SOUTH AMERICAN HOOFED MAMMALS 299–251 Triassic 251–199.6 Jurassic 199.6–145.5 Cretaceous 145.5–65.5 Paleogene 65.5–23 Neogene 23–present MESOZOIC 251–65.5 MYA CENOZOIC 65.5 MYA–present 249
MAMMALS AND THEIR ANCESTORS Arsinoitherium Two smaller URANOTHERES horns grew from the back ONE OF THE MOST PECULIAR groups of mammals is the Uranotheria, of the skull. a collection of plant-eating, hoofed mammals that includes elephants, seacows, and hyraxes. It might seem strange that these very different creatures are thought to be related, but they share features not seen in other mammals. The earliest members of these groups seem to have been similar in appearance. The first elephants, for example, were not giant creatures with trunks, but dog-sized animals probably similar to modern-day hyraxes. In fact, hyraxes may show how the earliest uranotheres lived – as small, grazing plant eaters. A later group of uranotheres, called the tethytheres, evolved an amphibious lifestyle. Some of these became committed to life in water and eventually evolved into the first seacows. Arsinoitherium had a comparatively small and simple brain. Males had larger and more pointed horns than females. Arsinoitherium’s teeth had tall crowns and could have been used to chew very tough plant material. ARSINOITHERIUM Scientific name: Arsinoitherium ARSINOITHERES Size: 12 ft (3.5 m) long Diet: Tough leaves and stems These rhinoceros-like uranotheres lived in Asia, Habitat: Woodland, wooded grassland Europe, and Africa 40–30 million years ago, in Where found: Egypt and Oman the Eocene and Oligocene epochs. The best- Time: Late Paleogene (Eocene) known arsinoithere is Arsinoitherium – Related genera: Crivadiatherium, Paleoamasia a large, heavy animal with two massive horns on its skull. The largest individuals of Arsinoitherium Cambrian 542–488.3 Ordovician 488.3–443.7 (probably old males) were about the size of small elephants. Unlike rhinoceros horns, arsinoithere horns were hollow. They have grooves on their surface, showing that there were blood vessels on the outside, and that the horns were probably covered in skin. Silurian 443.7–416 Devonian 416–359.2 Carboniferous 359.2–299 Permian PALEOZOIC 542–251 MYA 250
URANOTHERES Its shoulders Tusklike were massive front teeth and powerfully Arsinoitheres probably muscled. digested their food in an enormous hind gut region. Short, stocky limbs HYRAXES Kvabebihyrax Modern hyraxes are small African mammals that look like guinea pigs. Fossil hyraxes, however, were quite different and came in a huge range of shapes and sizes. Kvabebihyrax, shown here, was the shape of a hippopotamus and may have been amphibious. Titanohyrax was the size of a small rhinoceros. Antilohyrax was a long-legged runner resembling an antelope. Other fossil hyraxes had long heads and would have looked similar to modern-day pigs. Arsinoitherium’s limbs were stout and heavy. Each foot had five blunt toes, each tipped with a small hoof. Broad, heavy It had tusklike front body teeth and cylindrical chewing teeth. DESMOSTYLIANS The front legs Desmostylians, such as Paleoparadoxia, were may have been sea-dwelling uranotheres. They lived on the used as paddles edges of the Pacific Ocean during Oligocene when swimming. and Miocene times (33.9–5.3 million years ago) and probably fed on seaweeds. Skeleton of Paleoparadoxia Desmostylians perhaps looked like a cross between a hippopotamus and a walrus. Although they lived mainly in the sea, they could probably walk clumsily on land. 299–251 Triassic 251–199.6 Jurassic 199.6–145.5 Cretaceous 145.5–65.5 Paleogene 65.5–23 Neogene 23–present MESOZOIC 251–65.5 MYA CENOZOIC 65.5 MYA–present 251
MAMMALS AND THEIR ANCESTORS In Hipparion a large bony pocket called the HORSES preorbital fossa grew on the side of the PERISSODACTYLS ARE A GROUP of snout. This was larger hoofed, odd-toed, plant-eating in males than females, mammals, and include horses, though its function tapirs, rhinoceroses, and remains unknown. brontotheres. Horses, the most TEETH FOR GRASS-EATING suited to open grasslands, Advanced horses, such as appeared in the Eocene Hipparion, had large, high- (55.8–33.9 million years ago) crowned molar teeth with and about eight species of them complicated chewing surfaces survive today. Successive groups made up of loops of enamel. of horse species evolved Their premolars became different features and body large and squarish and came sizes to suit their environments. to look like the molars. These Hipparion, shown here, was one powerful, resistant teeth allowed of several species of three-toed advanced horses to eat rough horse that lived on the grasses, although they may Northern Hemisphere initially have evolved in grasslands during the response to the accidental Miocene (23–5.3 chewing of sand and grit. million years ago). Life in open, grassland environments favored the evolution of large body size and long limbs in horses. THREE-TOED FEET Like modern horses, Hipparion was a grassland animal. Earlier horses were probably inhabitants of forests. Unlike modern horses, which only have one toe on each foot, Hipparion had three-toed feet. However, most of its weight was borne on the enlarged central toe. Hipparion and its relatives were distant cousins of horses such as Merychippus, which were the probable ancestors of modern horses. Cambrian 542–488.3 Ordovician 488.3–443.7 Silurian 443.7–416 Devonian 416–359.2 Carboniferous 359.2–299 Permian PALEOZOIC 542–251 MYA 252
HORSES HIPPARION Scientific name: Hipparion Modern horses have special Size: 5 ft (1.5 m) long locking mechanisms, called Diet: Leaves and grasses stay apparatus, on their Habitat: Grasslands, open woodlands limb bones that allow them Where found: North America, Europe, Asia, to remain standing with and Africa a minimum of effort. Time: Neogene (Miocene–Pleistocene) Hipparion did not Related genera: Cormohipparion, Nannippus, have stay apparatus. Neohipparion 299–251 Triassic 251–199.6 Jurassic 199.6–145.5 Cretaceous 145.5–65.5 Paleogene 65.5–23 Neogene 23–present MESOZOIC 251–65.5 MYA CENOZOIC 65.5 MYA–present 253
MAMMALS AND THEIR ANCESTORS BRONTOTHERES AND CHALICOTHERES THESE TWO GROUPS OF CREATURES were odd-toed hoofed mammals, Weak teeth suggest or perissodactyls. Brontotheres were large, rhino-like animals known that brontotheres only from North America and Asia in the Eocene (53–33.7 million mostly ate years ago). Early brontotheres were about the size of sheep, but soft leaves. later ones were giants, up to 8 ft (2.5 m) tall at the shoulder. Some sported horns on the end of their snouts that grew in V or Y shapes. Chalicotheres were horselike perissodactyls with long forelimbs and curved claws on their fingers and toes. First appearing in the Eocene in Asia and Europe, they later spread to Africa and North America and survived into the early Quaternary. BRONTOTHERE HORNS SKULL OF BRONTOPS As brontotheres evolved, their horns became larger. Later brontotheres, such as In later brontotheres, males had larger horns than Brontops, had very shortenend females. This suggests that brontothere males used Injuries found on skulls and faces and eyes positioned close their horns for displaying and fighting, like the two ribs suggest that brontotheres to the nose. Brontops had two male Brontops shown here. Early brontotheres had fought each other with their short horns that stuck upward horns, perhaps for dominance, and outward. tusk-like front teeth, but territory, or mating rights. Brontops and other later brontotheres lacked these and instead had a mobile upper lip. The surface texture Powerful muscles were of the horns shows attached to the massive that they were spines on the shoulder covered in skin. vertebrae and formed a prominent hump. In some brontotheres one of the wrist bones, a bone called the trapezium, was absent. No one knows why. Cambrian 542–488.3 Ordovician 488.3–443.7 Silurian 443.7–416 Devonian 416–359.2 Carboniferous 359.2–299 Permian PALEOZOIC 542–251 MYA 254
BRONTOTHERES AND CHALICOTHERES BRONTOPS Its skeleton suggests that Scientific name: Brontops Moropus could Size: 17 ft (5 m) long stand up on its Diet: Leaves Habitat: Open woodland back legs. Where found: Western North America Time: Paleogene (Eocene) Its long, flexible Related genera: Menops, Megacerops, neck allowed Duchesneodus Moropus to reach up into Brontops branches. The powerful front legs were longer than the back legs. The hip bones were Moropus could very broad, probably hold its claws up to help support the off the ground weight of the body. when it walked. A CLAWED “HOOFED MAMMAL” Moropus was a chalicothere from North America. Like all chalicotheres, it had massive, powerful front legs and curving claws on its hands. Chalicotheres may have been digging animals that fed on roots and tubers. However, their wrists were not very mobile, and their teeth lack the wear marks normally created by soil and grit. It is more likely that chalicotheres were browsers that pulled branches down from trees. Brontothere tails probably ended in a tuft of hairs. Giant brontotheres LAST OF THE had stout, short BRONTOTHERES limbs suited to Embolotherium and carrying the its relatives were an immense weight advanced group of of their bodies. brontotheres, found only in Asia. They were related to Brontops. Embolotherium was one of the last and largest of the brontotheres. It was similar in size to Brontops, but was equipped with a large, forked nose horn. Its broad mouth contained small, rounded incisor teeth. 299–251 Triassic 251–199.6 Jurassic 199.6–145.5 Cretaceous 145.5–65.5 Paleogene 65.5–23 Neogene 23–present MESOZOIC 251–65.5 MYA CENOZOIC 65.5 MYA–present 255
MAMMALS AND THEIR ANCESTORS RHINOCEROSES ELASMOTHERIUM The surviving species of rhinoceros are members of TODAY THERE ARE FIVE SURVIVING SPECIES of the rhinocerotid group. The biggest rhinocerotid was Elasmotherium, which reached 16 ft (5 m) in length and rhinoceros. They are plant-eaters with horns had an immense conical horn 6 ft 6 in (2 m) tall on its made from keratin – the same structure that forehead. Elasmotherium lived in the Pleistocene skin, nails, and claws are made from. Fossil (1.81–0.01 million years ago). rhinoceroses were diverse and evolved many different lifestyles and body shapes. Perhaps the most primitive rhinoceroses were the hyracodontids, or running rhinoceroses. These were hornless, long-legged creatures with simple teeth suited for browsing. Another family, the amynodontids, included amphibious rhinoceroses with short mobile trunks, like the trunks of modern-day tapirs. Amynodontids appear to have been very primitive rhinoceroses, and it has even been suggested that they are not rhinoceroses at all, but part of the tapir group of perissodactyls instead. THE BIGGEST LAND MAMMAL Paraceratherium was a gigantic hyracodontid rhinoceros. In contrast to the small early hyracodontids, Paraceratherium was 18 ft (6 m) tall at the shoulder and weighed around 15.7 tons (16 tonnes), making it the biggest land mammal of all time. Its skull alone was about 4 ft (1.3 m) long. Paraceratherium was probably a browser that ate leaves from the tops of trees. PARACERATHERIUM Scientific name: Paraceratherium Despite its great size, Size: 9 m (30 ft) long Paraceratherium had Diet: Leaves and twigs long, slim legs and Habitat: Open woodland could probably run. Where found: Eastern Europe, Asia Time: Palaeogene–Neogene (Oligocene–Miocene) Related genus: Forstercooperia Cambrian 542–488.3 Ordovician 488.3–443.7 Silurian 443.7–416 Devonian 416–359.2 Carboniferous 359.2–299 Permian PALEOZOIC 542–251 MYA 256
RHINOCEROSES Paraceratherium Its skull structure Hollows in the sides suggests that of Paraceratherium’s Paraceratherium had back bones made a flexible upper lip. them light but strong. Its long neck enabled WOOLLY RHINOCEROS Paraceratherium to Coelodonta, the woolly rhinoceros of Asia and browse from trees. Europe, lived in the Pleistocene (1.75–0.01 Some Teleoceras million years ago). It is known from bodies species had a preserved in frozen soil and from prehistoric flexible upper lip. cave paintings. We therefore have a good idea of what Coelodonta would have looked like. It had two large horns, a hump over its shoulders, thick stocky limbs, and long, dark fur. Barrel-shaped body Tall cheek teeth Teleoceras probably for chewing walked along the tough grass bottom of rivers. Three stout toes LIVING LIKE A HIPPOPOTAMUS supported its Teleoceras was a long-bodied rhinoceros from North America weight. that lived in the Miocene (23.5–5.3 million years ago). It had very short legs and a small nose horn. Teleoceras had long teeth, which show that it was a grass-eater. It probably lived like a hippopotamus, wallowing in water but grazing on land at night. Its fossils are frequently found in the beds of ancient streams. 299–251 Triassic 251–199.6 Jurassic 199.6–145.5 Cretaceous 145.5–65.5 Paleogene 65.5–23 Neogene 23–present MESOZOIC 251–65.5 MYA CENOZOIC 65.5 MYA–present 257
MAMMALS AND THEIR ANCESTORS PROBOSCIDEANS THE TWO MODERN-DAY species of elephant are the living representatives of a much larger group of hoofed mammals, called the proboscideans. The earliest known proboscidean was Phosphatherium from the Paleocene Phiomia probably had a short trunk. (65–56 million years ago). It weighed only about 33 lbs (15 kg) and was just 2 ft (60 cm) tall at the shoulder. Later proboscideans increased in size and evolved straight, columnlike legs. They also grew massive tusks in their upper jaws, which they used for fighting and gathering food. The structure of their PHIOMIA skulls shows that nearly all fossil This primitive proboscidean proboscideans had a trunk. lived in northern Africa during the Oligocene (33.9–23 million years ago). Phiomia was larger than Moeritherium but still only about as big as a large modern horse. It had MOERITHERIUM columnlike legs, a shorter neck, and a much bigger skull One of the most primitive known proboscideans is Moeritherium. than more primitive elephants. It lived in Africa in the Eocene. Moeritherium’s skull indicates Like later elephants, Phiomia that it had an enlarged upper lip, but experts do not know had air-filled spaces, called whether this was a true trunk. Its bulky body was similar in diploe, in its skull. This meant shape to a hippopotamus, and its legs were short. These features that its skull was light, despite suggest that Moeritherium wallowed in lakes and rivers, perhaps its size. feeding on water plants. Enlarged incisor teeth in both the upper and lower jaws formed small tusks that probably protruded from its mouth. Primitive proboscideans like Moeritherium had not yet developed the columnlike legs of an elephant. Cambrian 542–488.3 Ordovician 488.3–443.7 Silurian 443.7–416 Devonian 416–359.2 Carboniferous 359.2–299 Permian PALEOZOIC 542–251 MYA 258
ELEPHANTS Gomphotherium SHOVEL-TUSKERS was about as big Like nearly all primitive proboscideans, Phiomia as an Asian elephant. had tusks in both its upper and lower jaws. Its lower jaw was long and had flattened, square- tipped tusks. This formed a shovel-shaped jaw that could have been used to scoop up water plants, or cut branches or bark from trees. Later, more advanced kinds of proboscidean, such as Gomphotherium, had similar lower jaws. Skull of Phiomia GOMPHOTHERIUM DEINOTHERES A successful group of elephants called These strange proboscideans had no gomphotheres spread around the world tusks in their skulls and two down- in the Miocene and Pliocene (23–1.8 curved tusks in their lower jaws. million years ago). Gomphotheres, These may have been used to dig such as Gomphotherium, inhabited up roots, strip bark, or wrench marshes, grasslands, and branches down from trees. forests. Gomphotherium’s upper Deinotheres seem to have jaw tusks were probably used had shorter trunks than for fighting and display and, living elephants. as in living elephants, were larger in males than in Deinotherium was females. Elephants similar 13 ft (4 m) tall at to Gomphotherium were the the shoulder. ancestors of mammoths and of modern-day elephants. MOERITHERIUM The enlarged upper Scientific name: Moeritherium lip and nose may Size: 10 ft (3 m) long have formed a Diet: Water plants very short trunk. Habitat: Lakes, rivers, riverside forest Where found: Northern Africa Moeritherium’s neck was Time: Paleogene (Eocene–Oligocene) longer than that of more Related genus: Phosphatherium advanced proboscideans. 299–251 Triassic 251–199.6 Jurassic 199.6–145.5 Cretaceous 145.5–65.5 Paleogene 65.5–23 Neogene 23–present MESOZOIC 251–65.5 MYA CENOZOIC 65.5 MYA–present 259
MAMMALS AND THEIR ANCESTORS PLATYBELODON PLATYBELODON WAS A WIDESPREAD “shovel-tusker” gomphothere. It had a long, scooplike tip to its lower jaw, formed by the tusk and mandible. In some specimens the lower jaw curved downward along its length, while in others it was straighter and curved upward at the tip. Platybelodon was once thought to have lived in marshes, where it used its lower jaw to scoop up water plants. Wear patterns on its tusks suggest instead that Platybelodon mostly lived in grasslands and forests and cropped tough vegetation from trees. A FLEXIBLE TRUNK Old reconstructions of Platybelodon show it with a short, wide trunk that would not have been very flexible. This is because the reconstructions were based on evidence from the more primitive Phiomia. The nasal openings in Phiomia’s skull show that its trunk was short and poorly developed. However, Platybelodon had the same kind of nasal openings as modern elephants. Like modern elephants therefore, Platybelodon probably had a long, flexible trunk. Cambrian 542–488.3 Ordovician 488.3–443.7 Silurian 443.7–416 Devonian 416–359.2 Carboniferous 359.2–299 Permian PALEOZOIC 542–251 MYA 260
PLATYBELODON PLATYBELODON Scientific name: Platybelodon Size: 3 m (10 ft) tall at shoulder Diet: Leaves, grasses, bark Habitat: Grasslands, forests Where found: North America, Africa, Asia, Europe Time: Neogene (Miocene–Pliocene) Related genera: Amebelodon, Torynobelodon LOWER JAW The wear marks on Platybelodon’s lower jaw tusks show that vegetation was often pulled across the tips of the tusks. Platybelodon probably used the tusks as blades. It would have grabbed a branch with its long trunk and pulled it repeatedly across the tusks until they sliced through the wood. It seems that most of this branch- cutting happened where the two tusks touched one another, as this is usually the most heavily worn area. LEGS AND FEET Like all advanced elephants, Platybelodon had straight legs. Its knees were positioned directly above its ankles, so each leg formed a column beneath its body. Platybelodon’s ankles were very close to the ground because the bones that formed its feet were very short. Fatty pads under its feet supported the foot bones and helped to spread the animal’s great weight as it walked. 299–251 Triassic 251–199.6 Jurassic 199.6–145.5 Cretaceous 145.5–65.5 Paleogene 65.5–23 Neogene 23–present MESOZOIC 251–65.5 MYA CENOZOIC 65.5 MYA–present 261
MAMMALS AND THEIR ANCESTORS Mammoth hair, found on the frozen specimens, can MAMMOTHS be up to 3 ft (90 cm) long. THE EIGHT SPECIES OF MAMMOTH were all true elephants, closely related Shoulder hump to the two living species. Mammoth genetic material, or DNA, was discovered in 1994 and found to be almost identical to that of living elephants. The woolly mammoth (Mammuthus primigenius) is perhaps the most famous fossil animal from the Pleistocene (1.75–0.01 million years ago). An inhabitant of the cold Ice Age grasslands, it had a shaggy coat, huge, curving tusks, and a tall, domed skull, but was small compared to some other extinct elephants. As in living elephants, male mammoths probably wandered on their own and fought for dominance with their tusks. Some males even died locked in combat and have been preserved this way as fossils. The last mammoths, a population of dwarf woolly mammoths, lived on Wrangel Island north of Siberia and died out only 4,000 years ago. WOOLLY MAMMOTH Mammuthus primigenius These mammoths lived in herds and fed on grasses and other small plants, which they plucked with Both male and female woolly the two “fingers” on the mammoths had long, curving tusks. They used their tusks in combat and display, and tips of their trunks. as tools for gathering food. Several woolly mammoths have been found preserved in the frozen ground of Siberia. Their fur, skin, muscles, and even their stomach contents are still intact. These frozen specimens show that, unlike living elephants, mammoths had very short tails. This is probably because a long tail would be vulnerable to frostbite. DIMA THE MAMMOTH Silurian 443.7–416 Devonian 416–359.2 Carboniferous 359.2–299 Permian Dima was the name given to a frozen male baby woolly mammoth, recovered in 1977 on the bank of the Berelekh River, Russia, and preserved in a remarkably complete condition. How Dima died has been the subject of debate. It has been suggested that he drowned, fell into a crack in the frozen ground, or became caught in wet mud. X-rays show that Dima had not yet grown the domed skull and tall shoulder hump seen in adult mammoths. Cambrian 542–488.3 Ordovician 488.3–443.7 PALEOZOIC 542–251 MYA 262
MAMMOTHS CAVE PAINTINGS WOOLLY MAMMOTH About 400 prehistoric cave paintings and sculptures of Scientific name: Mammuthus primigenius woolly mammoths are known. Size: 11 ft (3.3 m) tall Most have been found in French Diet: Grasses and other plant material or Spanish caves and are about Habitat: Woodlands and grasslands 30,000 years old. The paintings Where found: North America, Europe, Asia show mammoths with the same Time: Late Neogene (Pleistocene–Holocene) features as the mammoths Related genus: Elephas (includes preserved in frozen ground, living elephants) such as a shoulder hump and small, rounded ears. Some paintings depict mammoths moving in groups while others show mammoths fighting. The tusks were so long and curved that they crossed over. Huge columnlike limbs supported its weight. IMPERIAL MAMMOTH Mammuthus imperator (Imperial mammoth) was a giant North American Pleistocene mammoth and one of the biggest elephants that ever lived. It was 12 ft (3.7 m) tall at the shoulder and its huge, curving tusks could be 14 ft (4.3 m) long. Many fossils have been found in California, where this mammoth inhabited warm grassland and woodland environments. Warm-weather mammoths probably lacked the furry coats of their cold-climate cousins. 299–251 Triassic 251–199.6 Jurassic 199.6–145.5 Cretaceous 145.5–65.5 Paleogene 65.5–23 Neogene 23–present MESOZOIC 251–65.5 MYA CENOZOIC 65.5 MYA–present 263
MAMMALS AND THEIR ANCESTORS PIGS, HIPPOS, AND PECCARIES THE LARGEST AND MOST SUCCESSFUL group of hoofed mammals are the artiodactyls, or even-toed hoofed DAEODON mammals. All artiodactyls have distinctive ankle and foot bones that allow many of them to run fast. Their feet are symmetrical, and most forms have two or four toes, hence the group’s name. Artiodactyls include three major groups that all first appeared in the Eocene. The first group is the suiforms, including pigs, hippopotamuses (hippos for short), peccaries, and various extinct relations. Unspecialized teeth and flexible snouts have allowed them to become Scientific name: Daeodon omnivores that can eat most kinds of plant material Size: 10 ft (3 m) long Diet: Vegetation, carrion, smaller animals and fungi, as well as carrion and small animals. Many Habitat: Grasslands, open woodland suiforms had, or have, large fang- or tusk-like teeth Where found: North America used for fighting and display. These tusks have Time: Paleogene–Neogene (Oligocene–Miocene) become massively enlarged in the hippos. The Related genera: Archaeotherium, Choerodon second artiodactyl group, the tylopods, includes the camels. The third group, the pecorans, includes giraffes, deer, and cattle. Skeleton of Entelodonts had small Archaeotherium brains, but parts of the brain devoted to smell HIPPOPOTAMUSES were well developed. The first hippos appeared in the Late Miocene. Two kinds survive today – the large, amphibious Hippopotamus and the small, land-living Hexaprotodon. The recently extinct Hippopotamus lemerlei was a pygmy river-dwelling hippo from Bony cheek flanges Madagascar. Hippos have huge, curving tusks at the front of were especially large, the mouth. These are larger in males than females and are with swollen ends. used in fighting and displaying. Genetic studies suggest that hippos may be related to whales, but this is controversial. Eyes located on ANCIENT BEAST top of the head Entelodonts were pig- to bison-sized Amphibious hippos suiforms known have elongated snouts from Eocene and and lower jaws. Miocene Europe, Skull of Hippopotamus lemerlei Asia, and North America. They had long legs and deep bodies. Their huge skulls have bony bumps on the cheeks and lower jaws, crushing teeth, and huge, curving canine teeth. Archaeotherium was a successful pig- sized entelodont that lived across North America and Asia. Incisors protrude Unlike many other suiforms, forward, and can entelodonts had only two be used for digging. toes on each foot. Cambrian 542–488.3 Ordovician 488.3–443.7 Silurian 443.7–416 Devonian 416–359.2 Carboniferous 359.2–299 Permian PALEOZOIC 542–251 MYA 264
PIGS, HIPPOS, AND PECCARIES Macrogenis Powerful neck tendons skull attached to the shoulder spines helped to support head. Elongated bony spines grew from shoulder vertebrae. PECCARIES Daeodon Another group of suiforms, which appeared in the Late Eocene (around 40 million years ago), and lived mostly in North America, are the peccaries. They survive to the present as three species. Peccaries look much like pigs and live in similar ways. They have large, vertical canine teeth. Some extinct peccaries, like the Late Miocene Macrogenis, had massive triangular protrusions of bone growing out from their cheeks. Skull bumps became larger with age. Curving canine teeth with serrated edges Big crushing molars suggest Daeodon ate bones KILLER BUFFALO PIG Unlike in smaller Unlike in smaller entelodonts, the lower entelodonts, the Daeodon, formerly called Dinohyus, leg bones of Daeodon lower leg bones of is one of the biggest, best known, and were fused together Daeodon were fused last of the entelodonts. Like others of its group, for strength. together for strength. it had tall shoulders, a deep body, and long legs. The bumps on its skull and jaws were probably used for fighting – some fossil specimens have wounds that appear to have resulted from such battles. The bony bumps in Daeodon were actually smaller than those of most other entelodonts, though they were still prominent. The teeth and muscle scars of Daeodon suggest that it was an omnivore, easily able to break bones and eat animal carcasses. Entelodonts may have been scavengers, finding their food in a similar way to modern hyenas. 299–251 Triassic 251–199.6 Jurassic 199.6–145.5 Cretaceous 145.5–65.5 Palaeogene 65.5–23 Neogene 23–present MESOZOIC 251–65.5 MYA CENOZOIC 65.5 MYA–present 265
MAMMALS AND THEIR ANCESTORS CAMELS CAMELS AND THEIR RELATIVES EVOLVED in the Eocene HIGH CAMEL (56–34 million years ago), and include nearly 100 fossil species. Although modern camels inhabit deserts, they Scientific name: Aepycamelus were once abundant grassland and woodland herbivores. Size: 2 m (7 ft) tall at shoulder Two groups of artiodactyls – the bovoids and the camels Diet: Leaves from trees and relatives – evolved a special way of digesting plant Habitat: Open woodland, grassland with trees material called rumination. Once swallowed, food passes Where found: North America to the first of three or four stomach chambers, and is Time: Neogene (Miocene) later regurgitated to be chewed a second time (“chewing Related genera: Oxydactylus, Hesperocamelus the cud”). In each stomach chamber, microorganisms break down the plant material further. Ruminant mammals recycle urea, one of the body’s waste products, and use it to feed these microorganisms. As a result, less urine is produced, and less water wasted, which is why ruminants have adapted to dry environments like deserts more successfully than other hoofed mammals. Stenomylus probably lived in large herds. Skull very short compared to other camels Long, slender legs would have made Aepycamelus a fast runner. NARROW TOOTH When standing Aepycamelus walked The six living camel species are native to Africa, Asia, and South Stenomylus was less on the whole length of America. However, most of camel history occurred in North America, than 3 ft 3 in (1 m) its toes, unlike earlier and camels still lived here as recently as 11,000 years ago. The first tall at the shoulder. Stenomylus. camels were small, and perhaps lived like modern gazelles. Stenomylus from the Oligocene (around 30 million years ago) was a small early camel with enormous chewing teeth. Unlike advanced camels, Stenomylus had pointed hooves and walked on the tips of its toes. Cambrian 542–488.3 Ordovician 488.3–443.7 Silurian 443.7–416 Devonian 416–359.2 Carboniferous 359.2–299 Permian PALEOZOIC 542–251 MYA 266
CAMELS Features of teeth and FEEDING STRATEGIES skull suggest a closer Living camels are grazers that relation to living llamas Pointed mostly eat grasses. The skulls than to modern camels. front teeth were small. and teeth of fossil camels, however, show that many of them were browsers, feeding on shrubs and trees. Oxydactylus from Miocene GIANT GIRAFFE CAMEL North America had long legs and a long neck. Aepycamelus (“high camel”) Perhaps Oxydactylus fed was a large camel with on trees by standing tremendously long leg and tall on its back legs like the modern neck bones. It was probably Gerenuk antelope. a browsing herbivore that, Feeding Gerenuk like modern giraffes, fed from trees. Eight Aepycamelus species are known, each with a slightly different skull and jaws. They inhabited grasslands with scattered trees, and may have become extinct as the tree cover gradually disappeared. Aepycamelus probably shared a number of features with living camels, including a divided upper lip, a long, curved neck, unusual two-toed feet, and legs that are not joined by a sheet of skin to the side of the body. Camels also have special fanglike teeth Neck bones that the males use in fighting. longer than those of any other camel Front and back legs Fossil Oxydactylus foot of camels are more Two “metapodial” bones equal in size than are fused together in they are in other advanced camels. hoofed mammals. Advanced camels like this have only two toes. Like living kinds Primitive Eocene camels of camel, fossil still had four toes. species perhaps had dense, CAMEL FEET AND WALKING STYLE woolly fur. Advanced camels have unique feet. Unlike other artiodactyls, they do not walk on the tips of their As in modern toes, but on the whole length of their toes. Soft toe camels, two pads help them walk on rocks or sand with ease. main hand Camels walk using a technique called pacing. Both bones were legs on one side of the body move in the same fused together. direction at once. 299–251 Triassic 251–199.6 Jurassic 199.6–145.5 Cretaceous 145.5–65.5 Paleogene 65.5–23 Neogene 23–present MESOZOIC 251–65.5 MYA CENOZOIC 65.5 MYA–present 267
MAMMALS AND THEIR ANCESTORS In the largest males, the antlers spanned DEER AND KIN 12 ft (3.7 m). GIANT ANTLERS SEVERAL NEW GROUPS OF SMALL, forest-dwelling The largest antlers of all time belong to Megaloceros, a giant herbivores diversified during the Miocene Pleistocene deer that was still alive (23–5 million years ago). The spread of 9,000 years ago. Despite the huge grasslands allowed some of them to move size of these antlers, microscopic out of the forest, becoming larger and stress marks show that they were more successful. Like bovoids, many used for fighting and not just evolved horns or equivalent structures display. Antlers are shed each year for fighting and displaying. The most and, except in reindeer, are grown successful of these mammals were the only by males. Antlers may have deer – antelope-like animals distinguished first developed from scar tissue by their antlers. Early antlers were resulting from injuries. simple in shape – any fighting was probably done with fanglike teeth, and some living deer still fight in this way. Giraffids (the giraffes and their relatives) were one of the first groups to move to the grasslands and evolve large body size. Some fossil giraffes were deerlike, and all possessed bony horns called ossicones. Several other groups of deerlike mammals, including the paleomerycids and protoceratids, evolved their own impressive bony horns. Syndyoceras Protoceras Synthetoceras EARLY HORNS A dappled coat The deerlike protoceratids may have helped (“early horns”) lived in North Cranioceras to hide America from about 45 to 4 million in dense foliage. years ago, from the Eocene to the Pliocene. Male protoceratids displayed some of the most spectacular and complex All weight horns ever evolved. In earlier protoceratids, such as Protoceras, the was carried horns looked most impressive in side view. Later kinds, including by the middle Syndyoceras and Synthetoceras grew horns more suited for display two toes. from the front. Protoceratids had stout limbs and bodies, and their teeth suggest that they ate soft vegetation and they probably had a flexible upper lip like a camel’s. Cambrian 542–488.3 Ordovician 488.3–443.7 Silurian 443.7–416 Devonian 416–359.2 Carboniferous 359.2–299 Permian PALEOZOIC 542–251 MYA 268
DEER AND KIN THREE-HORNED DEER RELATIVE Second Long rear Giraffokeryx restoration pair of ossicones Cranioceras was a paleomerycid – one of a group of deerlike hoofed mammals that lived from about 34 to 4 ossicones million years ago, from the Oligocene to the Pliocene. Many, on nose but not all, paleomerycids had bony horns that grew backward, forward, or upward from above their eyes. In the group that includes Cranioceras, a third horn grew upward and back from the rear of the skull. Healed injuries seen on paleomerycid horns show that they were used for fighting – probably to establish mating rights and social dominance. As in most horned mammals, only males possessed horns. While some paleomerycids had long limbs and probably lived in the open, Cranioceras and its Paleomerycid relatives were short-legged horns may have denizens of dense woodlands, been more about 10 million years ago. Later like ossicones, paleomerycids grew larger, but suggesting GIRAFFIDS then smaller just before a close link Giraffokeryx was a primitive giraffid that lived to giraffids. in Miocene and Pliocene Asia, Europe, and Africa, around 5 million years ago. It had two their final extinction. pairs of pointed, furry, hornlike structures called ossicones. The giraffids are cud-chewing hoofed mammals that probably share an ancestor with bovoids. Only two survive today – the African giraffe (Giraffa camelopardalis) and the Okapi (Okapia johnstoni). As well as distinctive ossicones, both use a flexible tongue to bring foliage to the mouth. However, many other giraffids thrived from the early Miocene era (around 20 million years ago) and the recent past. One major group of extinct giraffids, the sivatheres, had enormous branching ossicones, and would have looked more like deer than giraffes. CRANIOCERAS Limbs were Scientific name: Cranioceras not as long or Size: 1 m (3 ft) tall at shoulder slim as those Diet: Leaves of grassland- Habitat: Subtropical woodland dwelling Where found: North America relatives. Time: Neogene (Miocene) Third horn Related genera: Procranioceras, Yumaceras could have been used for display as well as fighting. 299–251 Triassic 251–199.6 Jurassic 199.6–145.5 Cretaceous 145.5–65.5 Paleogene 65.5–23 Neogene 23–present MESOZOIC 251–65.5 MYA CENOZOIC 65.5 MYA–present 269
MAMMALS AND THEIR ANCESTORS CATTLE, SHEEP, AND GOATS CATTLE AND THEIR RELATIVES are Sharp, strong horns the most plentiful large, hoofed, that are never shed grazing animals alive today. Wild and domestic cattle, sheep, goats, antelopes, and musk oxen (but not the antelopelike pronghorns) are grouped together as bovoids, named from the Latin word for ox. All these animals probably evolved more than 20 million years ago from small, hornless, deerlike ancestors. Early forms resembling gazelles gave rise to huge variety, with more than 100 genera by about two million years ago, but all bovoids share some common features. These include strong, defensive horns which never shed on the heads of both sexes, and teeth and stomachs adapted for eating and digesting grass. Legs and two-toed feet are also designed for fast running or agile leaping to escape enemies. The cattle family spread first through Europe, Africa, and Asia. By one million years ago, some crossed the Bering Land Bridge into North America where bison, bighorn sheep, and Rocky Jaws have high- Mountain goats persist today. crowned teeth, which evolved for chewing. PREHISTORIC ANIMALS IN CAVE PAINTINGS ANCESTRAL OX An artist depicted this great ox on a cave wall at Lascaux in southwest France some 15,000 years ago. At the time, Bos primigenius, also known as the Old Stone Age hunter-gatherers hunted wild herds of aurochs, was the ancestor of most cattle for their meat. Perhaps another 7,000 years went domesticated cattle. It was larger by before people learned to tame and farm the least than modern cattle, wild and fierce, fierce individuals for meat, milk, and hides. The old wild and roamed the forests of Europe, cattle strain is now extinct, but millions of its descendants Asia, and Africa, dying out in recent graze peacefully in pastures worldwide. times. The last wild aurochs was killed in Poland in 1627. Bovoids such as Bos and Bison priscus left large fossil horn cores of bone. In life, horny sheaths covered these cores, making the horns even longer. Fossils of the prehistoric ox Pelorovis, related to the modern African buffalo, have 6 ft 6 in (2 m) horn cores. With the addition of their sheaths, its horns could have spanned as much as 13 ft (4 m). These awesome horns were used by rival males for display and fighting, as well as for self-defense. Although these prehistoric creatures are no more, wild cattle of several genera survive. The best known kinds are bison, buffaloes, and yak. Cambrian 542–488.3 Ordovician 488.3–443.7 Silurian 443.7–416 Devonian 416–359.2 Carboniferous 359.2–299 Permian PALEOZOIC 542–251 MYA 270
CATTLE, SHEEP, AND GOATS Muscular neck Bos stood up to ANCESTRAL OX and shoulders 6 ft 6 in (2 m) tall at the shoulder. Scientific name: Bos primigenus Size: 10 ft (3 m) long Diet: Plants Habitat: Forest glades Where found: Europe, Africa, Asia Time: Neogene (Pliocene–Holocene) Related species: Bison, Pelorovis Branched horns with sheaths shed annually Hind limbs with short thigh bones, but long shin and foot bones Long foot with high ankle, a design for fast running Sturdy limbs to Foot has two Tiny early support weight large toes tipped pronghorn EARLY SHEEP AND GOATS with hooves. Ramoceros Goats and sheep, including PREHISTORIC PRONGHORN mountain sheep like Ovis canadensis Ovis canadensis skull Miocene and Pliocene North America shared a common ancestor with was home to the antelopelike pronghorn other bovoids. This animal or antilocaprid family, of which only one probably existed in the member now survives – the second fastest Oligocene, more than mammal in the world. Ramoceros was a small, 20 million years ago. Its prehistoric relative of the living pronghorn. descendants gave rise first to Its long, forked horns may have been used antelopes, then to sheep and by rival males in pushing contests. Like other goats, and finally to cattle. antilocaprids, Ramoceros shed the sheaths of its horns every year, and new horns formed from hair sprouting on the bony cores. This difference between pronghorns and cattle persuades many scientists that these agile mammals may be more closely related to deer. 299–251 Triassic 251–199.6 Jurassic 199.6–145.5 Cretaceous 145.5–65.5 Paleogene 65.5–23 Neogene 23–present MESOZOIC 251–65.5 MYA CENOZOIC 65.5 MYA–present 271
MAMMALS AND THEIR ANCESTORS HOOFED PREDATORS WHEN SOMEONE MENTIONS “HOOFED MAMMALS,” we tend to think of unaggressive plant-eating creatures: cattle, antelope, and sheep. Early in their evolution, however, the ancestors of these even-toed hoofed mammals included very different creatures – the so-called mesonychians. Like sheep or cows, these had toes tipped with hooves, not claws. But instead of molars shaped for munching leaves, the creatures had massive teeth designed for slicing meat or crushing bones. Mesonychians looked rather like, and played the same role as, wolves, hyenas, and bears. They lived across Europe, Asia, and North America for around 30 million years, from the middle Paleocene (60 MYA) to the early Oligocene (30 MYA). Long, narrow jaw with Toes tipped teeth rather like a bear’s with short hooves instead Head broad at the back of long sharp with powerful jaw muscles. claws Andrewsarchus skull Massive molars with blunt Long, curved, GIGANTIC OMNIVORE cusps for crushing pointed canine tooth for piercing flesh The enormous Andrewsarchus (“Andrews’ flesh- eater”) lived in Eocene Mongolia more than POWERFUL JAWS 40 million years ago, and was the biggest known The skull of Andrewsarchus shows its powerful jaws and carnivorous land mammal of all time. Its skull formidable collection of teeth. The canines at the front alone was 33 in (83 cm) long and 22 in (56 cm) were long, curved, and piercing for delivering the killer bite wide. The rest of the skeleton has not been to its prey. The back teeth were also pointed – the lower molars discovered, but its body probably grew up to bladelike for cutting, and the upper molars broad, designed for 19 ft (6 m) long or even more. Andrewsarchus’s crushing. Notches in the top and bottom teeth were much like jaws were equipped with long, sharp, curved canine those in living carnivores, for gripping meat to tear it from the teeth, and massive, crushing back teeth, powerful bone. The back teeth, however, were generally not as well- enough to have killed and crunched the bones of designed for slicing flesh as those of modern carnivores. young hoofed mammals, but this enormous beast was probably as unfussy about its diet as a grizzly bear. It would have munched juicy leaves and berries, insect grubs, and small rodents, as well as scavenging from large corpses that it came across. Cambrian 542–488.3 Ordovician 488.3–443.7 Silurian 443.7–416 Devonian 416–359.2 Carboniferous 359.2–299 Permian PALEOZOIC 542–251 MYA 272
HOOFED PREDATORS Long, heavy body Skull shape shows Long, lean body (exact shape is unknown) Mesonyx had shaped something a strong bite. like a wolf’s Like Mesonyx, MIDDLE NAIL Andrewsarchus Mesonyx (“middle nail”) was a member of the may have had mesonychids, the best-known mesonychian family. a long tail. It was a wolflike predator from the Middle Eocene Strong limbs must of Wyoming and East Asia (around 45 million years have supported ago). Agile limbs made it a fast runner, and it probably its great weight. hunted hoofed plant eaters, moving lightly on its toes, not flat-footed like the less advanced mesonychians. Yet, Andrewsarchus may instead of claws, Mesonyx’s toes ended in small hooves. have been flat-footed, Its long skull had a crest above the braincase to anchor like a bear. large jaw muscles and give it a powerful bite. Archaeocetes skull ANDREWS’ FLESH-EATER Long, low, narrow jaws like those of mesonychians Scientific name: Andrewsarchus WHALE ANCESTORS Size: Up to 19 ft (6 m) long Mesonychians probably became extinct when outcompeted by Diet: Meat, plants, insects new kinds of more efficient predators, including the ancestors Habitat: Scrub and open woodland of carnivores alive today. Meanwhile, though, according to a once Where found: East central Asia popular theory, they gave rise to a much longer-lived and more Time: Paleogene (Eocene) successful group of mammals – the whales. The skulls and teeth Related species: Eoconodon, Goniaconodon of early whales such as Archaeocetes strongly resemble those of certain mesonychians. But other anatomical features and molecular evidence place whales with the artiodactyls. 299–251 Triassic 251–199.6 Jurassic 199.6–145.5 Cretaceous 145.5–65.5 Paleogene 65.5–23 Neogene 23–present MESOZOIC 251–65.5 MYA CENOZOIC 65.5 MYA–present 273
MAMMALS AND THEIR ANCESTORS EARLY WHALES Square tail vertebrae Long tail flukes show that the tail provided the main LIVING CETACEANS – whales, dolphins, and their kin – had flukes (fins swimming thrust. along its sides) are fish-shaped, with flipperlike forelimbs, powerful fluked tails for Long tail propulsion, and no hind legs. The earliest Eocene whales were very different: Large hindlimbs Tiny, three-toed they had four legs and ran around on land. made Ambulocetus hindlimbs would Yet skeletons show some had started to swim a strong swimmer have projected with an up and down motion of the tail. By the from sides. end of the Eocene, fully aquatic whales like Basilosaurus had evolved. Eocene whales were predators that mostly inhabited shallow tropical seas. They evidently descended from close kin of, or actual, artiodactyls that had taken to foraging for food in shallow water. Mesonychians’ skulls are strikingly like those of the first whales, but early whales’ ankle bones show features only seen in artiodactyls. DNA studies even hint that whales could have arisen from the artiodactyl group that includes hippopotamuses. Long skull, with nostrils close to the tip of snout ECHOLOCATION AT WORK Throughout their evolution, whales developed a method called echolocation to gain a mental picture of their surroundings. Many project Ambulocetus probably noises through a structure on the forehead spent most of its life on land, taking to the called the melon. Echoes of the noises are seas to hunt. then transmitted to the whale’s ears via a fatty pad in its lower jaw. Eocene whales do not appear to have had melons though they Artist’s restoration of Ambulocetus do show evidence of FIRST WHALES sensitive hearing. The first known whale, Pakicetus, comes from the Middle Eocene of Pakistan, a site rich in early whale fossils. From one of these fossilized skeletons, we now know that Pakicetus was small – probably less than 6 ft 6 in (2 m) long – and not specialized for life in water. However, its ear bones possessed features unique to whales, and its delicate front teeth suggest that it caught fish, perhaps while paddling in shallow water. Other early whales were formidable predators – Ambulocetus looked something like a cross between a The melon sits Any nearby wolf and a seal and had a long crocodile-like head. Its in a large bony skull was strong enough to resist the struggles of large depression on the mammalian prey. top of the skull. object, such as another animal, creates an echo. Cambrian 542–488.3 Ordovician 488.3–443.7 Silurian 443.7–416 Devonian 416–359.2 Carboniferous 359.2–299 Permian PALEOZOIC 542–251 MYA 274
EARLY WHALES Dorudon skull Nostrils were not located on the forehead in Eocene whales. Dorudon and other Eocene whales SKULL EVOLUTION have two different kinds of teeth. In early whales like Pakicetus, the nostrils were located close to the tip of the snout, as they are in most land mammals. More advanced whales like Basilosaurus and Dorudon have their nostrils located midway along their snouts. In advanced whales like the Miocene Prosqualodon, the nostrils are on the top of the head where they form the blowhole. Whales’ nostrils have therefore gradually moved backward, making room for the developing melon in the forehead. It is unknown whether fossil whales had dorsal fins or not. Prosqualodon skull Teeth of advanced whales are all similar in shape. Unique long body. EOCENE GIANT Other Eocene whales were much shorter. Basilosaurus is one of the biggest fossil whales known, growing to more than 60 ft (20 m). The vertebrae BASILOSAURUS that make up Basilosaurus’ back and tail are unusual, elongated bones, unlike the shortened vertebrae seen in most whales. These could have made Basilosaurus more flexible than living whales. Its huge skull, which can be more than 3 ft 3 in (1 m) long, had curved front teeth and triangular, serrated cheek teeth. Using these, Basilosaurus could have grabbed and sliced up fish as well as other marine mammals. Scientific name: Basilosaurus Ribs were made Size: 66–83 ft (20–25 m) long of very thick, Diet: Other marine mammals, fish heavy bone. Habitat: Shallow tropical seas Where found: North America, North Africa, Unlike modern whales, Wear on teeth shows that South Asia basilosaurs had a Basilosaurus preyed on Time: Paleogene (Eocene) flexible elbow. large animals. Related genera: Basiloterus Dorudon 299–251 Triassic 251–199.6 Jurassic 199.6–145.5 Cretaceous 145.5–65.5 Paleogene 65.5–23 Neogene 23–present MESOZOIC 251–65.5 MYA CENOZOIC 65.5 MYA–present 275
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Reference Section In this section, you can scan a pictorial summary of life through time, with pauses for dramatic mass extinctions. Retrace the journeys of prehistoric creatures, from their deaths millions of years ago to their stunning resurrection in museum halls. Along the way, step into the shoes of a paleontologist – spend time on an exotic dig, then learn what goes on when fossils reach the laboratory. Watch scientists reassemble fossil bones and see sculptors recreate lifelike replicas of ancient animals. Get tips on conducting your own fossil hunt. Read about personalities who helped solve puzzles of the past. Then discover where to see exciting fossils on display. 277
REFERENCE SECTION GEOLOGICAL TIME CHART ROCKS ARE DEPOSITED IN LAYERS, and the layers such as the Triassic and Jurassic. Periods are at the bottom of a sequence are the oldest. grouped together to form eras, such as the The actual ages in years of different rock Mesozoic and Cenozoic. Eras are one of the layers are worked out by techniques that biggest subdivisions of geological time and involve measuring the decay of radioactive are separated from one another by major elements within the rock. Groups of rock extinction events in which important fossil layers are classified together to form periods, groups disappear from the geological record. MILLION YEARS AGO PRECAMBRIAN First single-celled and Mawsonites Collenia multicellular life. 4,600-542 PALEOZOIC ERA 542-488.3 CAMBRIAN PERIOD All major animal Olenellus groups appear. Orthoceras 488.3-443.7 ORDOVICIAN PERIOD First nautiloids and jawed vertebrates. Cyrtoceras First plants and 443.7-416 SILURIAN PERIOD arachnids on land. Baragwanathia Pseudocrinites First vertebrates with four limbs 416-359.2 DEVONIAN PERIOD and distinct digits. Pteraspis 359.2-299 CARBONIFEROUS 299-251 PERIOD First reptiles and flying insects Sandalodus PERMIAN PERIOD on land. Sail-back synapsids Edaphosaurus Diplocaulus appear on land. MESOZOIC ERA 251-199.6 TRIASSIC PERIOD First dinosaurs, mammals, Lystrosaurus turtles, and frogs. 199.6-145.5 JURASSIC PERIOD First birds appear, dinosaurs Pterodactylus rule the land. Proceratosaurus 145.5-65.5 CRETACEOUS First modern mammals. PERIOD Non-avian dinosaurs die out. Triceratops CENOZOIC ERA 65.5-55.8 PALEOGENE PERIOD PALEOCENE EPOCH First owls, shrews, and Taeniolabis Phenacodus hedgehogs. 55.8-33.9 EOCENE EPOCH First horses, elephants, Palaeochiropteryx Hyracotherium dogs, and cats. 33.9-23 OLIGOCENE EPOCH First monkeys, deer, Phiomia 23-5.3 MIOCENE EPOCH and rhinoceroses. 5.3-1.81 1.81-0.01 PLIOCENE EPOCH First apes, mice, and many Samotherium 0.01-Present PLEISTOCENE new mammals. EPOCH NEOGENE PERIOD HOLOCENE EPOCH First cattle and sheep. Bison Balaena Whales diversify. First modern humans Gigantopithecus appear. Extinctions caused Homo sapiens by human activity. Formation of the Earth PRECAMBRIAN TIME 4,600 MYA 278
FOSSIL TIMELINE PRECAMBRIAN TIME 4,600-542 MYA AQUATIC ANIMALS COLLENIA Stromatolites are layered structures that THE RISE OF LIFE resemble stony pillars or platforms. They were widespread in Precambrian times The first living things were and still exist today in Australia and prokaryotes – microscopic, elsewhere. These bizarre structures single-celled bacteria-like were built by colonial microorganisms, organisms. They evolved in the Precambrian, probably in the hot such as Collenia, which grew in mats using water around deep-sea volcanic sunlight for energy. When studied under the vents. Their fossils first appear microscope, stromatolites can be seen to be made around 3,500 million years ago. up of fossil prokaryotes and other microorganisms. Multicellular organisms, or eukaryotes, arose late in the Precambrian, perhaps Round, flattened when single-celled forms took to body, with projection living in colonies. By the end of the at the center Precambrian, soft-bodied animals were present. Some of these may be ancestors Body made of of animals, such as jellyfish and worms, three connected but others might be bizarre dead ends segments unrelated to later animals. EARTH FACTS EDIACARA This simple, disk-shaped Precambrian organism probably lived a static life on the seafloor, absorbing oxygen directly from the surrounding water. It formed part of the Vendian fauna – a group of soft-bodied organisms first discovered in rocks in southern Australia, and later found worldwide. Such fossils are highly controversial because they are difficult to interpret. Scientists are not even sure if they were animals or plants. DICKINSONIA NOGORTNHDEWRANNA This 5-in (13-cm) long SOUTHERN segmented animal appears GONDWANA to have lived on or in sandy parts of the seafloor. The Precambrian represents more Dickinsonia is known from than 85 percent of geological time the Vendian faunas of during which the Earth changed Australia and Russia. Its from a molten ball to a planet identity is controversial. with continents, oceans, and an Some experts argue that it atmosphere. The first tectonic plates was a kind of flat-bodied developed 3,800 million years ago worm. Others suggest that it and life evolved. Oxygen built up in was a soft-bodied coral. One the atmosphere much later. expert has even argued that Dickinsonia and similar fossils CENOZOIC were actually lichens. MESOZOIC This timeline shows each era as a proportion of all geological time. The Precambrian era is more than eight times longer than all the time elapsed since. PALEOZOIC 542 MYA 251 MYA 65.5 MYA TODAY 279
REFERENCE SECTION CAMBRIAN PERIOD METALDETES AQUATIC ANIMALS The appearance of shells and other hard parts was a EXPLOSION OF LIFE key event in animal evolution that occurred at the very end of the Precambrian. Microscopic fossils Most of the major groups of animals of early shelled animals are found worldwide that exist today evolved in the Cambrian in Cambrian rocks. They include mollusks Period. This huge growth in diversity with coiled shells and worms that lived occurred only in the seas – the land in straight tubes. Archaeocyathans, such was still bare of life other than as Metaldetes, had porous cone-shaped micro-organisms – and is called the calcite skeletons. They probably lived “Cambrian explosion.” Soft-bodied fixed to the seafloor. animals and stromatolites (bacterial Metaldetes probably colonies) of the Precambrian were resembled a largely replaced by species with hard modern sponge. parts, especially trilobites. Their fossils have been found in abundance in the Metaldetes taylori Burgess Shale, a famous fossil-rich Large eyes area in the Canadian Rocky Mountains that dates back around 505 million years. XYSTRIDURA PIKAIA Trilobites were a tremendously Chordates, the group successful and varied group of that includes vertebrates, arthropods, and they make up evolved in the Early one third of all fossils known Cambrian. A later kind was Pikaia, from the Cambrian period. a swimming eel-like animal, Types such as Xystridura had 2 in (5 cm) in length. Pikaia had a many legs, complex eyes flexible rod called a notochord made of numerous stiffening its body. In later animals, individual lenses, and this developed into the backbone. long antennae. Some Pikaia swam by contracting blocks burrowing trilobites of muscle around the notochord from the Cambrian, to produce a wavelike motion. however, lacked eyes. 280 Other species were good swimmers, while others could roll into balls for protection. Each body segment supported a walking leg and a gill-bearing leg. V-shaped muscle blocks around the notochord
FOSSIL TIMELINE 542–488.3 MYA WIWAXIA Ridged scale Perhaps distantly related to the The head shield supported large mollusks, Wiwaxia backward-pointing horns. was 1 in (3 cm) long, dome shaped, and MARRELLA covered in scales. It The most common was equipped with long arthropod in the Burgess spines, probably for self- Shale is Marella – more than defense. Wiwaxia is from 25,000 specimens have been the Burgess Shale, but new collected to date. Up to 1 in discoveries show that such (2 cm) long, this animal had animals were widespread a large head shield, and two in the Cambrian. pairs of long antennae, although it seems to have Wiwaxia probably lacked eyes. Its body was ate algae. made up of 24–26 segments, each of which carried a two- branched appendage. The lower branch was a walking leg, while the upper branch carried long gills. The animal was able to both walk and swim. Indentations may have been sites of muscle attachment. EARTH FACTS Concave G ONDWANA IAPETUS OCEAN NA inner surface indicates an G O N DWA older individual. During the Cambrian, most of the MOBERGELLA world’s landmasses were united as Among the early the supercontinent Gondwana. This shelled fossils of the was surrounded by the vast Iapetus Cambrian are tiny limpetlike Ocean. Smaller landmasses that today forms, such as Mobergella from form Europe, North America, and Scandinavia. These might not have Siberia lay in tropical and temperate been separate, individual animals but zones. There were no ice caps in the scalelike structures that covered the bodies seas, and water levels were high. of larger species. Members of another Cambrian group with hard parts, the halkieriids, were elongate with scaly bodies. It is possible that Mobergella may have actually been halkieriid body scales. 281
REFERENCE SECTION ORDOVICIAN PERIOD AQUATIC ANIMALS Estonioceras Uncoiled final ESTONIOCERAS perforatum whorl of shell This swimming mollusc was a nautiloid – a FILTER FEEDERS group which survives today as the animal Nautilus. It had a loosely coiled shell and Another burst of evolution in was adapted for hunting in fairly deep the Ordovician gave rise to European waters, grabbing prey with its thousands of new animals. tentacles. Estonioceras was small, just Many were filter feeders that 4 in (10 cm) across, but straight- fed on increasing numbers shelled relatives had shells up to 16 ft of plankton – microscopic (5 m) in length. At the time, these free-floating organisms – in intelligent Ordovician predators were the water. These included the largest animals ever to have lived. mosslike bryozoans, Shell could be bivalves, and corals, which locked shut by formed reefs that were home internal pegs and to swimming molluscs and sockets other animals. Trilobites diversified greatly into STROPHOMENA swimming forms equipped Among the most abundant with huge eyes and bottom- of Ordovician animals were feeders with shovel-like brachiopods – filter-feeders whose snouts for plowing through two shells were joined at a hinge. mud. The earliest jawed Strophomena was a small vertebrates – a group that brachiopod that probably lived includes sharks and bony on sand or mud, although many fish – probably also appeared of its relatives cemented in the Ordovician. themselves to submerged rocks. AQUATIC PLANTS ALGAE AND LIFE ON LAND MASTOPORA This reef-forming green alga grew in Colonial blue-green algae – the stromatolites – which rounded clusters with a characteristic had evolved in the Precambrian, were still widespread honeycombed surface pattern. during the Ordovician. True algae, including globular Fossils of the clusters, each about forms that resembled sponges, lived alongside other 3 in (8 cm) across, are found reef builders, such as corals, while green worldwide. Limestone algae, the ancestors of land plants, secreted by the algae covered colonized freshwater habitats. the surface, protecting Most significantly, plants Mastopora from hungry similar to liverworts and herbivores. Superficially mosses evolved late in resembling a sponge, the Ordovician and Mastopora was originally began to colonize thought to be an animal. the land, which until this time had been barren of life. These plants were still strongly tied to water, needing it for reproduction. Honeycomb Mastopora favus pattern on surface of cluster 282
FOSSIL TIMELINE 488.3–443.7 MYA Colony about 3 in (8 cm) long Serrations on CONODONT the interlocking Long known only from their small serrated teeth were used teeth, conodonts were eel-like vertebrates, to bite and cut or close relatives of vertebrates. With large up prey. eyes, they probably hunted and ate small animals. At 16 in (40 cm) in length, Promissum, an Ordovician conodont from ORTHOGRAPTUS South Africa, was the largest known Graptolite fossils first appeared in the Cambrian, conodont. The group has a fossil record but most groups arose in the Ordovician. that extends from the Late Cambrian to Graptolites formed colonies of interlinked cuplike the Triassic. structures called thecae, each inhabited by a soft- bodied filter-feeding animal called a zooid. Some kinds of graptolites were attached to the seafloor while others floated in the surface waters. Orthograptus was a common graptolite whose colonies consisted of two parallel strips of thecae. Surface protected and strengthened by calcium carbonate EARTH FACTS IAP AN G WANA ONDWANA ETU S O CE GOND ACANTHOCHONIA The Gondwanan supercontinent The surface of Acanthochonia was made up of numerous remained separate from the diamond-shaped cells arranged in a spiral pattern. All of these landmasses that would become cells originated from a single central stem, which anchored the North America and Europe, though alga to rocks or corals. The rounded alga was only about 2 in the Iapetus Ocean had started to (5 cm) in diameter and lived in coral reefs. close. The Ordovician was a time of global cooling. Late in the Period a 283 huge ice sheet covered much of the southern hemisphere.
REFERENCE SECTION SILURIAN PERIOD AQUATIC ANIMALS Small body, about 3 in NEW LIFE (6 cm) in length A large-scale extinction event at BIRKENIA Birkenia elegans the end of the Ordovician greatly reduced the richness of animal Primitive jawless fish evolved in the Cambrian, but were life early in the Silurian. However, still thriving well into the Silurian. The small, spindle- surviving groups – including shaped Birkenia lived in European lakes and rivers. Like brachiopods, mollusks, trilobites, other jawless fish, it lacked paired fins, making it unstable and graptolites – soon recovered when swimming. This poor swimmer was unable to catch and increased in diversity in the fast prey and probably foraged in mud taking in tiny food warm, shallow continental seas of particles through its vertical slitlike mouth. Its body was the period. Entirely new aquatic covered by deep, overlapping scales arranged in rows. A invertebrates, such as primitive sea urchins, also appeared for the first time. Jawless fish still thrived while jawed fish – including armored placoderms and acanthodians or “spiny sharks” – diversified, becoming increasingly important. The very first land-living animals – arthropods including millipedes, centipedes, and scorpions – evolved from aquatic ancestors during the Silurian. row of taller defensive scales grew along the top of its back. LAND PLANTS COOKSONIA Cooksonia Best known from hemisphaerica THE PIONEERS Silurian rocks of southern Ireland, Cooksonia was the first Branching The Silurian marks the upright vascular plant. Lacking stems appearance of the first leaves and roots, it was composed of formed true land plants, cylindrical stems that branched into two Y-shapes. making it a critical time in at several points along their length. At PARKA the evolution of plants. The the ends of the stems were cap-shaped Low-growing, liverwortlike Parka was a land first land-living plants were spore-bearing structures. Compared to plant, or maybe green alga, from Silurian and mosses and liverworts that later vascular plants Cooksonia was very Devonian North America and Europe. It had grew along the edges of small, growing to just 4 in (10 cm) in a flattened, loosely branching shape and was ponds and streams. Among height, and very simple in shape. It only 2 in (4 cm) or so in diameter. A thick later Silurian forms were grew along pond and lake margins. protective covering on its outer surface may the first vascular plants. have helped prevent Parka from drying out, These contained internal suggesting that it may have grown on land hollow tubes with a woody rather than in water. lining, which helped support the plant and also carried water around its body. Because of the vessels, vascular plants were able to grow to larger sizes and farther from water than mosses and liverworts. 284
FOSSIL TIMELINE 443.7–416 MYA Each arm was free The paired legs and could be moved could have been by strands of muscle. used for walking or for handling prey. Abdomen PARACARCINOSOMA SAGENOCRINITES Sea scorpions were Silurian arthropods – Crinoids, or sea lilies, were relatives of the spiders. They had long tails and important animals of the many were equipped with large pincers, which Silurian seas. Many species they used to grab and dismember their prey. survive today in deeper waters. Paracarcinosoma was a small sea scorpion, only Attached to the seafloor by long, about 2 in (5 cm) long. It may have lived in cylindrical stalks, their tentacled brackish and fresh water as well as in the sea. heads collect plankton and The squarish head, body, and its six pairs of suspended food from the sea. limbs were encased in a hard external skeleton. Sagenocrinites was a small crinoid The last pair of limbs were paddle-shaped and from Silurian Europe and North would have been used for swimming. America. Its head and tentacles were very compact in shape, suggesting that it lived in shallower waters that were constantly churned by waves. Base of the stem attached to the seafloor Stem covered with short fine leaves EARTH FACTS BARAGWANATHIA LAURENTIA Closely resembling a club moss, Baragwanathia was a GONDWANA relatively complex Silurian Distinct rounded plant. It had branching stems During the Silurian, the southern spore capsules about 10 in (25 cm) high, which continent Gondwana was fringed by grew on its grew upward from creeping branches other landmasses. Smaller plate surface. that spread across the ground. The fragments moved northward and stems were clothed in short leaves, collided, producing new mountain Parka decipiens giving the plant a “furry” look. ranges in North America and Europe. Baragwanathia lived in the Sea levels rose as Ordovician ice southern hemisphere and melted and the climate became is known from Devonian warmer and less changeable. as well as Silurian rocks. Baragwanathia longifolia 285
REFERENCE SECTION DEVONIAN PERIOD LAND ANIMALS LIMBS ON LAND Sharp teeth suggest a diet The Devonian was one of the most of fish and other important periods of vertebrate animals. evolution. The first vertebrates with ACANTHOSTEGA four limbs and distinct digits evolved Among the earliest of from lobe-finned fish during this time, four-limbed vertebrates was and by the Late Devonian they Acanthostega from Greenland. Like its had spread widely around the world. lobe-finned fish relatives, it was a pond- Land-dwelling arthropods increased dwelling predator that still had gills and a paddlelike tail. in number throughout the period. Its limbs suggest that it would not have been good at walking Primitive, wingless insects and even on land. However, fossilized tracks show that some four- winged forms arose while spiders and footed vertebrates had ventured onto land by this time. their relatives became more diverse. AQUATIC ANIMALS Pointed fins with a prominent central DEVONIAN DIVERSITY row of bones. DIPTERUS Heavily armored jawless fish Lungfish such flourished in the Devonian seas as Dipterus were and jawed fish were by now also one of the most abundant. Among the bony fish, abundant groups of the Devonian. lobe-finned fish were numerous Five species of these lobe-finned fish and diverse while ray-finned fish survive in modern times. Dipterus swam in began to become more important. European waters and, like all lungfish, had large Several groups of trilobites were crushing teeth. Fossilized stomach contents still widespread and ammonoids show that it was preyed on by placoderms. and modern-type horseshoe crabs appeared. Their descendants survive to this day. LAND PLANTS ARCHAEOPTERIS This widespread and highly successful LEAVES AND ROOTS Late Devonian plant was one of the first to resemble modern trees. It had The Devonian Period saw the an extensive root system and its most important steps so far in the trunk had branches with reinforced development of land plants. Leaves joints at its crown. Archaeopteris was and roots evolved independently in also one of the first plants to reach a number of different groups. For great size, reaching about 65 ft (20 the first time, plants displayed m). Scientists once thought that its secondary growth – their stems woody trunk belonged to a different could not only grow in length, species and named it Callixylon. but also in diameter. These developments allowed plants to Branching, Archaeopteris grow far larger than before. The fernlike leaves early reedlike pioneers on land gave way to gigantic trees and species with complex leaves. Horsetails, seed ferns, and conifer ancestors appeared late in the Devonian, and it was these forms that would evolve into species that later made up the lush forests of the Carboniferous. 286
FOSSIL TIMELINE 416–359.2 MYA Seven toes ICHTHYOSTEGA FOSSIL on each foot Ichthyostega was an early four-footed vertebrate. It probably hunted fish and Ichthyostega other prey in shallow pools. Features of its Limbs served limbs suggest that it was relatively advanced as props for and was related to the ancestor of all later walking on land. four-footed vertebrates. Ichthyostega had a short, broad skull and very broad ribs, which helped support its body when it crawled on land. Large eye for excellent vision EASTMANOSTEUS PHACOPS Phacops Placoderms were jawed fish that This small were abundant in Devonian seas. trilobite lived in They included predators, armored warm, shallow seas. bottom-dwellers, and flattened ray- Like many arthropods, like forms. Some Late Devonian each of its body segments placoderms reached 33 ft (10 m) supported two sets of limbs. in length, making them the For protection against predators largest vertebrates yet to evolve. it could roll up its body and tuck its Eastmanosteus, known from tail beneath its head. Seven of the Australia, North America, eight groups of trilobites, including and Europe, was less than the one to which Phacops belonged, 6 ft 6 in (2 m) long but died out at the end of the Devonian. would still have been a formidable hunter. EARTH FACTS ZOSTEROPHYLLUM Clusters of EURAMERICA Lacking roots and leaves, spore-bearing Zosterophyllum was a primitive land plant. Its stems GONDWANA erect, branching stems grew not from roots, Zosterophyllum but from a complex underground rhizome llanoveranum The Devonian world was warm and (stem). The sides of the stems carried small mild. The huge continent Gondwana kidney-shaped capsules in which spores 287 lay over the South Pole while modern were produced. Reaching a height of Europe and North America were around 10 in (25 cm), the plant probably positioned close to the equator. Sea grew along the swampy edges of lakes. levels were high, and much of the land lay under shallow waters, where tropical reefs flourished. Deep ocean covered the rest of the planet.
REFERENCE SECTION CARBONIFEROUS PERIOD LAND ANIMALS ORIGIN OF THE AMNIOTES WESTLOTHIANA Sharp teeth Primitive four-footed vertebrates are well suggest Amniotes, vertebrates whose embryos known from fossils in North America and a diet of are enclosed by a watertight membrane, Europe. Westlothiana was discovered in insects. evolved in the Carboniferous. Both major Scotland, in rocks formed in a lake fed by amniote groups – reptiles and the mammal- hot, volcanic springs. Its fossils were found like synapsids – appeared at the time, while alongside those of four-legged and other more primitive land-living vertebrates snake-like tetrapods as well as diversified. At 10 ft (3 m) in length, the millipedes, scorpions, and synapsid Ophiacodon was one of the biggest spiders. Westlothiana was about known land animals of the time. Flying 12 in (30 cm) long. Its long insects evolved and together with arachnids body was carried on short legs. – spiders, scorpions, and mites – increased in size and diversity. Westlothiana lizziae AQUATIC ANIMALS DIVERSE DEPTHS SYMMORIUM Internal rodlike structures called Many of the Carboniferous sharks ceratotrichia supported the fins. Sharks and bony fish were bizarre compared to modern dominated Carboniferous forms. Some were decorated with seas, but ray-finned fish, peculiar spiky crests and spines. the actinopterygians, also Stethacanthus had a spine on its diversified greatly during this back covered with toothlike period. Trilobites were still material, while Symmorium present, but survived as only looked similar but lacked a handful of groups. Crinoids, the spine. Some brachiopods, echinoderms, scientists think that and swimming mollusks it was the female inhabited the tropical of Stethacanthus. coral reefs of the time. LAND PLANTS Pointed teeth Fossilized show that tubers and FORESTS AND FLOODPLAINS Symmorium roots of was a predator. Equisetites Lush tropical forests forming vast swamps and forested deltas were EQUISETITES widespread in the Carboniferous. Equisetites is an extinct Clubmosses and horsetails were horsetail, which came from important components of these a group that survives today forests, and some grew to immense in the form of Equisetum. sizes. Lepidodendron was a clubmoss It grew to a height of 130 ft (40 m) tall, while Calamites around 20 in (50 cm) from was a 50-ft (15-m) horsetail. underground stems (tubers), Gymnosperms – the group of and its straight stem carried plants with naked seeds that leaves arranged in regular rings includes conifers and cycads – or whorls. Equisetites dominated the river began to diversify during the banks and lake edges of the Carboniferous, Carboniferous. Toward the end of and these habitats are still favored today the period, the huge European and by modern forms of Equisetum. North American floodplains began to shrink as the climate became less wet. The clubmosses were then replaced by ferns and seed ferns from drier habitats. 288
FOSSIL TIMELINE Two eyes on a 359.2–299 MYA projecting bump GRAEOPHONUS Arachnids, the arthropod group that includes spiders, scorpions, and their relatives, are well represented in the Carboniferous fossil record and many new kinds made their first appearance at this time. Graeophonus was an early member of a group that survives to this day – the whip scorpions. These have six walking legs and a front pair of serrated pincers that they use to grab prey. They lack poisonous fangs, but have sharp jaws. Growth lines, or sutures, on shell Skeleton made Forked tail of cartilage suggests fast swimming speeds. GONIATITES This animal is a type of swimming mollusk with a coiled shell. It was a member of a group of ammonoids (ammonitelike animals) that was dominant throughout the Paleozoic. Like all ammonoids, Goniatites had gas- filled shell chambers that allowed it to float. It probably had complex eyes and beaklike mouthparts. Goniatites lived in large swarms over reefs in shallow seas. Straplike leaves EARTH FACTS CORDAITES Cordaites EURAMERICA This coniferlike angulostriatus G O N D WA N A land plant grew in Carboniferous 289 The Carboniferous is known as the mangrove swamps, “Age of Coal” because decaying but died out in vegetation from the vast forests was the Permian. It had transformed into coal. The main characteristic long, leathery landmasses present were the two leaves and its straight main trunk grew to a height huge continents of Gondwana and of up to 100 ft (30 m), although other species of Euramerica. Oxygen levels were high, Cordaites were shrublike. It produced seeds in loose and this may have allowed giant cones. The leaves, stems, and seeds of Cordaites had terrestrial arthropods to evolve. been given different names because they were originally thought to belong to different species.
REFERENCE SECTION PERMIAN PERIOD LAND ANIMALS Fragment of EDAPHOSAURUS palate with teeth This Permian synapsid had SYNAPSIDS a broad, rounded body with The most important and diverse a tall fin on the back. This land-dwelling vertebrates of fin was richly supplied with the Permian were the primitive blood vessels and so could synapsids – early members absorb or radiate heat. It of the group that includes mammals. Most were small or may have been used to control medium-sized animals equipped the animal’s body temperature. with powerful skulls and sharp Edaphosaurus had two types of teeth to deal with a diet of flesh teeth. Peglike teeth lined its jaws or insects. They thrived, along and crushing teeth were located with reptiles and arthropods, on its palate. These features including spiders and insects. suggest that it was a herbivore that fed on ferns and other AQUATIC ANIMALS tough Permian plants. DERBYIA SEAS OF LIFE Brachiopods were shelled, filter-feeding animals that evolved in the Cambrian Immense reefs, mostly built by and still survive today as lamp shells. bryozoans (tiny, colonial animals) Their larvae could swim, but and sponges, teemed with marine life adults lived a static life on the in Permian times. Shelled animals sea floor. Derbyia was a large, called brachiopods burgeoned, heavy brachiopod that grew important new fish groups evolved, to 3 in (8 cm) in diameter and some reptiles – the mesosaurs in the Carboniferous and – returned to live in water. The Permian seas. Brachiopods Permian ended with the biggest were plentiful in the Devonian, mass extinction of all time. Many but there were few left by animals died out, although some, the end of the Permian. such as fish, were not badly affected. Sword-shaped leaves LAND PLANTS Glossopteris NAKED SEEDS Apart from lacking flowering plants, Permian plant communities resembled some that thrive today. The clubmosses and horsetails that had formed the vast Carboniferous forests largely disappeared and were replaced by gymnosperms, plants that produce their seed “naked,” not enclosed in a fruit. Conifers (gymnosperms with GLOSSOPTERIS needlelike leaves) One of the most flourished and two other important Permian gymnosperm groups – gymnosperms was Glossopteris. This tree, which grew to 26 ft cycads and ginkgos – also (8 m), and its close evolved. Late in the relatives dominated Permian, many of the the southern part of the conifers developed thick supercontinent Pangea. fleshy leaves protected by Fossils of Glossopteris have an important place in scientific hairs. These features helped history. They were found across all the the plants to tolerate the southern continents, thereby providing hot and dry climate that one of the first pieces of evidence to typified Late Permian times. support the theory of continental drift. 290
FOSSIL TIMELINE Eye socket 299–251 MYA Dimetrodon loomisi skull DIMETRODON A stout skull and large, pointed teeth made Dimetrodon an awesome predator. This fin- backed synapsid grew to around 10 ft (3 m) in length. It had lightly built limbs so could run fast to catch its prey. Animals like Dimetrodon were important because they gave rise to an entirely new group of synapsids, the therapsids, which became dominant in the late Permian. PALEONISCUM Long, streamlined body Asymmetric Ray-finned fish, which evolved in suited to fast swimming tail fin the Devonian, continued to diversify during the Permian, and a major new group – the neopterygians – appeared. Paleoniscum was a primitive ray-finned fish distantly related to the neopterygians. It grew to around 8 in (20 cm) in length. Covered by a coat of overlapping scales, this spindle-shaped predator was a strong swimmer. Paleoniscum magnus Small, pointed MARIOPTERIS EARTH FACTS leaflets Found in late Carboniferous and PANG EA early Permian swamps, Mariopteris grew to a The two major landmasses of the height of around 16 ft early Permian – Euramerica in the (5 m). Its stem consisted north and Gondwana in the south – partly of old leaf bases. collided late in the period to form Some species were the supercontinent Pangea. The treelike, while others Pangean climate became hotter were climbing plants. and drier. While the south seems to have been relatively cool, tropical Mariopteris maricata conditions prevailed in the north. 291
REFERENCE SECTION PERMIAN EXTINCTION THE END OF THE PERMIAN PERIOD, 251 million years ago, saw the greatest mass extinction of all time, a period of crisis that has been called “the time of great dying.” Perhaps as few as five percent of all species survived. In the seas, reef- dwelling animals were severely affected and trilobites, sea scorpions, and key coral groups disappeared. On land, many synapsid and reptile groups vanished. Some experts think that the Permian extinction happened quickly, but it seems unlikely that a single catastrophe was responsible. It is more probable that a series of several events gradually resulted in the mass extinction. The formation of the supercontinent Pangea, for example, would have destroyed important shallow coastal seas and coastline habitats. Climate changes and volcanic eruptions undoubtedly contributed to the extinction. FIERY END A possible cause of the end-Permian extinction is volcanic activity. Huge eruptions of volcanic material are known to have happened in Siberia at this time. Around one million cubic miles of lava poured out, covering enormous areas of the land surface, and pumping vast amounts of gases and dust into the air. This volcanic dust blocked the Sun’s rays, causing the air to chill. Then, later, as dust settled, carbon dioxide trapped so much of the Sun’s heat that temperatures soared with lethal effects. CLIMATIC CRISIS Branching colonies Climate change characterized the end of the Permian. Rocks from the of bryozoans, or period indicate that cooling occurred in some areas and ice sheets built moss animals up at the poles, causing the global sea level to drop. The white ice sheets reflected sunlight back into space, lowering global temperatures Rugose corals even further. Falling sea levels may have exposed huge areas of coal on completely the seafloor. This would have released large amounts of carbon dioxide disappeared into the atmosphere, so reducing oxygen content. Less oxygen in the in the Permian atmosphere may have contributed to the extinction of animals with extinction. active lifestyles, such as the mammal-like synapsids. Brachiopods like Edriostege, grew DESERT DEVASTATION on the tops of When the Permian landmasses the reefs. collided, they produced the vast continent of Pangea. Rains and mists that arose at sea could no longer reach the interior of the land, with the result that some parts of the Permian world became drier and hotter. Deserts grew ever larger, and animals not adapted for life in the arid conditions became extinct. Cambrian 542–488.3 Ordovician 488.3–443.7 Silurian 443.7–416 Devonian 416–359.2 Carboniferous 359.2–299 Permian PALEOZOIC 542–251 MYA 292
FOSSIL TIMELINE DEATH OF A REEF The huge tubular sponge A Permian coral reef is shown healthy Heliospongia on the left-hand side of the image – and was one of the dying as it would have appeared during largest reef the Permian extinction – on the right- organisms of hand side. Permian reefs – complex the Permian. environments built by corals and sponges – were inhabited by thousands Reef animals of different animals and plants. Lowered would have died sea levels and reduced areas of shallow off as oxygen seafloor that resulted from the formation levels in the sea of Pangea destroyed the areas in which dropped. reefs could grow. The reefs were gradually killed off, causing an Gastropods enormous drop in the diversity grazed on algae of marine life. Reduced quantities that grew on of oxygen in the Permian atmosphere the reefs. would also have meant that sea water contained less oxygen than before. Entire oceans would have slowly become stagnant, suffocating the life they contained. 299–251 Triassic 251–199.6 Jurassic 199.6–145.5 Cretaceous 145.5–65.5 Paleogene 65.5–23 Neogene 23–present MESOZOIC 251–65.5 MYA CENOZOIC 65.5 MYA–present 293
REFERENCE SECTION TRIASSIC PERIOD LAND ANIMALS CYNOGNATHUS This aggressive Early Triassic THE AGE OF REPTILES predator was a member of the cynodonts – a group of advanced The Triassic was the start of the synapsids. Growing to 6 ft 6 in “Age of Reptiles” and a time when (2 m) in length, it possessed animals from very different lineages prominent canine teeth, shearing lived alongside one another. Synapsid cheek teeth, and large jaw muscles relatives of the mammals, were still that allowed it to hunt other large important but were gradually being synapsids. It may have been warm- driven to extinction. Archosaurs, the blooded and fur-covered like a “ruling reptiles,” became important and mammal, and its close relatives the first crocodylomorphs, pterosaurs, were themselves the ancestors and dinosaurs all arose late in the of mammals. Triassic. The first turtles, frogs, and mammal-like animals also appeared. AQUATIC ANIMALS Diamond-shaped DICELLOPYGE scales covered the Primitive ray-finned fish like SEA CHANGE streamlined body. Dicellopyge were important predators in the Triassic. This Modern-type corals formed small freshwater fish from southern the Triassic reefs and new Africa had a deeply notched tail groups of ammonoid suggesting that it was a fast swimmer. molluscs appeared. Its deep skull and jaws were armed Advanced ray-finned fish with conical teeth that allowed it to and the first modern sharks catch and eat smaller fish and and rays replaced slower, swimming invertebrates. older kinds of marine life. Primitive ichthyosaurs evolved and soon became dolphin- shaped predators. Some were as big as modern whales. LAND PLANTS DOMINANT CONIFERS Leaf fossilized in mudstone Most plants of the earlier Ginkgo Paleozoic Era reproduced by biloba spores and so relied on moist leaf habitats to reproduce. These groups – including the famous GINKGO glossopterids of the southern This tree, which evolved in the continents – suffered in the Triassic, survives essentially unchanged drier conditions of the Triassic, to this day. A native of China, it has when the landscape was been transported around the world for increasingly dominated by planting in urban parks and gardens, evergreen trees (conifers and partly because it grows well in heavily other gymnosperms). Among polluted air. Ginkgo grows to around 115 ft the more modern kinds of (35 m) in height. It is deciduous, and there plants, cycads became well are abundant fossils of its leaves. Members established. Ginkgos – relatives of the Ginkgo genus typically grew in damp of the conifers – became more habitats in temperate climates. successful and at least seven genera, including the modern genus Ginkgo, lived during Triassic times. 294
Enlarged, stabbing FOSSIL TIMELINE canine teeth 251–199.6 MYA MEGAZOSTRODON One of the earliest of all mammaliaforms, Megazostrodon, was only about 5 in (12 cm) long and lived in Africa, although similar forms inhabited Europe and Asia. Like other early mammals, it was probably nocturnal and resembled a modern shrew in appearance and lifestyle. Its teeth and jawbones suggest that it hunted insects and other small animals and hid in burrows. As in later mammals, some of its jawbones formed parts of its ear. Deep jaws could Small, hold struggling prey. delicate bones Limbs were NEUSTICOSAURUS used as The sauropterygians were a group of marine reptiles paddles that evolved during the Triassic. The most famous when in members of the group were the plesiosaurs. the water. Neusticosaurus was a small, amphibious, predatory sauropterygian that lived in the shallow seas of Triassic Europe,. where it hunted invertebrates and fish. Fossils of babies preserved without eggshells suggest that these reptiles may have given birth to live young. Thick, heavy ribs helped keep EARTH FACTS Neusticosaurus submerged. PACHYPTERIS TETHYS Seed ferns (pteridosperms), OCEAN such as Pachypteris, were not ferns at all, but primitive PANGEA seed plants that lived in swampy areas. They had Typical height The vast supercontinent of Pangea woody stems studded 6 ft 6 in (2 m) straddled the equator during the with dried-out leaf bases. Triassic. Hints that it would later split Their tops had fernlike 295 up came from the narrow seaway that fronds that carried the separated North America from seeds. The group was Europe and another tongue of sea, very successful late in the Tethys, which encroached on the Paleozoic, but gradually Europe. The climate was generally declined in importance during the warm and dry. Mesozoic, finally becoming extinct in the Cretaceous. Pachypteris, which grew worldwide in tropical forests, was one of the last seed ferns to die out in the Cretaceous.
REFERENCE SECTION JURASSIC PERIOD LAND ANIMALS PTERODACTYLUS Pterosaurs were reptiles that RULE OF THE DINOSAURS developed wings and powered flight. They evolved in the Triassic Life on land took on a very different and diversified during the Jurassic, appearance in the Jurassic. Most when shorter-tailed, longer-winged crocodile-group archosaurs and forms appeared. Pterodactylus was a synapsids were wiped out at the small Jurassic pterosaur that lived end of the Triassic (but some small along shorelines, feeding on fish and synapsids, including early mammals, crustaceans. Just 12 in (30 cm) long, it survived). Giant dinosaurs took over as had a lightly built skeleton and thin, the dominant animals. On land, plated hollow bones. Its wing membrane was stegosaurs and long-necked sauropods were supported by the long fourth finger. preyed upon by large theropods. In the air, new kinds of pterosaurs were seen and the first This specimen preserves its wing birds evolved from small predatory dinosaurs. membranes, throat pouch, and foot webbing. AQUATIC ANIMALS Slender clawed toes JURASSIC DIVERSITY ICHTHYOSAURUS Advanced ichthyosaurs (“fish lizards”), swimming crocodylomorphs, and Important marine animals that appeared in new kinds of plesiosaurs all added to the diversity of marine reptiles in the Jurassic include ammonites, belemnites, the Jurassic. Ichthyosaurus was a European fish lizard. It had large, sensitive and modern-type sharks and rays. Teleosts – bony fish – diversified greatly into long- eyes and a slim snout containing many cone-shaped teeth, both bodied predatory forms as well as gigantic of which helped it catch fish and swimming mollusks. There filter-feeders, which may have been the were several species of Ichthyosaurus ranging in size biggest fish of all time. Crinoids, from 3–10 ft (1–3 m) long. relatives of today’s sea lilies, were important during the period. Giant forms, some with stalks as long as 50 ft (15 m), grew from floating driftwood. LAND PLANTS THE AGE OF CYCADS Ring of bony plates Nostril was located close to Cycads, conifers, and ginkgos were Cycas fronds the eye socket. important Jurassic plant made of many groups. So many cycads parallel leaflets CYCAS grew in the forests of the Cycads are gymnosperms that reached time that the Jurassic is their greatest diversity and abundance often called “the Age of during the Jurassic. Cycas, of which Cycads.” One group of cycadlike plants – the about forty species survive today, first bennettitaleans – were appeared in the Jurassic. Some significant because they may species grew into giant tree- have been the ancestors of flowering like forms, while others were plants. Jurassic conifers included smaller and more fernlike. close relatives of living pines, yews, Jurassic cycads lived redwoods, and cypresses. Ferns worldwide, but today’s formed much of the ground cover, species are restricted to and all three major fern groups had the tropical and probably evolved by the Jurassic. Club subtropical zones. mosses, horsetails, and seed ferns continued to survive but were not Many cycad as important as in earlier times. leaves were poisonous. 296
Serrated teeth in FOSSIL TIMELINE lightly built skull 196.6–145.5 MYA Long hind limbs COMPSOGNATHUS ARCHAEOPTERYX with four toes The chicken-sized Perhaps the first true bird, Compsognathus was an Archaeopteryx evolved in the advanced predatory dinosaur Jurassic from small theropod from Late Jurassic Europe. ancestors. It retains many It walked on its two long, slim features of its reptile past – hind limbs and probably stalked three working fingers, true its prey of small lizards teeth, and a breastbone and insects. It was similar without a keel. This famous fossil – known as the Berlin in size, habit, and specimen – was discovered distribution to the bird in the Solnhofen limestones Archaeopteryx, and the of Germany in 1877. two may have lived side- Remarkably, it preserves by-side in the woodlands complete impressions of the of southern Germany. wing and tail feathers. ASPIDORHYNCHUS Triangular skull Among the new Jurassic teleosts were long- with forward- facing eyes bodied predators, such as Aspidorhynchus. Growing to around 20 in (50 cm) in length, it had a pointed skull, large eyes, and sharp teeth. Its upper jaw was extended beyond the lower jaw, forming a prominent toothless “beak.” Its tail was symmetrical, and thick rectangular scales protected its body. Aspidorhynchus was widely distributed in shallow, subtropical seas. EARTH FACTS WILLIAMSONIA TETHYSPAN This plant, which lived OCEAN Flowers probably throughout the GEA pollinated by insects Mesozoic, was a member of the Bennettitales – a The Pangean supercontinent split as group that may include the Atlantic opened up between the the ancestors of flowering areas that today form Africa and plants. It had a robust stem North America. The land that would that was covered in diamond- later become Antarctica, India, and shaped scales and grew large Australia started to move away from flowerlike structures. The the rest of Pangea. The climate was Bennettitales had leaves that warm, and, with no polar ice caps, sea resembled those of cycads, levels were high worldwide. and were probably related to this group. Williamsonia 297
REFERENCE SECTION CRETACEOUS PERIOD LAND ANIMALS Zalambdalestes Stiff tail helped balance probably fed the heavy body. NEW DINOSAURS on insects Dinosaurs remained the dominant land ZALAMBDALESTES animals during the Cretaceous. New Early Cretaceous mammals were small and groups, including tyrannosaurs, duck- insignificant, just as they had been throughout billed hadrosaurs, and horned the Triassic and Jurassic, but an important new dinosaurs, spread across the northern group emerged later in the Period. Characterized by continents, while birds evolved from three-cusped teeth, this group included Zalambdalestes, toothed Archaeopteryx-like species into which probably resembled a modern elephant shrew. forms that resembled modern kinds. Snakes developed from lizard ancestors and new groups of insects, including moths, ants, and bees, appeared feeding on and pollinating the newly evolved flowering plants. AQUATIC ANIMALS MESOZOIC MARINE REVOLUTION MACROPOMA Mobile skull Coelacanths – fleshy-finned fish that first appeared joint Marine invertebrates took on a distinctly in the Devonian – grew to sizes of up to 10 ft (3 m) in allowed modern look during the Cretaceous. Crabs Cretaceous seas, though the group was in decline later in jaws to and other modern crustaceans appeared, the Period. Macropoma was a European coelacanth, less open wide as did predatory gastropod mollusks and than 24 in (60 cm) long. It had a short, deep body and burrowing sea urchins. Among fish, the large fins that would have aided maneuverability. Its tail teleosts (advanced bony fish) underwent a had three lobes – a feature common to all coelacanths. massive increase in diversity and relatives of modern herrings, eels, carps, and perches all appeared. Swimming lizards called mosasaurs and the first marine turtles and aquatic birds also evolved. LAND PLANTS FLOWER POWER BETULITES Betulites is an extinct member of the birch family, and a close Cretaceous forests were largely dominated by several groups of relative of Betula, the familiar modern birch tree. Like gymnosperms, particularly living birches, it grew in temperate climates, conifers, including cypresses, favoring lakesides and other damp bald cypresses, and monkey- habitats. Betulites had round or oval puzzle trees. Other leaves that had teeth along their gymnosperm groups, such as margins – typical of plants that grow cycads and ginkgos, declined in cool or dry environments. in importance. Flowering Because fossilized Betulites leaves plants – the angiosperms – are frequently detached from arose, first as small, weedlike twigs, this tree was probably forms in areas of land deciduous. disturbed and trampled by herds of dinosaurs. Later in the Leaf fossil period, flowering plants, in ironstone including birches, willows, nodule and magnolias, formed forests in which the dinosaurs lived, Betulites but ferns remained important wherever rainfall was high. 298
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