals have received scant attention, but the mammals' development before the Cenozoic is important for understanding their rise to dominance. The therapsids that led to mammals, called cynodonts, first appeared in the late Permian, about 260 mya, and they had key mammalian characteristics. Their jaws and teeth were markedly different from other reptiles; their teeth were specialized for more thorough chewing, which extracts more energy from food, and that was likely a key aspect of ornithischian success more than 100 million years later. Cynodonts also developed a secondary palate so that they could chew and breathe at the same time, which was more energy efficient. Cynodonts eventually ceased the reptilian practice of continually growing and shedding teeth, and their specialized and precisely fitted teeth rarely changed.[363] Mammals replace their teeth a maximum of once. Along with tooth changes, jawbones changed roles. Fewer and stronger bones anchored the jaw, which allowed for stronger jaw musculature and led to the mammalian masseter muscle (clench your teeth and you can feel your masseter muscle). Bones previously anchoring the jaw were no longer needed and became bones of the mammalian middle ear.[364] The jaw's rearrangement led to the most auspicious proto-mammalian development: it allowed the braincase to expand. Mammals had relatively large brains from the very beginning, and it was probably initially related to developing a keen sense of smell. Mammals are the only animals with a cerebral cortex, which eventually led to human intelligence. As dinosaurian dominance drove mammals to the margins, where they lived underground and emerged to feed at night, mammals needed improved senses to survive, and auditory and olfactory senses heightened, as did the mammalian sense of touch. Increased processing of stimuli required a larger brain, and brains have high energy requirements. In humans, only livers use more energy than brains.[365] Cynodonts also had turbinal bones, which suggest that they were warm-blooded. Soon after the Permian extinction, a cynodont appeared that may have had a diaphragm; it was another respiratory innovation that served it well in those low-oxygen times, functioning like pump gills in aquatic environments.
Further along the evolutionary path, here are two animals (1, 2) that may be direct ancestors of mammals; one herbivorous and the other carnivorous/insectivorous. They both resembled rats and probably lived in that niche as burrowing, nocturnal feeders. Mammaliaformes included animals that were probably warm-blooded, had fur, and nursed their young, but laid eggs, like today's platypus. Nursing one's offspring is the defining mammalian trait today, but there has been great controversy over just which mammaliaformes are mammals' direct ancestors and which one can be called the first mammal.[366] According to the most commonly accepted definition of a mammal, the first ones appeared in the mid-Triassic, about 225 mya, nearly 20 million years after dinosaurs first appeared. The only remaining therapsids after a mass extinction at 230 mya were small (the largest was dog-sized), including the mammalian clade, and archosaurs dominated all Earthly biomes from that extinction event until the end-Cretaceous extinction.
Dinosaurs fortunately never became as small as typical Mesozoic mammals, or else mammals might have been out-competed into extinction. Mammals stayed small in the Mesozoic, and the largest Mesozoic mammal yet known was raccoon-size, and its diet included baby dinosaurs. Dinosaurs returned the favor, and digging up mammals from their burrows to snack on them is known dinosaurian behavior.[367]
The issue of early mammalian thermoregulation is controversial and unsettled; even today, mammals engage in a wide array of thermoregulatory practices. Today's primitive mammals have lower metabolic levels than modern ones. Therapsids did not overcome Carrier's Constraint as dinosaurs did; they were not high-performance animals. However, early mammals did not see the Sun, and their larger brains required more energy. Early mammals probably were endothermic, but the condition may have included regular torpor, when they went into a brief "hibernation" phase, and their active body temperature may have been several degrees Celsius lower than today's modern mammals. Birds and mammals are often born without endothermy, but develop it as they grow.[368] Mammals solved Carrier's Constraint when they adopted erect postures in the early Jurassic.[369]
Mammalian reproductive practices separate them into their primary categories. Some "primitive" mammals still lay eggs. The first placental mammal appeared about 160 mya, the marsupial split began about 35 million years later, and the first true marsupial appeared about 65 mya. The marsupial/placental "decision," as with many other lines of evolution, seems to have been a cost-benefit one rooted in energy. Marsupials have far less energy invested in their young at birth than placentals do. Marsupials and birds readily abandon their offspring when hardship strikes. Placentals have a great deal more invested in giving birth to offspring and are therefore less likely to "cut their losses" as easily as birds and marsupials do.[370] In certain environments, marsupials had the advantage over placentals. The earliest known marsupial-line mammal appeared in China 125 mya, and marsupials and placentals co-existed on the fringes. From there they migrated to North America, and then South America. About when the end-Cretaceous holocaust happened, South America separated from North America, but South America was still connected to Antarctica. About 50 mya, marsupials crossed from Antarctica to Australia, perhaps by crossing a narrow sea, and placental mammals died out in Australia, probably outcompeted by marsupials. Earth's only egg-laying mammals today live in New Guinea, Australia, and Tasmania. An entire order of early mammals, which were like marsupial and monotreme rodents, existed for about 120 million years, longer than any other mammalian lineage, to only go extinct in the Oligocene, probably outcompeted by rodents. They were probably the first mammals to disperse nuts, and were probably responsible for a great deal of coevolution between nut trees and animals.[371] All living marsupials have ancestors from South America. In North America and Eurasia, marsupials died out, probably outcompeted by placentals. Africa was not connected to any of those landmasses during those times and thus never hosted marsupials. In South America, marsupials and birds were apex predators (1, 2), but a diverse and unique assemblage of placental ungulates flourished in South America during about 60 million years of relative isolation from all other landmasses.
As with the origins of animals, the molecular evidence shows that virtually all major orders of mammals existed before the end-Cretaceous extinction. The Paleocenes Mammalian Explosion appears to have not been a genetic event, but an ecological one; mammals quickly adapted to empty niches that non-avian dinosaurs left behind.[372] The kinds of mammals that appeared in the Paleocene and afterward illustrate the idea that body features and size are conditioned by their environment, which includes other organisms. With the sauropods' demise, high grazers of conifers never reappeared, but many mammals developed ornithischian eating habits and many attained a similar size. That phenomenon illustrates the ecological concept of guilds, in which assemblages of vastly different animals can inhabit similar ecological niches. The guild concept is obvious with the many kinds of animals that formed reefs in the past; the Cambrian, Ordovician, Silurian, Devonian, Permian, Triassic, Jurassic, and Cretaceous reefs all had similarities, particularly in their shape and location, but the organisms comprising them, from reef-forming organisms to reef denizens and the apex predators patrolling them, had radical changes during the eon of complex life. If you squinted and blurred your vision, most of those reefs from different periods would appear strikingly similar, but when you focused, the variation in organisms could be astounding. The woodpecker guild is comprised of animals that eat insects living under tree bark. But in Madagascar, where no woodpeckers live, a lemur fills that niche, with a middle finger that acts as the woodpecker's bill. In New Guinea, a marsupial fills that role. In the Galapagos Islands, a finch uses cactus needles to acquire those insects. In Australia, cockatoos have filled the niche, but unlike the others, they have not developed a probing body part, nor do they use tools, but just rip off the bark with the brute force of their beaks.[373]
After the dinosaurs, empty niches filled with animals that looked remarkably like dinosaurs, if we squinted. Most large browsing ornithischians weighed in the five-to-seven metric ton range, and by the late Paleocene, uintatheres appeared in North America and China and attained about rhinoceros size, to be supplanted in the Eocene by larger titanotheres, and by the Oligocene, in Eurasia lived the largest land mammals of all time, including the truly dinosaur-sized Paraceratherium. The largest yet found weighed 16 metric tons and was about five meters tall at the shoulders and eight meters in length. Even a T-rex might have thought twice before attacking one of those. It took about 25 million years for land mammals to reach their maximum size, and for the succeeding 40 million years, the maximum size remained fairly constant.[374] Scientists hypothesize that mammalian growth to dinosaurian size was dependent on energy parameters, including continent size and climate, and cooler climates encouraged larger bodies.
Huge mammals persist to this day, although the spread of humans was coincident with the immediate extinction of virtually all large animals with the exception of those in Africa and, to a lesser extent, Asia. The five-to-seven-metric-ton browser formed a guild common to dinosaurs and mammals, and is probably related to metabolic limits and the relatively low calorie density that browsing and foraging affords.[375] Sometimes, the similarity between dinosaurs and mammals could be eerie, such as ankylosaurs and glyptodonts, which is a startling example of convergent evolution, which is the process by which distantly related organisms develop similar features to solve similar problems. They were even about the same size, at least for the most common ankylosaurs, which were about the size of a car. Ankylosaurs appeared in the early Cretaceous and abounded all the way to the Cretaceous's end. Glyptodonts appeared in the Miocene and prospered for millions of years.
The Cenozoic equivalent of a bolide impact was the arrival of humans, as glyptodonts went extinct with all other large South American megafauna shortly after human arrival. The largest endemic South American animals to survive the Great American Interchange of three mya, when North American placentals prevailed over South American marsupials, and the arrival of humans to the Western Hemisphere beginning less than 15 kya, are the capybara and giant anteater, which are tiny compared to their ancient South American brethren. The giant anteater is classified as a sloth, and sloths were a particularly South American animal. The largest sloths were bigger than African bush elephants, which are Earth's largest land animals today. After car-sized glyptodonts went extinct, dog-sized giant armadillos became the line's largest remaining representative.
Among herbivores, their mode of digestion was important. Hindgut fermenters attained the largest size among land mammals, and elephants, rhinos, and horses have that digestive process. Cattle, camels, deer, giraffes, and many other herbivorous mammals are foregut fermenters and many are ruminants, which have four-chambered stomachs, while the others have only three chambers. While foregut fermenters are more energy efficient, hindgut fermenters can ingest more food. Hindgut fermenters gain an advantage when forage is of low quality. What they lack in efficiency they more than make up for in volume. There are drawbacks to that advantage, however, such as when there is not much forage or its quality is poor, such as dead vegetation. A cow, for instance, digests as much as 75% of the protein that it eats, while a horse digests around 25%. Live grass contains about four times the protein as dead grass. Cattle can subsist on the dead grass of droughts or hard winters and horses cannot, which was a tradeoff in pastoral societies.[376]
Angiosperms began overtaking gymnosperms in the early Cenozoic, but it did not immediately happen. In Paleocene coal beds laid down in today's Wyoming, gymnosperms still dominated the swamps, and the undergrowth was mainly comprised of ferns and horsetails.[377] But angiosperms were on their way to dominance, and mammals, birds, and insects began major adaptations to them.
The present consensus is that primates appeared in the late Cretaceous between 85 mya and 65 mya, perhaps in China, but the earliest known primate fossils are from the late Paleocene around 55 mya and were found in Northern Africa. The first primates were tree-dwellers that ate insects, nectar, seeds, and fruit. Their eyes point forward (they rely on sight more than other senses, and have pronounced binocular vision), and most have opposable digits on their hands/feet, which are ideal for canopy-living. Primates generally have larger brains than other mammals, which may have developed to rely more on eyesight and process the stimuli of binocular vision, and primates rely less on the olfactory sense. That change assisted the primate increase in intelligence. Lemurs diverged early in the primate line and rafted over to the newly isolated Madagascar in the early Eocene, and lemurs were Madagascar's only primates until humans arrived about two thousand years ago (and the largest lemurs, which were gorilla-sized, immediately went extinct). A rodent-like sister group to primates that lived in North America and Europe went extinct in the Paleocene, as did many early mammalian lines. In general, Paleocene mammals had relatively small brains, and many from that epoch are called "primitive," although it did not necessarily mean functionally primitive when compared to modern mammals. However, evolutionary "progress" is a legitimate concept. The energy efficiency of ray-finned fish is probably responsible for their success, and the change from "primitive" to "modern" was usually related to the energy issue. Evolutionary progress is an unfashionable concept in some scientific circles, but it is a clear trend over life's history on Earth, and can be quite obvious during the eon of complex life.[378]
Paleocene mammals were rarely apex predators. Crocodilians survived the end-Cretaceous extinction and remained dominant in freshwater environments, although turtles lived in their Golden Age in the Paleocene Americas, and might have even become apex predators for a brief time. The largest snakes ever recorded (1, 2) lived in the Paleocene and could swallow crocodiles whole. In addition to birds' being among South America's apex predators, a huge flightless bird in North America and Europe was also probably a Paleocene apex predator and survived to the mid-Eocene, although the herbivore hypothesis regarding it is currently debated. When the Great American Interchange began three mya, one of those flightless South American birds quickly became a successful North American predator.
People are usually surprised to hear that grass is a relatively recent plant innovation. Grasses are angiosperms and only became common in the late Cretaceous, along with flowering plants. With grass, some dinosaurs learned to graze, and grazers have been plentiful Cenozoic herbivores. According to GEOCARBFSULF, carbon dioxide levels have been falling nearly continuously for the past 150-100 million years. Not only has that decline progressively cooled Earth to the point where we live in an ice age today, but carbon starvation is currently considered the key reason why complex life may become extinct on Earth in several hundred million years. In the Oligocene, between 32 mya and 25 mya some plants developed a new form of carbon fixation during photosynthesis known as C[SUB]4[/SUB] carbon fixation. It allowed plants to adapt to reduced atmospheric carbon dioxide levels. C[SUB]4[/SUB] plants became ecologically prevalent about 6-7 mya in the Miocene, and grasses are today's most common C[SUB]4[/SUB] plants and comprise more than 60% of all C[SUB]4 [/SUB]species. The rest of Earth's photosynthesizers use C[SUB]3 [/SUB]carbon fixation or CAM photosynthesis, which is a water-conserving process used in arid biomes.
In Paleocene oceans, sharks filled the empty niches left by aquatic reptiles, but it took coral reefs ten million years to begin to recover, as usual. As Africa and India moved northward, the Tethys Ocean shrank, and in the late Paleocene and early Eocene, the last Tethyan anoxic events laid down Middle East oil, and the last Paleocene climate event is called the Paleocene-Eocene Thermal Maximum ("PETM"). The PETM has been the focus of a great deal of recent research because of its parallels to today's industrial era, when carbon dioxide and other greenhouse gases are massively vented to the atmosphere, causing a warming atmosphere and acidifying oceans. The seafloor communities suffered a mass extinction, and the PETM's causes are uncertain, but the release of methane hydrates when the global ocean warmed sufficiently is a prominent hypothesis, but scientists also look to the usual suspects of volcanism, changes in oceanic circulation, and a bolide impact.
The PETM, according to carbon isotope excursions, "only" lasted about 120-170 thousand years. The early Eocene (c. 56 to 34 mya), which followed the PETM, is also known as one of Earth's Golden Ages of Life. It has also been called a Golden Age of Mammals, but all life on Earth thrived then. In 1912, the doomed Scott Expedition spent a day collecting Antarctic fossils and still had them a month later when the entire team died in a blizzard. The fossils were recovered and examined in London, and surprisingly yielded evidence that tropical forests once existed near the South Pole. They were Permian plants. That was not long after Wegener first proposed his continental drift hypothesis, and was generations before orthodoxy accepted Wegener's idea. Antarctica has rarely strayed far from the South Pole during the past 500 million years, so the fossils really represented polar forests. A generation before the Scott Expedition's Antarctic fossils were discovered, scientists had been finding similar evidence of polar forests in the Arctic, within several hundred kilometers of the North Pole, on Ellesmere Island and Greenland. Scientists were finding Cretaceous plants in the Arctic, which were much younger than Permian plants.[379]
Polar forests reappeared in the Eocene after the PETM, and the Eocene's first ten million years was the Cenozoic's warmest time and even warmer than the dinosaurian heyday.[380] Not only did alligators live near the North Pole, but the continents and oceans hosted an abundance and diversity of life that Earth may have not seen before or since. When that ten million year period ended, as Earth began cooling off and headed toward the current ice age, has been called the original Paradise Lost.[381] One way that methane has been implicated in those hot times is that leaves have stomata, which regulate the air they take in to obtain carbon dioxide and oxygen, needed for photosynthesis and respiration. Plants also lose water vapor through their stomata, so balancing gas input needs against water losses are key stomata functions, and it is thought that in periods of high carbon dioxide concentration, plants will have fewer stomata. Scientists can count stomata density in fossil leaves, which led some scientists to conclude that carbon dioxide levels were not high enough to produce the PETM, so methane became a candidate greenhouse gas that produced the PETM and Eocene Optimum, and the controversy and research continues.[382]
However the hot times were created and sustained, Earth's life reveled in the conditions. Similar to reptiles' beating the heat and migrating into the oceans, some mammals did the same thing nearly 200 million years later, and cetaceans appeared. Scientists were surprised when molecular studies found that whales share a common ancestor with even-toed ungulates and the hippopotamus in particular. Whales evolved in and near India, beginning about 50 mya, when the earliest "whale" surely did not resemble one and lived near water. By 49 mya, whales could walk or swim. A few million years later they resembled amphibians, and by 40 mya they became fully aquatic, for a transition from land to sea that "only" took 10 million years. Whales quickly became the dominant marine predators. However, sharks did not go quietly and began an arms race with whales, which culminated 28 mya in C. megalodon, the most fearsome marine predator ever: a shark reaching nearly 20 meters in length and weighing 50 metric tons. It could have swallowed a great white shark whole, as seen below (C. megalodon in gray, great white shark in green, and next to that is a man taking a break in C. megalodon's mouth). (Source: Wikimedia Commons)
![[Image: megalodon.jpg]](http://www.ahealedplanet.net/megalodon.jpg)
C. megalodon preyed on whales and had the greatest bite force in Earth's history (although some estimates of T-rex bite strength equal it). C. megalodon went extinct less than two mya, due to the current ice age's vagaries.
Because of early Eocene Arctic forests, animals moved freely between Asia, Europe, Greenland, and North America, which were all nearly connected around the North Pole, and great mammalian radiations occurred in the early Eocene. Many familiar mammals first appeared by the mid-Eocene, such as modern rodents, elephants, bats, and horses. The earliest monkeys may have first appeared in Asia and migrated to India, Africa, and the Americas. Europe was not yet connected with Asia, however, as the Turgai Strait separated them. Modern observers might be startled to know where many animals originated. Camels evolved in North America and lived there for more than 40 million years, until humans arrived. Their only surviving descendants in the Western Hemisphere are llamas. As with lemurs migrating to Madagascar from Africa, or marsupials to Australia via Antarctica, or monkeys migrating from Africa to the Americas, or Eocene mammalian migrations via polar routes, the migrants often involuntarily "sailed" on vegetation mats that crossed relatively short gaps between the continents. Such a migration depended on fortuitous prevailing currents and other factors, but it happened often enough.
Several of the Eocene's geologic events had long-lasting impact. About 50 mya, the plates under India and Southern Asia began their epic collision and started creating the Himalayas, and Australia split from Antarctica. The collisions of the African, Arabian, and Indian plates with the Eurasian plate created the mountain ranges that stretch from Western Europe to New Guinea. After the Pacific Ring of Fire, it is the world's most seismically active region. Those colliding plates eventually squeezed the Tethys Ocean out of existence. That event ended more than 500 million years of Tethyan sedimentation, beginning with the Proto-Tethys Ocean in the Ediacaran, continuing with the Paleo-Tethys Ocean in the Ordovician, and the Tethys Ocean appeared in the late Permian. The Tethys Ocean's existence spanned the entire Mesozoic and finally vanished less than six mya, at the Miocene's end.[383] Most of the world's oil formed in the sediments of those Tethyan oceans and very little has formed since the Eocene.
The process of transforming anoxic sediments into oil requires millions of years. When organic sediments are buried, most of the oxygen, nitrogen, hydrogen, and sulfur of dead organisms is released, leaving behind carbon and some hydrogen in a substance called kerogen, in a process that is like reversed photosynthesis. Plate tectonics can subduct sediments, particularly where oceanic plates meet continental plates. There is an "oil window" roughly between 2,000 and 5,000 meters deep, where if kerogen-rich sediments are buried at those depths for long enough (millions of years), geological processes (which produce high temperature and pressure) break down complex organic molecules and the result is the hydrocarbons that comprise petroleum. If organic sediments never get that deep, they remain kerogen. If they are subducted deeper than that for long enough, all carbon-carbon bonds are broken and the result is methane, which is also called natural gas. Today, the geological processes that make oil can be reproduced in industrial settings that can turn organic matter into oil in a matter of hours. Many hydrocarbon sources touted today as replacements for conventional oil were never in the oil window, so were not "refined" into oil and remain kerogen. The so-called oil shales and oil sands are made of kerogen (bitumen is soluble kerogen). It takes a great deal of energy to refine kerogen into oil, which is why kerogen is an inferior energy resource. Nearly a century ago in East Texas oil fields it took less than one barrel of oil energy to produce one hundred barrels, for an energy return on investment ("EROI" or "EROEI") of more than 100, in the Golden Age of Oil. Global EROI is declining fast and will fall to about 10 by 2020. The EROIs of those oil shales and oil sands are less than five and as low as two.
During the early Eocene's Golden Age of Life, forests blanketed virtually all lands all the way to the poles, modern orders of most mammals appeared, today's largest order of sharks appeared, and coral reefs again appeared beyond 50 degrees latitude. Many animals would also appear bizarre today. One crocodile developed hooves, and an order of hooved mammalian predators lived, including the largest terrestrial mammalian predator/scavenger ever, which looked like a giant wolf with hooves. The ancestors of modern carnivores began displacing those primitive predatory mammals in the Eocene, after starting out small. A family of predatory placentals called bear dogs lived from the mid-Eocene to less than two mya. Rhino-sized uintatheres and their bigger cousins the brontotheres were the Eocene's dominant herbivores in North America and Asia. Primates flourished in the tropical canopies of Africa, Europe, Asia, and North America. Deserts are largely an Icehouse Earth phenomenon, and during the previous Greenhouse Earths, virtually all lands were warm and moist. Australia was not a desert in the early Eocene, but was largely covered by rainforests. It must have been a marsupial paradise, as it would have been in Antarctica and South America, but the fossil record is currently thin as rainforests are poor fossil preservers.
In the late Cretaceous, about 75 mya, New Zealand split from Gondwana, and by the end-Cretaceous event it, Madagascar, and India were alone in the oceans. Madagascar was close enough to Africa for lemurs to migrate to it, but the only animals that repopulated New Zealand's lands after the end-Cretaceous holocaust were those that flew. From the end-Cretaceous event until the Maoris arrived around 1250-1300 CE (CE stands for "Common Era," formerly designated with AD), birds were New Zealand's dominant animals and had no rivals. The only mammals were a few species of bat that migrated there in the Oligocene. A recent finding of a mouse-sized mammal fossil shows that some land mammals lived in New Zealand long ago, possibly Mesozoic survivors and unrelated to any living mammals, but they died out many millions of years ago. A few small reptiles and amphibians also lived there, and even a crocodile that died out in the Miocene, but New Zealand, unlike any other major landmass in Earth's history, was the realm of birds. The Maoris encountered giant birds, ecological niches filled with mammals elsewhere were filled by birds, and gigantic moas were the equivalent of mammalian browsers. Before the arrival of humans, moas were only preyed upon by the largest eagle ever. Of all ecosystems that would have appeared strange to modern eyes, New Zealand's pre-human ecosystem has been perhaps the most intriguing to me, perhaps because it still existed less than a millennium ago. It seemed like something that sprang out of Dr. Seuss's imagination. The Seuss-like kiwi is one of the few surviving specialized birds of that time. The Maoris drove all moas to extinction in less than a century, and quickly destroyed about half of New Zealand's forests via burning.
For several million years, life in the Eocene was halcyonic, and at 50 mya, the Greenhouse Earth state had prevailed for 200 million years, ever since the end-Permian extinction. But just as whales began invading the oceans 49 mya, Earth began cooling off. The ultimate reason was atmospheric carbon dioxide levels that had been steadily declining for tens of millions of years. The intense volcanism of the previous 200 million years waned and the carbon cycle inexorably sequestered carbon into Earth's crust and mantle. While falling carbon dioxide levels were the ultimate cause, the first proximate cause was probably the isolation of Antarctica at the South Pole and changes in global ocean currents. During the early Eocene, the global ocean floor's water temperature was about 13[SUP]o[/SUP]C (55[SUP]o[/SUP]F), warm enough to swim in, which was a far cry from today's near-freezing and below-freezing temperatures. The North Sea was warm as bathwater. Radical current changes accompanied the PETM of about 56 mya, warming the ocean floor, and perhaps that boiled off the methane hydrates. Whatever the causes were, the oceans were warm from top to bottom, from pole to pole. But between 50 to 45 mya, Australia made its final split from Antarctica and moved northward, India began crashing into Asia and cut off the Tethys Ocean and the global tropical circulation, and South America also moved northward, away from Antarctica. Although the debate is still fierce over the cooling's exact causes, the evidence (much is from oxygen isotope analyses) is that the oceans cooled off over the next twelve million years, very consistently, and a brief small reversal transpired at about 40 mya.[384] By 37-38 mya, the 200-million-year-plus Greenhouse Earth phase ended and the transition into today's ice age was underway. In the late Eocene, as the trend toward Icehouse Earth conditions began, deserts such as the Saharan, South African, and Australian formed.
That cooling caused the greatest mass extinction of the entire Cenozoic Era, at least until today's incipient Sixth Mass Extinction. With continents now scattered across Earth's surface, there was no event that wiped nearly everything out as the end-Permian extinction did, nor were bolide events convincingly implicated. But mass extinctions punctuated a 12-million-year period when Earth's global ocean and surface temperatures steadily declined. When it was finished, there were no more polar forests, no more alligators in Greenland or palm trees in Alaska, and Antarctica was developing its ice sheets. A few million years later, another mass extinction event in Europe marked the Eocene's end and the Oligocene's beginning, but the middle-Eocene extinctions were more significant.[385] All in all, there was about a 14-million-year period of cooling and extinction, which encompassed the mid-Eocene to early Oligocene, and Icehouse Earth conditions reappeared after a more-than-200-million-year hiatus.[386]
The Oligocene Epoch (c. 34 to 23 mya) was relatively cold. In the 1960s, a global effort was launched to drill deep sea cores, the Glomar Challenger recovered nearly 20,000 cores from Earth's oceans, and scientists had paradigm-shift learning experiences from studying those cores. One finding was that Antarctica developed its ice sheets far earlier than previously supposed, and the cores pushed back the initial ice sheet formation by 20 million years, to about 34-35 mya; the first Antarctic glaciers formed as early as 49 mya. The evidence included dropstones in Southern Ocean sediments, which meant icebergs.[387] The event that led to Antarctic ice sheets was the formation of the Antarctic Circumpolar Current, which began to form about 40 mya and was firmly established by 34 mya, when the Antarctic ice sheets grew in earnest. The current's formation was caused by Antarctica's increasing isolation from Australia and South America, which gradually allowed an uninterrupted current to form that circled Antarctica and isolated it so that it no longer received tropical currents. That situation eventually turned Antarctica into the big sheet of ice that it is today. It also radically changed global oceanic currents. Antarctic Bottom Water formed, which cooled the oceans as well as oxygenated its depths, and it comprises more than half of the water in today's oceans. North Atlantic Deep Water began forming around the same time.[388]
Those oceanic changes profoundly impacted Earth's ecosystems. Not only did most warm-climate species go extinct, at least locally, but new species appeared, adapted to the new environment. Early whales all died out about 35 mya and were replaced by whales adapted to the new oceanic ecosystems that are still with us today: toothed whales, which include dolphins, orcas and porpoises; and baleen whales, which adapted to the rich plankton blooms caused by upwellings of the new circulation and in the Southern Ocean in particular.[389] Sharks adapted to the new whales, which culminated with C. megalodon in the Oligocene. With the land bridges and small seas between the northern continents unavailable in colder times, the easy travel between those continents that characterized the Eocene's warm times ended and the continents began developing endemic ecosystems. Europe became isolated from all other continents by the mid-Eocene and developed its own peculiar fauna. At the Oligocene's beginning, the Turgai Strait was no longer a barrier between Europe and Asia and the more cosmopolitan Asian mammals replaced the provincial European mammals, although from competition, an extinction event, or other causes is still debated, although competition is favored. About half of European mammalian genera went extinct, replaced by immigrants from Asia, and some from North America via Asia.[390]
Africa was also isolated from other continents during those times and developed its own unique fauna. The first proboscideans evolved in Africa about 60 mya, and Africa remained their evolutionary home, and the one leading to today's elephants lived in Africa in the mid-Oligocene. Hyraxes are relatives of elephants, they have never strayed far from their initial home in Africa, and were Africa's dominant herbivore for many millions of years, beginning in the Oligocene. Some reached horse size, and a close relative looked very much like a rhino, with rhino size. The rhinoceros line itself seems to have begun in North America in the early Eocene, and rhinos did not reach Africa until the Miocene.
But the African Oligocene event of most interest to humans was African primate evolution. By the Eocene's end, primates were extinct in Europe and North America, and largely gone in Asia. Africa became the Oligocene's refuge for primates as they lived in the remaining rainforest. The first animals that we would call monkeys evolved in the late Eocene, and what appears to be a direct ancestor of Old World monkeys and apes appeared in Africa at the Oligocene's beginning, about 35-33 mya. But ancestral to that creature was one that also led to those that migrated to South America, probably via vegetation rafts (with perhaps a land bridge helping), around the same time. Those South American monkeys are known as New World monkeys today and they evolved in isolation for more than 30 million years. For those that stayed behind in Africa, what became apes first appeared around the same time as those New World monkeys migrated; they diverged from Old World monkeys. Scientists today think that somewhere between about 35 mya and 29 mya the splits between those three lineages happened. Old World and New World monkeys have not changed much in the intervening years, but apes sure have.
The size issue is dominant in evolutionary inquiries, and scientists have found that in Greenhouse Earth conditions, animal size is relatively evenly distributed, and all niches are taken. When Icehouse Earth conditions prevail, the cooling and drying encourages some animal sizes and not others, and mid-sized animals suffer, such as those early primates. That may be why primates went extinct outside the tropics in the late Eocene.[391] Tropical canopies are rich in leaves, nectar, flowers, fruit, seeds, and insects, while temperate canopies are not, particularly in winter. Large herbivores lost a great deal of diversity in late-Eocene cooling, but the survivors were gigantic, and the largest land mammal ever thundered across Eurasia in the Oligocene. Mid-sized species were rare in that guild.[392]
The earliest bears appeared in North America in the late Eocene and early Oligocene, and raccoons first appeared in Europe in the late Oligocene. It might be amusing to consider, but cats and dogs are close cousins and a common ancestor lived about 50 mya. Canines first appeared in the early Oligocene in North America about 34 mya, and felines first appeared in Eurasia in the late Oligocene about 25 mya. Beavers appeared in North America and Europe in the late Eocene and early Oligocene, and the first deer in Europe in the Oligocene. The common ancestor of today's sloths lived in the late Eocene; South American giant ground sloths appeared in the late Oligocene. The kangaroo family may have begun in the Oligocene. The horse was adapting and growing in North America in the Oligocene. By the late Eocene, the pig and cattle suborders had appeared, and squirrels had appeared in North America.
In summary, numerous mammals appeared by the Oligocene that resemble their modern descendants. They were all adapted to the colder, dryer Icehouse Earth conditions, the poorer quality forage, and the food chains that depended on them. In subsequent epochs, conditions warmed and cooled, ice sheets advanced and retreated, and deserts, grasslands, woodlands, rainforests, and tundra grew and shrank, but with a few notable exceptions, Earth's basic flora and fauna has not significantly changed during the past 30 million years.
The Oligocene ended with a sudden global warming that continued into the Miocene Epoch (c. 23 to 5.3 mya). The Miocene was also the first epoch of the Neogene Period (c. 23 to 2.6 mya). Although nowhere near as warm as the Eocene Optimum, England had palm trees again, Antarctic ice sheets melted, and oceans rose. The Miocene is also called the Golden Age of Mammals. Scientists still wrestle with why Earth's temperature increased in the late Oligocene, but there is no doubt that it did. As the study of ice ages has demonstrated, many dynamics impact Earth's climate, and positiv
World map in early-Eocene (c. 50 mya) (Source: Wikimedia Commons) (map with names is here)
World map in early-Miocene (c. 20 mya) (Source: Wikimedia Commons) (map with names is here)
Chapter summary:
- Recovery from the Cretaceous extinction
- Development of mammals
- Mammalian reproductive practices
- Ecological guilds
- Mammalian convergent evolution with dinosaurs
- Mammals reach maximum size
- Hindgut and foregut digestion
- Primate development
- Non-mammalian apex predators of Cenozoic
- Rise of grass and C4 carbon fixation
- Paleocene-Eocene Thermal Maximum
- Eocene's Golden Age of Life
- Mammals migrate to oceans and become whales
- Mammals easily migrate between continents via Arctic region
- India, Africa, and Arabia begin colliding with Eurasia, forming mountain ranges
- How geological processes make oil
- New Zealand's bird-dominated biomes evolve in isolation until humans arrive
- 200-million-year Greenhouse Earth phase ends, and Earth begins cooling
- Mid-Eocene extinction
- End-Eocene extinction largely confined to Europe
- Antarctic ice sheet begins developing
- Original whales go extinct, and whales adapted to new biomes appear
- Africa evolves in isolation; elephants appear
- Monkeys appear
- Many modern mammal families appear
- Oligocene warms into Miocene
- Global currents dramatically change
- Asian invasion of North America
- Africa collides with Eurasia, and mass cross-migration begins
- South America and Australia evolve in isolation
- Mid-Miocene cooling; Greenland ice sheets begin to develop
- Cause of mid-Miocene cooling
- Mountain-building events
- Grasslands appear
- Mammals adapt to eating dry-climate plants
- Tethys Ocean finally disappears
- Pliocene Epoch and Great American Interchange
- Changing ocean currents initiate current ice age
- Ice Age begins, along with Quaternary Period
Compared to the recovery from the mass extinctions that ended the Devonian, Permian, and Triassic periods, the recovery from the end-Cretaceous extinction was relatively swift. The seafloor ecosystem was fully reestablished within two million years.[361] But the story on land was spectacularly different. By the Paleocene's end, ten million years after the end-Cretaceous event, all mammalian orders had appeared in what I will call the "Mammalian Explosion." While the fossil record for Paleocene mammals is relatively thin, the Mammalian Explosion is one of the most spectacular evolutionary radiations on record.[362] Because of its younger age, the Cenozoic Era's fossil record is generally more complete than those of previous eras.
So far in this essay, mammals have received scant attention, but the mammals' development before the Cenozoic is important for understanding their rise to dominance. The therapsids that led to mammals, called cynodonts, first appeared in the late Permian, about 260 mya, and they had key mammalian characteristics. Their jaws and teeth were markedly different from other reptiles; their teeth were specialized for more thorough chewing, which extracts more energy from food, and that was likely a key aspect of ornithischian success more than 100 million years later. Cynodonts also developed a secondary palate so that they could chew and breathe at the same time, which was more energy efficient. Cynodonts eventually ceased the reptilian practice of continually growing and shedding teeth, and their specialized and precisely fitted teeth rarely changed.[363] Mammals replace their teeth a maximum of once. Along with tooth changes, jawbones changed roles. Fewer and stronger bones anchored the jaw, which allowed for stronger jaw musculature and led to the mammalian masseter muscle (clench your teeth and you can feel your masseter muscle). Bones previously anchoring the jaw were no longer needed and became bones of the mammalian middle ear.[364] The jaw's rearrangement led to the most auspicious proto-mammalian development: it allowed the braincase to expand. Mammals had relatively large brains from the very beginning, and it was probably initially related to developing a keen sense of smell. Mammals are the only animals with a cerebral cortex, which eventually led to human intelligence. As dinosaurian dominance drove mammals to the margins, where they lived underground and emerged to feed at night, mammals needed improved senses to survive, and auditory and olfactory senses heightened, as did the mammalian sense of touch. Increased processing of stimuli required a larger brain, and brains have high energy requirements. In humans, only livers use more energy than brains.[365] Cynodonts also had turbinal bones, which suggest that they were warm-blooded. Soon after the Permian extinction, a cynodont appeared that may have had a diaphragm; it was another respiratory innovation that served it well in those low-oxygen times, functioning like pump gills in aquatic environments.
Further along the evolutionary path, here are two animals (1, 2) that may be direct ancestors of mammals; one herbivorous and the other carnivorous/insectivorous. They both resembled rats and probably lived in that niche as burrowing, nocturnal feeders. Mammaliaformes included animals that were probably warm-blooded, had fur, and nursed their young, but laid eggs, like today's platypus. Nursing one's offspring is the defining mammalian trait today, but there has been great controversy over just which mammaliaformes are mammals' direct ancestors and which one can be called the first mammal.[366] According to the most commonly accepted definition of a mammal, the first ones appeared in the mid-Triassic, about 225 mya, nearly 20 million years after dinosaurs first appeared. The only remaining therapsids after a mass extinction at 230 mya were small (the largest was dog-sized), including the mammalian clade, and archosaurs dominated all Earthly biomes from that extinction event until the end-Cretaceous extinction.
Dinosaurs fortunately never became as small as typical Mesozoic mammals, or else mammals might have been out-competed into extinction. Mammals stayed small in the Mesozoic, and the largest Mesozoic mammal yet known was raccoon-size, and its diet included baby dinosaurs. Dinosaurs returned the favor, and digging up mammals from their burrows to snack on them is known dinosaurian behavior.[367]
The issue of early mammalian thermoregulation is controversial and unsettled; even today, mammals engage in a wide array of thermoregulatory practices. Today's primitive mammals have lower metabolic levels than modern ones. Therapsids did not overcome Carrier's Constraint as dinosaurs did; they were not high-performance animals. However, early mammals did not see the Sun, and their larger brains required more energy. Early mammals probably were endothermic, but the condition may have included regular torpor, when they went into a brief "hibernation" phase, and their active body temperature may have been several degrees Celsius lower than today's modern mammals. Birds and mammals are often born without endothermy, but develop it as they grow.[368] Mammals solved Carrier's Constraint when they adopted erect postures in the early Jurassic.[369]
Mammalian reproductive practices separate them into their primary categories. Some "primitive" mammals still lay eggs. The first placental mammal appeared about 160 mya, the marsupial split began about 35 million years later, and the first true marsupial appeared about 65 mya. The marsupial/placental "decision," as with many other lines of evolution, seems to have been a cost-benefit one rooted in energy. Marsupials have far less energy invested in their young at birth than placentals do. Marsupials and birds readily abandon their offspring when hardship strikes. Placentals have a great deal more invested in giving birth to offspring and are therefore less likely to "cut their losses" as easily as birds and marsupials do.[370] In certain environments, marsupials had the advantage over placentals. The earliest known marsupial-line mammal appeared in China 125 mya, and marsupials and placentals co-existed on the fringes. From there they migrated to North America, and then South America. About when the end-Cretaceous holocaust happened, South America separated from North America, but South America was still connected to Antarctica. About 50 mya, marsupials crossed from Antarctica to Australia, perhaps by crossing a narrow sea, and placental mammals died out in Australia, probably outcompeted by marsupials. Earth's only egg-laying mammals today live in New Guinea, Australia, and Tasmania. An entire order of early mammals, which were like marsupial and monotreme rodents, existed for about 120 million years, longer than any other mammalian lineage, to only go extinct in the Oligocene, probably outcompeted by rodents. They were probably the first mammals to disperse nuts, and were probably responsible for a great deal of coevolution between nut trees and animals.[371] All living marsupials have ancestors from South America. In North America and Eurasia, marsupials died out, probably outcompeted by placentals. Africa was not connected to any of those landmasses during those times and thus never hosted marsupials. In South America, marsupials and birds were apex predators (1, 2), but a diverse and unique assemblage of placental ungulates flourished in South America during about 60 million years of relative isolation from all other landmasses.
As with the origins of animals, the molecular evidence shows that virtually all major orders of mammals existed before the end-Cretaceous extinction. The Paleocenes Mammalian Explosion appears to have not been a genetic event, but an ecological one; mammals quickly adapted to empty niches that non-avian dinosaurs left behind.[372] The kinds of mammals that appeared in the Paleocene and afterward illustrate the idea that body features and size are conditioned by their environment, which includes other organisms. With the sauropods' demise, high grazers of conifers never reappeared, but many mammals developed ornithischian eating habits and many attained a similar size. That phenomenon illustrates the ecological concept of guilds, in which assemblages of vastly different animals can inhabit similar ecological niches. The guild concept is obvious with the many kinds of animals that formed reefs in the past; the Cambrian, Ordovician, Silurian, Devonian, Permian, Triassic, Jurassic, and Cretaceous reefs all had similarities, particularly in their shape and location, but the organisms comprising them, from reef-forming organisms to reef denizens and the apex predators patrolling them, had radical changes during the eon of complex life. If you squinted and blurred your vision, most of those reefs from different periods would appear strikingly similar, but when you focused, the variation in organisms could be astounding. The woodpecker guild is comprised of animals that eat insects living under tree bark. But in Madagascar, where no woodpeckers live, a lemur fills that niche, with a middle finger that acts as the woodpecker's bill. In New Guinea, a marsupial fills that role. In the Galapagos Islands, a finch uses cactus needles to acquire those insects. In Australia, cockatoos have filled the niche, but unlike the others, they have not developed a probing body part, nor do they use tools, but just rip off the bark with the brute force of their beaks.[373]
After the dinosaurs, empty niches filled with animals that looked remarkably like dinosaurs, if we squinted. Most large browsing ornithischians weighed in the five-to-seven metric ton range, and by the late Paleocene, uintatheres appeared in North America and China and attained about rhinoceros size, to be supplanted in the Eocene by larger titanotheres, and by the Oligocene, in Eurasia lived the largest land mammals of all time, including the truly dinosaur-sized Paraceratherium. The largest yet found weighed 16 metric tons and was about five meters tall at the shoulders and eight meters in length. Even a T-rex might have thought twice before attacking one of those. It took about 25 million years for land mammals to reach their maximum size, and for the succeeding 40 million years, the maximum size remained fairly constant.[374] Scientists hypothesize that mammalian growth to dinosaurian size was dependent on energy parameters, including continent size and climate, and cooler climates encouraged larger bodies.
Huge mammals persist to this day, although the spread of humans was coincident with the immediate extinction of virtually all large animals with the exception of those in Africa and, to a lesser extent, Asia. The five-to-seven-metric-ton browser formed a guild common to dinosaurs and mammals, and is probably related to metabolic limits and the relatively low calorie density that browsing and foraging affords.[375] Sometimes, the similarity between dinosaurs and mammals could be eerie, such as ankylosaurs and glyptodonts, which is a startling example of convergent evolution, which is the process by which distantly related organisms develop similar features to solve similar problems. They were even about the same size, at least for the most common ankylosaurs, which were about the size of a car. Ankylosaurs appeared in the early Cretaceous and abounded all the way to the Cretaceous's end. Glyptodonts appeared in the Miocene and prospered for millions of years.
The Cenozoic equivalent of a bolide impact was the arrival of humans, as glyptodonts went extinct with all other large South American megafauna shortly after human arrival. The largest endemic South American animals to survive the Great American Interchange of three mya, when North American placentals prevailed over South American marsupials, and the arrival of humans to the Western Hemisphere beginning less than 15 kya, are the capybara and giant anteater, which are tiny compared to their ancient South American brethren. The giant anteater is classified as a sloth, and sloths were a particularly South American animal. The largest sloths were bigger than African bush elephants, which are Earth's largest land animals today. After car-sized glyptodonts went extinct, dog-sized giant armadillos became the line's largest remaining representative.
Among herbivores, their mode of digestion was important. Hindgut fermenters attained the largest size among land mammals, and elephants, rhinos, and horses have that digestive process. Cattle, camels, deer, giraffes, and many other herbivorous mammals are foregut fermenters and many are ruminants, which have four-chambered stomachs, while the others have only three chambers. While foregut fermenters are more energy efficient, hindgut fermenters can ingest more food. Hindgut fermenters gain an advantage when forage is of low quality. What they lack in efficiency they more than make up for in volume. There are drawbacks to that advantage, however, such as when there is not much forage or its quality is poor, such as dead vegetation. A cow, for instance, digests as much as 75% of the protein that it eats, while a horse digests around 25%. Live grass contains about four times the protein as dead grass. Cattle can subsist on the dead grass of droughts or hard winters and horses cannot, which was a tradeoff in pastoral societies.[376]
Angiosperms began overtaking gymnosperms in the early Cenozoic, but it did not immediately happen. In Paleocene coal beds laid down in today's Wyoming, gymnosperms still dominated the swamps, and the undergrowth was mainly comprised of ferns and horsetails.[377] But angiosperms were on their way to dominance, and mammals, birds, and insects began major adaptations to them.
The present consensus is that primates appeared in the late Cretaceous between 85 mya and 65 mya, perhaps in China, but the earliest known primate fossils are from the late Paleocene around 55 mya and were found in Northern Africa. The first primates were tree-dwellers that ate insects, nectar, seeds, and fruit. Their eyes point forward (they rely on sight more than other senses, and have pronounced binocular vision), and most have opposable digits on their hands/feet, which are ideal for canopy-living. Primates generally have larger brains than other mammals, which may have developed to rely more on eyesight and process the stimuli of binocular vision, and primates rely less on the olfactory sense. That change assisted the primate increase in intelligence. Lemurs diverged early in the primate line and rafted over to the newly isolated Madagascar in the early Eocene, and lemurs were Madagascar's only primates until humans arrived about two thousand years ago (and the largest lemurs, which were gorilla-sized, immediately went extinct). A rodent-like sister group to primates that lived in North America and Europe went extinct in the Paleocene, as did many early mammalian lines. In general, Paleocene mammals had relatively small brains, and many from that epoch are called "primitive," although it did not necessarily mean functionally primitive when compared to modern mammals. However, evolutionary "progress" is a legitimate concept. The energy efficiency of ray-finned fish is probably responsible for their success, and the change from "primitive" to "modern" was usually related to the energy issue. Evolutionary progress is an unfashionable concept in some scientific circles, but it is a clear trend over life's history on Earth, and can be quite obvious during the eon of complex life.[378]
Paleocene mammals were rarely apex predators. Crocodilians survived the end-Cretaceous extinction and remained dominant in freshwater environments, although turtles lived in their Golden Age in the Paleocene Americas, and might have even become apex predators for a brief time. The largest snakes ever recorded (1, 2) lived in the Paleocene and could swallow crocodiles whole. In addition to birds' being among South America's apex predators, a huge flightless bird in North America and Europe was also probably a Paleocene apex predator and survived to the mid-Eocene, although the herbivore hypothesis regarding it is currently debated. When the Great American Interchange began three mya, one of those flightless South American birds quickly became a successful North American predator.
People are usually surprised to hear that grass is a relatively recent plant innovation. Grasses are angiosperms and only became common in the late Cretaceous, along with flowering plants. With grass, some dinosaurs learned to graze, and grazers have been plentiful Cenozoic herbivores. According to GEOCARBFSULF, carbon dioxide levels have been falling nearly continuously for the past 150-100 million years. Not only has that decline progressively cooled Earth to the point where we live in an ice age today, but carbon starvation is currently considered the key reason why complex life may become extinct on Earth in several hundred million years. In the Oligocene, between 32 mya and 25 mya some plants developed a new form of carbon fixation during photosynthesis known as C[SUB]4[/SUB] carbon fixation. It allowed plants to adapt to reduced atmospheric carbon dioxide levels. C[SUB]4[/SUB] plants became ecologically prevalent about 6-7 mya in the Miocene, and grasses are today's most common C[SUB]4[/SUB] plants and comprise more than 60% of all C[SUB]4 [/SUB]species. The rest of Earth's photosynthesizers use C[SUB]3 [/SUB]carbon fixation or CAM photosynthesis, which is a water-conserving process used in arid biomes.
In Paleocene oceans, sharks filled the empty niches left by aquatic reptiles, but it took coral reefs ten million years to begin to recover, as usual. As Africa and India moved northward, the Tethys Ocean shrank, and in the late Paleocene and early Eocene, the last Tethyan anoxic events laid down Middle East oil, and the last Paleocene climate event is called the Paleocene-Eocene Thermal Maximum ("PETM"). The PETM has been the focus of a great deal of recent research because of its parallels to today's industrial era, when carbon dioxide and other greenhouse gases are massively vented to the atmosphere, causing a warming atmosphere and acidifying oceans. The seafloor communities suffered a mass extinction, and the PETM's causes are uncertain, but the release of methane hydrates when the global ocean warmed sufficiently is a prominent hypothesis, but scientists also look to the usual suspects of volcanism, changes in oceanic circulation, and a bolide impact.
The PETM, according to carbon isotope excursions, "only" lasted about 120-170 thousand years. The early Eocene (c. 56 to 34 mya), which followed the PETM, is also known as one of Earth's Golden Ages of Life. It has also been called a Golden Age of Mammals, but all life on Earth thrived then. In 1912, the doomed Scott Expedition spent a day collecting Antarctic fossils and still had them a month later when the entire team died in a blizzard. The fossils were recovered and examined in London, and surprisingly yielded evidence that tropical forests once existed near the South Pole. They were Permian plants. That was not long after Wegener first proposed his continental drift hypothesis, and was generations before orthodoxy accepted Wegener's idea. Antarctica has rarely strayed far from the South Pole during the past 500 million years, so the fossils really represented polar forests. A generation before the Scott Expedition's Antarctic fossils were discovered, scientists had been finding similar evidence of polar forests in the Arctic, within several hundred kilometers of the North Pole, on Ellesmere Island and Greenland. Scientists were finding Cretaceous plants in the Arctic, which were much younger than Permian plants.[379]
Polar forests reappeared in the Eocene after the PETM, and the Eocene's first ten million years was the Cenozoic's warmest time and even warmer than the dinosaurian heyday.[380] Not only did alligators live near the North Pole, but the continents and oceans hosted an abundance and diversity of life that Earth may have not seen before or since. When that ten million year period ended, as Earth began cooling off and headed toward the current ice age, has been called the original Paradise Lost.[381] One way that methane has been implicated in those hot times is that leaves have stomata, which regulate the air they take in to obtain carbon dioxide and oxygen, needed for photosynthesis and respiration. Plants also lose water vapor through their stomata, so balancing gas input needs against water losses are key stomata functions, and it is thought that in periods of high carbon dioxide concentration, plants will have fewer stomata. Scientists can count stomata density in fossil leaves, which led some scientists to conclude that carbon dioxide levels were not high enough to produce the PETM, so methane became a candidate greenhouse gas that produced the PETM and Eocene Optimum, and the controversy and research continues.[382]
However the hot times were created and sustained, Earth's life reveled in the conditions. Similar to reptiles' beating the heat and migrating into the oceans, some mammals did the same thing nearly 200 million years later, and cetaceans appeared. Scientists were surprised when molecular studies found that whales share a common ancestor with even-toed ungulates and the hippopotamus in particular. Whales evolved in and near India, beginning about 50 mya, when the earliest "whale" surely did not resemble one and lived near water. By 49 mya, whales could walk or swim. A few million years later they resembled amphibians, and by 40 mya they became fully aquatic, for a transition from land to sea that "only" took 10 million years. Whales quickly became the dominant marine predators. However, sharks did not go quietly and began an arms race with whales, which culminated 28 mya in C. megalodon, the most fearsome marine predator ever: a shark reaching nearly 20 meters in length and weighing 50 metric tons. It could have swallowed a great white shark whole, as seen below (C. megalodon in gray, great white shark in green, and next to that is a man taking a break in C. megalodon's mouth). (Source: Wikimedia Commons)
C. megalodon preyed on whales and had the greatest bite force in Earth's history (although some estimates of T-rex bite strength equal it). C. megalodon went extinct less than two mya, due to the current ice age's vagaries.
Because of early Eocene Arctic forests, animals moved freely between Asia, Europe, Greenland, and North America, which were all nearly connected around the North Pole, and great mammalian radiations occurred in the early Eocene. Many familiar mammals first appeared by the mid-Eocene, such as modern rodents, elephants, bats, and horses. The earliest monkeys may have first appeared in Asia and migrated to India, Africa, and the Americas. Europe was not yet connected with Asia, however, as the Turgai Strait separated them. Modern observers might be startled to know where many animals originated. Camels evolved in North America and lived there for more than 40 million years, until humans arrived. Their only surviving descendants in the Western Hemisphere are llamas. As with lemurs migrating to Madagascar from Africa, or marsupials to Australia via Antarctica, or monkeys migrating from Africa to the Americas, or Eocene mammalian migrations via polar routes, the migrants often involuntarily "sailed" on vegetation mats that crossed relatively short gaps between the continents. Such a migration depended on fortuitous prevailing currents and other factors, but it happened often enough.
Several of the Eocene's geologic events had long-lasting impact. About 50 mya, the plates under India and Southern Asia began their epic collision and started creating the Himalayas, and Australia split from Antarctica. The collisions of the African, Arabian, and Indian plates with the Eurasian plate created the mountain ranges that stretch from Western Europe to New Guinea. After the Pacific Ring of Fire, it is the world's most seismically active region. Those colliding plates eventually squeezed the Tethys Ocean out of existence. That event ended more than 500 million years of Tethyan sedimentation, beginning with the Proto-Tethys Ocean in the Ediacaran, continuing with the Paleo-Tethys Ocean in the Ordovician, and the Tethys Ocean appeared in the late Permian. The Tethys Ocean's existence spanned the entire Mesozoic and finally vanished less than six mya, at the Miocene's end.[383] Most of the world's oil formed in the sediments of those Tethyan oceans and very little has formed since the Eocene.
The process of transforming anoxic sediments into oil requires millions of years. When organic sediments are buried, most of the oxygen, nitrogen, hydrogen, and sulfur of dead organisms is released, leaving behind carbon and some hydrogen in a substance called kerogen, in a process that is like reversed photosynthesis. Plate tectonics can subduct sediments, particularly where oceanic plates meet continental plates. There is an "oil window" roughly between 2,000 and 5,000 meters deep, where if kerogen-rich sediments are buried at those depths for long enough (millions of years), geological processes (which produce high temperature and pressure) break down complex organic molecules and the result is the hydrocarbons that comprise petroleum. If organic sediments never get that deep, they remain kerogen. If they are subducted deeper than that for long enough, all carbon-carbon bonds are broken and the result is methane, which is also called natural gas. Today, the geological processes that make oil can be reproduced in industrial settings that can turn organic matter into oil in a matter of hours. Many hydrocarbon sources touted today as replacements for conventional oil were never in the oil window, so were not "refined" into oil and remain kerogen. The so-called oil shales and oil sands are made of kerogen (bitumen is soluble kerogen). It takes a great deal of energy to refine kerogen into oil, which is why kerogen is an inferior energy resource. Nearly a century ago in East Texas oil fields it took less than one barrel of oil energy to produce one hundred barrels, for an energy return on investment ("EROI" or "EROEI") of more than 100, in the Golden Age of Oil. Global EROI is declining fast and will fall to about 10 by 2020. The EROIs of those oil shales and oil sands are less than five and as low as two.
During the early Eocene's Golden Age of Life, forests blanketed virtually all lands all the way to the poles, modern orders of most mammals appeared, today's largest order of sharks appeared, and coral reefs again appeared beyond 50 degrees latitude. Many animals would also appear bizarre today. One crocodile developed hooves, and an order of hooved mammalian predators lived, including the largest terrestrial mammalian predator/scavenger ever, which looked like a giant wolf with hooves. The ancestors of modern carnivores began displacing those primitive predatory mammals in the Eocene, after starting out small. A family of predatory placentals called bear dogs lived from the mid-Eocene to less than two mya. Rhino-sized uintatheres and their bigger cousins the brontotheres were the Eocene's dominant herbivores in North America and Asia. Primates flourished in the tropical canopies of Africa, Europe, Asia, and North America. Deserts are largely an Icehouse Earth phenomenon, and during the previous Greenhouse Earths, virtually all lands were warm and moist. Australia was not a desert in the early Eocene, but was largely covered by rainforests. It must have been a marsupial paradise, as it would have been in Antarctica and South America, but the fossil record is currently thin as rainforests are poor fossil preservers.
In the late Cretaceous, about 75 mya, New Zealand split from Gondwana, and by the end-Cretaceous event it, Madagascar, and India were alone in the oceans. Madagascar was close enough to Africa for lemurs to migrate to it, but the only animals that repopulated New Zealand's lands after the end-Cretaceous holocaust were those that flew. From the end-Cretaceous event until the Maoris arrived around 1250-1300 CE (CE stands for "Common Era," formerly designated with AD), birds were New Zealand's dominant animals and had no rivals. The only mammals were a few species of bat that migrated there in the Oligocene. A recent finding of a mouse-sized mammal fossil shows that some land mammals lived in New Zealand long ago, possibly Mesozoic survivors and unrelated to any living mammals, but they died out many millions of years ago. A few small reptiles and amphibians also lived there, and even a crocodile that died out in the Miocene, but New Zealand, unlike any other major landmass in Earth's history, was the realm of birds. The Maoris encountered giant birds, ecological niches filled with mammals elsewhere were filled by birds, and gigantic moas were the equivalent of mammalian browsers. Before the arrival of humans, moas were only preyed upon by the largest eagle ever. Of all ecosystems that would have appeared strange to modern eyes, New Zealand's pre-human ecosystem has been perhaps the most intriguing to me, perhaps because it still existed less than a millennium ago. It seemed like something that sprang out of Dr. Seuss's imagination. The Seuss-like kiwi is one of the few surviving specialized birds of that time. The Maoris drove all moas to extinction in less than a century, and quickly destroyed about half of New Zealand's forests via burning.
For several million years, life in the Eocene was halcyonic, and at 50 mya, the Greenhouse Earth state had prevailed for 200 million years, ever since the end-Permian extinction. But just as whales began invading the oceans 49 mya, Earth began cooling off. The ultimate reason was atmospheric carbon dioxide levels that had been steadily declining for tens of millions of years. The intense volcanism of the previous 200 million years waned and the carbon cycle inexorably sequestered carbon into Earth's crust and mantle. While falling carbon dioxide levels were the ultimate cause, the first proximate cause was probably the isolation of Antarctica at the South Pole and changes in global ocean currents. During the early Eocene, the global ocean floor's water temperature was about 13[SUP]o[/SUP]C (55[SUP]o[/SUP]F), warm enough to swim in, which was a far cry from today's near-freezing and below-freezing temperatures. The North Sea was warm as bathwater. Radical current changes accompanied the PETM of about 56 mya, warming the ocean floor, and perhaps that boiled off the methane hydrates. Whatever the causes were, the oceans were warm from top to bottom, from pole to pole. But between 50 to 45 mya, Australia made its final split from Antarctica and moved northward, India began crashing into Asia and cut off the Tethys Ocean and the global tropical circulation, and South America also moved northward, away from Antarctica. Although the debate is still fierce over the cooling's exact causes, the evidence (much is from oxygen isotope analyses) is that the oceans cooled off over the next twelve million years, very consistently, and a brief small reversal transpired at about 40 mya.[384] By 37-38 mya, the 200-million-year-plus Greenhouse Earth phase ended and the transition into today's ice age was underway. In the late Eocene, as the trend toward Icehouse Earth conditions began, deserts such as the Saharan, South African, and Australian formed.
That cooling caused the greatest mass extinction of the entire Cenozoic Era, at least until today's incipient Sixth Mass Extinction. With continents now scattered across Earth's surface, there was no event that wiped nearly everything out as the end-Permian extinction did, nor were bolide events convincingly implicated. But mass extinctions punctuated a 12-million-year period when Earth's global ocean and surface temperatures steadily declined. When it was finished, there were no more polar forests, no more alligators in Greenland or palm trees in Alaska, and Antarctica was developing its ice sheets. A few million years later, another mass extinction event in Europe marked the Eocene's end and the Oligocene's beginning, but the middle-Eocene extinctions were more significant.[385] All in all, there was about a 14-million-year period of cooling and extinction, which encompassed the mid-Eocene to early Oligocene, and Icehouse Earth conditions reappeared after a more-than-200-million-year hiatus.[386]
The Oligocene Epoch (c. 34 to 23 mya) was relatively cold. In the 1960s, a global effort was launched to drill deep sea cores, the Glomar Challenger recovered nearly 20,000 cores from Earth's oceans, and scientists had paradigm-shift learning experiences from studying those cores. One finding was that Antarctica developed its ice sheets far earlier than previously supposed, and the cores pushed back the initial ice sheet formation by 20 million years, to about 34-35 mya; the first Antarctic glaciers formed as early as 49 mya. The evidence included dropstones in Southern Ocean sediments, which meant icebergs.[387] The event that led to Antarctic ice sheets was the formation of the Antarctic Circumpolar Current, which began to form about 40 mya and was firmly established by 34 mya, when the Antarctic ice sheets grew in earnest. The current's formation was caused by Antarctica's increasing isolation from Australia and South America, which gradually allowed an uninterrupted current to form that circled Antarctica and isolated it so that it no longer received tropical currents. That situation eventually turned Antarctica into the big sheet of ice that it is today. It also radically changed global oceanic currents. Antarctic Bottom Water formed, which cooled the oceans as well as oxygenated its depths, and it comprises more than half of the water in today's oceans. North Atlantic Deep Water began forming around the same time.[388]
Those oceanic changes profoundly impacted Earth's ecosystems. Not only did most warm-climate species go extinct, at least locally, but new species appeared, adapted to the new environment. Early whales all died out about 35 mya and were replaced by whales adapted to the new oceanic ecosystems that are still with us today: toothed whales, which include dolphins, orcas and porpoises; and baleen whales, which adapted to the rich plankton blooms caused by upwellings of the new circulation and in the Southern Ocean in particular.[389] Sharks adapted to the new whales, which culminated with C. megalodon in the Oligocene. With the land bridges and small seas between the northern continents unavailable in colder times, the easy travel between those continents that characterized the Eocene's warm times ended and the continents began developing endemic ecosystems. Europe became isolated from all other continents by the mid-Eocene and developed its own peculiar fauna. At the Oligocene's beginning, the Turgai Strait was no longer a barrier between Europe and Asia and the more cosmopolitan Asian mammals replaced the provincial European mammals, although from competition, an extinction event, or other causes is still debated, although competition is favored. About half of European mammalian genera went extinct, replaced by immigrants from Asia, and some from North America via Asia.[390]
Africa was also isolated from other continents during those times and developed its own unique fauna. The first proboscideans evolved in Africa about 60 mya, and Africa remained their evolutionary home, and the one leading to today's elephants lived in Africa in the mid-Oligocene. Hyraxes are relatives of elephants, they have never strayed far from their initial home in Africa, and were Africa's dominant herbivore for many millions of years, beginning in the Oligocene. Some reached horse size, and a close relative looked very much like a rhino, with rhino size. The rhinoceros line itself seems to have begun in North America in the early Eocene, and rhinos did not reach Africa until the Miocene.
But the African Oligocene event of most interest to humans was African primate evolution. By the Eocene's end, primates were extinct in Europe and North America, and largely gone in Asia. Africa became the Oligocene's refuge for primates as they lived in the remaining rainforest. The first animals that we would call monkeys evolved in the late Eocene, and what appears to be a direct ancestor of Old World monkeys and apes appeared in Africa at the Oligocene's beginning, about 35-33 mya. But ancestral to that creature was one that also led to those that migrated to South America, probably via vegetation rafts (with perhaps a land bridge helping), around the same time. Those South American monkeys are known as New World monkeys today and they evolved in isolation for more than 30 million years. For those that stayed behind in Africa, what became apes first appeared around the same time as those New World monkeys migrated; they diverged from Old World monkeys. Scientists today think that somewhere between about 35 mya and 29 mya the splits between those three lineages happened. Old World and New World monkeys have not changed much in the intervening years, but apes sure have.
The size issue is dominant in evolutionary inquiries, and scientists have found that in Greenhouse Earth conditions, animal size is relatively evenly distributed, and all niches are taken. When Icehouse Earth conditions prevail, the cooling and drying encourages some animal sizes and not others, and mid-sized animals suffer, such as those early primates. That may be why primates went extinct outside the tropics in the late Eocene.[391] Tropical canopies are rich in leaves, nectar, flowers, fruit, seeds, and insects, while temperate canopies are not, particularly in winter. Large herbivores lost a great deal of diversity in late-Eocene cooling, but the survivors were gigantic, and the largest land mammal ever thundered across Eurasia in the Oligocene. Mid-sized species were rare in that guild.[392]
The earliest bears appeared in North America in the late Eocene and early Oligocene, and raccoons first appeared in Europe in the late Oligocene. It might be amusing to consider, but cats and dogs are close cousins and a common ancestor lived about 50 mya. Canines first appeared in the early Oligocene in North America about 34 mya, and felines first appeared in Eurasia in the late Oligocene about 25 mya. Beavers appeared in North America and Europe in the late Eocene and early Oligocene, and the first deer in Europe in the Oligocene. The common ancestor of today's sloths lived in the late Eocene; South American giant ground sloths appeared in the late Oligocene. The kangaroo family may have begun in the Oligocene. The horse was adapting and growing in North America in the Oligocene. By the late Eocene, the pig and cattle suborders had appeared, and squirrels had appeared in North America.
In summary, numerous mammals appeared by the Oligocene that resemble their modern descendants. They were all adapted to the colder, dryer Icehouse Earth conditions, the poorer quality forage, and the food chains that depended on them. In subsequent epochs, conditions warmed and cooled, ice sheets advanced and retreated, and deserts, grasslands, woodlands, rainforests, and tundra grew and shrank, but with a few notable exceptions, Earth's basic flora and fauna has not significantly changed during the past 30 million years.
The Oligocene ended with a sudden global warming that continued into the Miocene Epoch (c. 23 to 5.3 mya). The Miocene was also the first epoch of the Neogene Period (c. 23 to 2.6 mya). Although nowhere near as warm as the Eocene Optimum, England had palm trees again, Antarctic ice sheets melted, and oceans rose. The Miocene is also called the Golden Age of Mammals. Scientists still wrestle with why Earth's temperature increased in the late Oligocene, but there is no doubt that it did. As the study of ice ages has demonstrated, many dynamics impact Earth's climate, and positiv
"The philosophers have only interpreted the world, in various ways. The point, however, is to change it." Karl Marx
"He would, wouldn't he?" Mandy Rice-Davies. When asked in court whether she knew that Lord Astor had denied having sex with her.
“I think it would be a good idea” Ghandi, when asked about Western Civilisation.
"He would, wouldn't he?" Mandy Rice-Davies. When asked in court whether she knew that Lord Astor had denied having sex with her.
“I think it would be a good idea” Ghandi, when asked about Western Civilisation.