In part because a huge sea (the Tethys) formed an east-west connection between the oceans, Cretaceous climate was uniformly warm; even the poles lacked ice. In response, warm-adapted plants and dinosaurs expanded to within 15 degrees of the poles. Dinosaurs reached a peak of diversity and size (Figure below and Figure below).
Figure 11.44
Many kinds of reptiles and invertebrates lived during the Cretaceous Period. Mosasaurs (upper left), plesiosaurs (center) and ammonites (upper right) swam the seas with modern sharks. Triceratops (lower left) and duckbilled dinosaurs (lower right) show some of Cretaceous diversity in dinosaurs.
Figure 11.45
Moderate climate worldwide during the Cretaceous encouraged great size and diversification among dinosaurs. The herbivorous titanosaur, (above) may have been the largest of all the dinosaurs, weighing in at up to 100 tons. (below) probably preyed upon titanosaurs such as , but weighed only 5.2 tons, and despite a bath-tub-sized skull, operated on a brain the size of a banana.
Titanosaurs, including possibly the largest of all the dinosaurs, the 100-ton Argentinosaurus, were the dominant herbivores. A single Argentinosaurus vertebra was 1.3 meters long, and its tibia would have been as tall as some humans. Fossilized eggs, containing embryos with skin, indicate that titanosaurs were colonial nesters. Fossilized dung shows they ate cycads and conifers, but also palms and the ancestors of rice and bamboo; some scientists suggest that dinosaurs and grasses coevolved like insects and flowering plants.
One of the largest predatory dinosaurs,Giganotosaurus, weighed “only” 5.2 tons, but in length surpassed Tyrannosaurus rex by two meters (six feet). Giganotosaurus’ skull was the size of a bathtub, but its brain was the size and shape of a banana! What were they thinking?
The dramatic extinction of all dinosaurs (except the lineage which led to birds) marked the end of the Cretaceous. Dinosaurs had begun to decline earlier, perhaps due to reduction in atmospheric oxygen and global cooling. A worldwide iridium-rich layer, dated at 65.5 million years ago, provides evidence for an additional, more dramatic cause for their ultimate extinction. Iridium is rare in the Earth’s crust, but common in comets and asteroids. Scientists correlate this layer with a huge crater in the Yucatan and Gulf of Mexico. A collision/explosion between the Earth and a comet or asteroid could have spread debris which set off tsunamis, altered the climate (including acid rain), and reduced sunlight 10-20%. A consequent reduction in photosynthesis would have caused a drastic disruption in food chains. Some scientists believe that volcanism also contributed to the “K-T” (Cretaceous-Tertiary) extinction, but most agree that “an impact event” was at least a major cause (Figure below). The massive extinction and sharp geologic line led geologists to define the end of the Mesozoic and the beginning of our modern Era, the Cenozoic, with this event.
Figure 11.46
The extinction of the dinosaurs at the end of the Cretaceous is attributed at least in part to an impact event which could have involved a meteor, an asteroid, or a comet.
Cenozoic Era: Age of Modern Life
Figure 11.47
Neogene and Quaternary (Q) Periods share part of the Pliocene Epoch (Pl). Pleistocene (P) and Holocene (H) Epochs complete the Quaternary Period. Divisions in this part of the Time Scale are debated and may change.
The Cenozoic Era brings the history of life into the present, but not without drama, mystery, and the looming possibility of a “Sixth Extinction” (Figure above) You probably know the basic story: mammals took over where dinosaurs left off, branched to form primates, moved to the grasslands, became human-like, survived the ice ages, and the rest is – literally – history. Let’s look at some of the major events, focusing not only on our immediate ancestors but also on the world in which they evolved. More detailed stories of the evolution of humans and other groups will be told in later chapters.
Seven Epochs comprise the Cenozoic Era, with the Holocene continuing up to today. “Tertiary” refers to the 64 million years and five epochs before the Quaternary Period, well known for its recent ice ages and recognizable humans. Tertiary and Quaternary periods could be called suberas, but current organization of the Cenozoic segment of the Geologic Time Scale is the subject of current debate; it may well change.
The Paleocene Epoch provided a worldwide warm, humid climate for the rapid evolution which followed the extinction of the dinosaurs (Figure below). Many plants, herbivores, and carnivores had disappeared because they depended on photosynthesis, but omnivores, insectivores, and scavengers – which included many mammals and birds – survived because their food sources actually increased. Mammals radiated into the ecological niches opened up by the extinction of herbivores and carnivores, and larger species, up to bear- or hippopotamus- sized, began to appear In equatorial regions, the first recognizably modern rain forests appeared, and south of the equator, hot arid regions provided niches for new groups of plants, including cacti.
Figure 11.48
During the Paleocene, mammals and birds invaded ecological niches formerly occupied by the dinosaurs. Mammals included monotremes (A), marsupials, and hoofed placentals (B). Modern sharks (C) patrolled the seas. Birds included the giant flightless (D).
Volcanism or a massive release of methane gas trapped in the oceans may have triggered one of the most rapid global warming events ever measured at the beginning of the Eocene, 56 million years ago. CO2 from either volcanism or oxidation of methane would have caused the oceans to become more acidic, and Earth’s temperatures to rise. Warm temperatures allowed forests of dawn redwood, swamp cypress, and palms to extend toward both poles. In the interiors of the continents, seasonal temperature and moisture variations led to the evolution of grasses, expansive savannas and deciduous forests. Within these new ecosystems, modern mammals with specialized teeth evolved. Probably due to high temperatures, these mammals were smaller than those who preceded them – or those who followed:
Horses and tapirs evolved in North America, and rhinoceros evolved in Asia.
Primates, with their long arms and legs and grasping hands and feet, appeared.
Mammals returned to the sea; Basilosaurus was an ancestor of today’s whales.
At the beginning of the Eocene, Australia was still connected to Antarctica, but when they broke apart, ocean currents changed and cooling began in earnest, foreshadowing the ice ages to come. Tundra ecosystems developed near the poles. Falling sea levels, a land bridge immigration of mammals from Asia to North America, and perhaps several impact events led to an extinction which marks the end of this epoch.
As its name implies, the Oligocene Epoch produced a “few” new mammals, especially in grasslands and savannahs (Figure below).
Figure 11.49
The Oligocene produced fewer new mammals than the Eocene; most were adapted to grasslands. (A) showed small steps toward modern horses. (B) had the large, sharp teeth of a carnivore. (C) was a piglike scavenger, and (D) was a large relative of elephants and hyraxes. Perhaps the largest land mammal of all time resembled an overweight giraffe; (E) weighed up to 15 tons and reached 18 feet in height.
Pig-like entelodonts used massive skulls to crush bones of scavenged prey.
One of the largest land mammals of all time, the 18-foot, 15 ton Indricotherium, ate leaves from the tops of trees in the manner of a giraffe.
Horses, represented by Mesohippus remained small relative to today’s species.
Large terrestrial carnivores such as Hyaenodon, hunted mammals up to the size of sheep.
The rhinoceros-like Arsinoitherium wandered tropical rain forests and swamps.
By the beginning of the Miocene Epoch 23 million years ago, the continents had almost assumed their current configuration, except that North and South America did not connect. Oceans continued to cool, ice caps expanded at the poles, and consequently the climate dried. Grasslands, needing less rain, replaced forests, and large herbivores coevolved with the grasses. Modern mammals, including wolves, beaver, deer, camels, seals, dolphins, and p
orpoises, evolved. Up to 100 species of apes lived throughout Africa, Europe, and Asia. Almost all modern bird groups were represented.
The Earth’s climate continued to cool into the Pliocene, the epoch in which hominids first appeared. Seasons became more pronounced; deciduous forests and grasslands replaced tropical forests, and coniferous forests and tundra expanded. Large mammals, such as browsing mastodons and grazing mammoths, roamed the grasslands and tundra. Into this setting walked Australopithecines, such as Lucy who share common ancestry with humans. Fossil footprints dated as 3.7 million years old establish Australopithecenes as bipedal – perhaps the first apes to walk upright (Figure below). Later Pliocene hominids included two members of our own genus, Homo rudolfensis and Homo habilis. During this epoch, falling sea levels exposed two land bridges which allowed important migrations.
Figure 11.50
Lucy (D) is one of the most complete fossils of , a human relative also known for fossil footprints which establish an upright posture. Note that the brown bones are Lucys; others have been added to restore her skeleton. Australopithecines coexisted (but not necessarily on the same continent!) with browsing mastodons (A), grazing mammoths (B), and giant herbivorous sloths (C).
One allowed horses, mammoths, mastodons and more to migrate between Asia and North America.
A second allowed North American placental mammals, such as giant sloths, armadillos, and sabertooth cats, to migrate to South America. Placentals eventually out-competed all of South America’s marsupials except the opossum.
Repeated glaciations define the Pleistocene Epoch. Glaciation tied up huge volumes of water in ice packs; rainfall was less, because evaporation was less. Deserts were relatively dry. During interglacial periods, huge inland lakes and rivers held or carried the melt waters, and coastal flooding reduced land area. During the four major glaciations, these severe climate changes stressed animals and plants, encouraged the evolution of large animals (the Pleistocene megafauna), and forced life toward the equator.
Figure 11.51
That (A), and later (or in other parts of the world) , hunted mammoth (B) is shown by fossil evidence 1.8 million years old. Woolly mammoths (C), specially adapted to cold climate, were probably hunted, as well, as humans spread throughout the world. Nearly 40 woolly mammoth remains have been found preserved in permafrost, complete with soft tissue and DNA. To date, mitochondrial DNA has been sequenced. The calf (D) measures 2.3 meters (8 feet) long. A predatory competitor to humans was the saber-tooth tiger, (E).
Some adapted to the cold: the Woolly Mammoth grew thick, shaggy hair oiled by abundant sebaceous glands, a layer of fat beneath the skin, smaller ears, and even a convenient flap to cover the anus, keeping out the cold. Mammoth teeth ground tough tundra grasses, and their long, curved tusks may have helped to clear snow. Permafrosts have preserved nearly 40 mammoth remains, including soft tissues, and scientists actually hope to be able to recreate its genome; mitochondrial DNA for one species has already been sequenced! Using this sequence as a molecular clock, scientists calculate that mammoths diverged from African elephants about 6 million years ago, roughly the same time that humans diverged from chimpanzees.
Saber-tooth cats used dagger-like teeth to cut their prey’s windpipe and jugular veins, causing death by bleeding. Many saber-tooths have been found in the LaBrea Tar Pits in southern California, where they had tried to feed on mammoths trapped before them in the sticky tar/asphalt.
Homo erectus, the dominant hominid during the Pleistocene, migrated throughout Africa, Europe, and Asia, giving rise to a number of variations of hominids. Although Homo erectus was probably the first hominid to leave Africa, the species may not have been a direct ancestor of humans. Pleistocene hominids were hunter-gatherers; evidence dated at 1.8 million years ago supports their consumption of mammoth.
A major extinction of Pleistocene megafauna continued into the Holocene. Some attribute the extinction to changing climate or disease, but others have connected the migrations of humans to each continent’s time of extinction The “overkill” theory suggests that humans hunted large animals with too much success. Agreement is not yet universal, but most scientists admit the evidence is strong.
Figure 11.52
This data comparing the arrival of humans to the decline of the Pleistocene megafauna supports the overkill theory that human predation contributed to the extinction of large mammals throughout the world. Other theories involve climate change and disease.
The current Holocene Epoch began 11,550 years ago (about 9600 B.C.) with the retreat of the Pleistocene glaciers. During the Holocene, melting ice has raised sea level over 180 meters (600 feet). Geologists believe that we are currently experiencing an interglacial warming, and that glaciers will return – unless continued human burning of fossil fuels raises CO2 levels to bring about global warming. All of human civilization has occurred within the Holocene; Homo sapiens have passed through Mesolithic, Neolithic, and Bronze Age civilizations. Human evolution will be discussed in more detail in a future chapter, but here we will examine the possibility that humans are currently causing a mass extinction which some compare to the Permian. Many would include the Pleistocene megafauna in this “Sixth Extinction,” citing the “overkill theory” data in Figure above. Some even call the period of time from that loss to the present the “Anthropocene epoch” to describe the major impact humans have had on the planet and its life. Human population has surpassed 6.6 billion, and over-fishing, climate change, industrialization, intensive agriculture, and clearance of grasslands and rainforests contribute to a startlingly high loss of life.
Figure 11.53
(A) Changes in CO levels (green) are clearly associated with temperature changes (blue); the graph shows the four major Ice ages of the Pleistocene. Graphs B (long time scale) and C (recent time) show increases in CO and global temperature over the past 150 years, suggesting that the Industrial Revolution, which began our major fossil fuel burning and release of CO, may account for much of the increase in temperature -- a new, humaninduced global warming.
Paleontologists estimate that background extinction rates throughout most of life’s history averaged between 1 and 10 species per year (Figure above) . The present rate of extinction is thought to be 100 to 1000 times "background" rates, suggesting that the number of species which currently disappear each year could exceed 1,000! Biologist E.O. Wilson has predicted that current rates will result in the loss of over half of life’s biodiversity within the next one hundred years. In contrast, Earth’s shortest previous extinctions spanned several hundred thousand to several million years, and evidence for cause is entirely geological in nature. No other species has influenced the Earth and its life as powerfully as Homo sapiens.
Others say there is ample evidence – evidence discussed in this lesson – to show that extinction is a natural phenomenon which has occurred repeatedly throughout the history of life on Earth. They point to the recoveries – indeed, radiations – which filled vacated ecological niches after each event.
However, those who are concerned about the current extinction wonder whether or not humans will be one of the species to become extinct. Because we are the only surviving members of our family, recovery or radiation would not be an option.
What do you think?
Lesson Summary
After 3 billion years, life was unicellular but included all 3 major lineages: Bacteria, Archaea and Eukaryotes; all of multicellular evolution occurred within the last billion years.
Plant, animal, and fungal ancestors diverged as solitary cells.
Colonies of eukaryotic cells and specialized cells evolved; Volvox illustrates this level of evolution.
Sexual reproduction appeared a little over 1 billion years ago, providing more variation for natural selection.
By about 1 billion years ago, true (multicellular) plants emerged, and 100 million years later, true animals.
600 million year old Ediacaran fossils illustrate diverse early animal life, b
ut few are related to modern animals.
The Cambrian explosion was the sudden appearance of great diversity in animals, plants, and fungi clearly related to modern species, due to lower O2, global warming, plate tectonics, and a critical mass of biotic change.
During the Ordovician, liverworts became the first land plants, and jawless, bony-plated fish joined a variety of invertebrates in warm seas.
Ferns and the first seed plants forested the land in the Devonian; in shallow seas, jawed fish evolved lobed fins.
Extensive coal deposits and an all-time high level of atmospheric O2 characterized the Carboniferous Period. Giant insects, early gymnosperms with pollen, and vertebrates with shelled eggs colonized dry land.
During the Permian, all the major landmasses of earth combined into a single supercontinent, Pangaea. A continental climate of seasons and drought favored reptiles, gymnosperms, and insects such as beetles. The Permian ended with the most massive extinction of all time; 99.5% of all species disappeared, opening the door for a new radiation of species in the Mesozoic.
The Permian extinction, stable temperatures and continental breakup created niches for a great radiation of life in the Triassic Period. Reptiles diversified – on land as the archosaurs, in the air as pterosaurs, and in the seas as ichthyosaurs. Other reptilian lines gave rise to early turtles, crocodiles, and finally, mammals and birds.
During the Cretaceous, primitive birds began to radiate and out-compete the pterosaurs; dinosaurs reached their largest size and greatest diversity.
Record-high temperatures during the Eocene made way for hoofed animals and primates within grasslands, savannas, and deciduous and coniferous forests.
CK-12 Biology I - Honors Page 50