Prehistoric animals refer to any animal that existed more than 5,500 years before, before human beings began recording their history in writing. Some prehistoric animals greatly resembled their present day relatives. Many ancient creatures, however, were unlike anything alive today. Dinosaurs rank among the best known prehistoric animals. The largest dinosaurs may grow forty meters in length. Other unusual creatures included flying reptiles, sea scorpions 2.4 meters long, and hairy relatives of elephants called mammoths. Not all of these animals lived at the same time.
The long history of animals is recorded by fossils. Fossils are the remains of prehistoric life. They include the tracks, shells and bones of animals as well as petrified wood and impressions of leaves. Fossils help scientists called paleontologists learn what prehistoric animals looked like and how, where and why they lived. Paleontologists use fossils to compare prehistoric animals with living ones. Such comparisons help people understand how ancient and modern animals are related.
The oldest known animal remains date from about 600 million years before. However, some paleontologists think that simple, tiny animals may have existed much earlier, perhaps as early as one billion years before.
Most animals have developed during a time in earth’s history that scientists call the Phanerozoic Eon, which means “time of visible animal life.” It began about 544 million years before and continues to the present day. Scientists divide the Phanerozoic Eon into three eras: 1) the Paleozoic Era, which lasted from about 544 million to 248 million years before; 2) the Mesozoic Era, which lasted from 248 million to 65 million years before; and 3) the Cenozoic Era, which began 65 million years before and continues today. Throughout each era, climates kept changing, continents drifted and changed in outline, mountains rose up and were leveled, and sea levels rose and fell. These changes helped determine which living things survived and which died out.
Each of the three eras is divided into shorter intervals called periods. Periods, in turn, can be divided into still shorter intervals called epochs. Different layers of rock with different kinds of fossils formed during each period. Such rocks help scientists understand how earth’s surface and climates have changed. The fossils tell paleontologists what animals and plants lived during each period.
Early Forms of Animal Life
Animal life evolved and flourished only after plants became available as food. Like animals today, prehistoric animals depended on green plants. Green plants can use energy from sunlight to make food. Animals cannot do so. They must eat either plants or plant-eating animals.
The earliest animals lived in the seas and were probably tiny. Although their bodies were small, they were made up of many cells. Some groups of those cells became adapted for feeding or reproduction, while others helped the animals move and detect changes in their environment.
The first animals are known from fossils preserved in rocks about 600 million to 570 million years old. They include jellyfishes, worms, and leaf like creatures called sea pens. All these creatures were invertebrates, i.e. they had no backbones. Their soft bodies also lacked shells or other hard parts. As a result, few of them were preserved as fossils.
According to fossils found by scientists, animals first become abundant at the beginning of the Paleozoic era. A tremendous variety of invertebrate animals suddenly developed in the oceans during this time. Unlike earlier animals, many of these creatures had hard shells or tough outer frames to protect them from enemies.
Trilobites ranked among the most common early Paleozoic animals. They were ancient arthropods, the group to which spiders and crabs belong. Most trilobites fed on small food particles from the sand or mud on which they lived. Such food may have included small worms and other bottom-dwelling animals. Another common group of Paleozoic animals, the brachiopods, had shells similar to those of claims. They lived on the sea floor or burrowed in the mud. Brachiopods ate tiny organisms. They fed by opening their shells and filtering food from the water with a comb like organ.
The first animals with backbones, called vertebrates, probably appeared at the end of the Cambrian period, more than 500 million years before. The earliest vertebrates were small. Plates of bone usually protected their heads and much of their bodies. In later vertebrates, a skeleton made up of many bones formed inside the body. The skeleton provided a solid internal frame to which muscles could attach.
The earliest vertebrates had mouths without jaws. They most likely fed on small bits of dead animals or tiny creatures on the sea floor or in the water. In the Silurian period, beginning 440 millions years before, vertebrates developed bony jaws and teeth. These mouth parts enabled them to catch and consume larger kinds of food. The vertebrates also developed movable paired fins, enabling them to become more active swimmers.
The Devonian period, sometimes called as the age of fishes, took place about 410 million years before. One of the largest Devonian fishes, Dunkleosteus, grew 23 feet long. Massive plates of bone protected Dunkleosteus head and the front of its body. Its jaw bones formed sharp edges for cutting up and eating smaller fish.
Body fines of the Devonian period had two types of fins, lobed and rayed. Rayed fins consist of a web of skin supported by a skeleton of rods called rays. Fish with this type of fin, known as ray-finned fishes, are fast swimmers. Lobed fins consist of a fleshy stalk fringed with rays. The Devonian period was the time of greatest abundance for fishes with this type of fin, called lobe-finned fishes. Lobed fins enabled the creatures to crawl along the bottom of the sea or over land. Many lobe-finned fishes also had lungs with which they gulped air when there was not enough oxygen in the water. All land living vertebrates, including human beings, descended from these fishes.
The move onto land, animals first established themselves on dry land by the beginning f the Devonian period. The oldest known land animals were arthropods, including scorpions, spiders, and mites. Vertebrates probably first moved onto land near the end of the Devonian Period. The sturdy fins of lobe-finned fishes evolved into the legs of land-living vertebrates, with angles, wrists, fingers, and toes suitable for leaving the water. Lungs became full-time suppliers of oxygen.
Four-footed land vertebrates are known as tetrapods. The first tetrapods, called amphibians, had to return to the water to lay eggs. These eggs were enclosed in a jellylike substance that, if left on land, would dry out and kill the eggs. Thus amphibian eggs developed and hatched in water, where the young lived until they changed into land-dwelling, air breathing adults. During the late Paleozoic era, a great variety of amphibians inhabited the shores of lakes and rivers, dividing their lives between land and water. Modern amphibians, including frogs and toads, are small, but some Paleozoic amphibians were large. The meat-eating Eryops grow more than 1.5 meters long and had a big head. Others, such as Ophiderpeton, were small and snakelike.
The earliest insects were wingless and appeared during the Devonian period. By the late carboniferous period, about 300 million years before, many insects had developed wings. They included meganeura, a spectacular animal resembling a dragonfly. It had a wingspan of up to 65 centimeters.
The first reptiles evolved from amphibians during the carboniferous period. Reptiles had an important advantage over earlier land living vertebrates: they could lay their eggs on land. Reptile eggs were protected by a hard leathery shell that prevented them from drying out. Special membranes inside the eggs enclosed and protected the developing young in a fluid filled chamber. This type of egg enabled reptiles to live entirely on land.
The Age Of Reptiles
During the Permian period at the end of the Paleozoic era, earth’s climate generally grew warmer and drier. Deserts spread over large areas, displacing many amphibians that needed water for their survival. At the end of the period, about 248 million years before, the seas shrunk as sea level helped cause one of earth’s largest known extinctions. Ninety-five percent of species may have perished in the seas at that time. But reptiles survived the new conditions on land and sea. Thus these animals came to dominate the land, sea, and air throughout most of the Mesozoic era, which lasted from about 248 million years before to about 65 million years before. This era often called the age of reptiles.
Dinosaurs ranked as the dominant reptiles of the Mesozoic. They varied greatly in size, including some of the largest animals that ever lived. One of the largest, the plant-eating Seismosaurus, may have measured about 40 meters in length. The smallest fully grown dinosaurs grew to about one meter in length. Many large dinosaurs were probably slow moving. But scientists believe some dinosaurs, such as ornithomimus, could run fairly fast. While dinosaurs ruled the land, many kinds of large reptiles inhabited the sea and air. Sea-dwelling reptiles included ichthyosaurs, which resembled dolphins in shape and probably could swim fast. Mosasaurs were enormous sea lizards that lived during the late cretaceous period, which ended about 65 million years before. They grew up to 9 meters long. Plesiosaurs had four large flippers and a short tail. Many plesiosaurs also had an extremely long neck.
Flying reptiles called pterosaurs became the first vertebrates to evolve flapping flight, the kind of flight later used by most birds. Each of a pterosaur’s wings consisted of a membrane of skin supported by a long finger made of hollow bones. Pterosaurs had no feathers, but hair like structures probably covered their bodies. Some pterosaurs ranked as the largest flying animals of all time, reaching a wingspan of 12 meters.
Invertebrates flourished during the age of reptiles, and many new kinds appeared. Numerous species of mollusks lived in the seas. They included clams, snails, squids, and ammonites, which formed spiral-shaped shells. Crustaceans, including lobsters, crabs, and shrimp, became common in the oceans during the cretaceous period. All major present day groups of insects had evolved by cretaceous times.
Fish remained abundant in the Mesozoic era. The first modern kinds of bony fish, called Teleosteans, evolved during the Mesozoic era, which began about 240 million years before.
Many of the larger amphibians had died out by the end of the Triassic period. But some kinds persisted well into the cretaceous period. Smaller amphibians, including frogs and salamanders, first appeared at the beginning of the Jurassic period, around 213 million years before.
Birds evolved from small meat-eating dinosaurs during the Jurassic period. The oldest known bird, archaeopteryx, lived around 145 million years before. It grew to about the size of a crow. Archaeopteryx was covered with feathers, but it resembled reptiles in many features. It had teeth, a long tail, and three grasping, clawed fingers on each wing.
The first modern birds appeared in the cretaceous period. They developed a toothless beak covered by hornlike material. The tail became short, and the fingers fused into a single structure supporting the wing.
Mammals evolved from mammal like reptiles near the close of the Triassic period. The first mammals were small, about 15 centimeters long, and probably ate insects and other small animals. These mammals likely had hair covering their bodies. Like their modern descendants, ancient mammals suckled their young from special milk glands. Most Mesozoic mammals probably laid eggs as their reptile ancestors had done. Today, the only egg-laying mammals, called monotremes, are the duck-billed platypus and the echidnas of Australia and New Guinea.
The two major groups of mammals, marsupials and placentals, first developed in the cretaceous period. Both these groups give birth to live young. Marsupial young are born in an underdeveloped state. The young crawl into a special pouch on the mother’s belly, where they continue to develop and grow. Most early marsupials resembled modern opossums.
Placentals give birth to fairly well-developed young. These young grow inside the mother’s body, where they receive nourishment from a special organ called the placenta. Many early placentals resembled shrews.
The Age of Mammals
By the end of the Mesozoic era, some 65 million years before, dinosaurs and other dominant reptiles had become extinct. Scientists have found evidence that a huge asteroid hit earth about 65 million years before. Dust from the impact of this asteroid, as well as soot from the wildfires caused by the impact, would have blocked sunlight from reaching earth’s surface. Lack of sunlight would have lowered temperatures worldwide and killed many of the plants and animals that dinosaurs ate.
The Cenozoic era, also called the age of mammals, followed the Mesozoic era and continues today. During this new era the climate grew gradually cooler and drier, and mammals became the dominant animals on earth. All mammals have warm-blooded bodies covered with hair and are equipped with a variety of teeth for chewing food. Warm bloodedness, which keeps the mammals’ bodies at constant high temperatures, probably enabled early mammals to adapt more easily than other vertebrates to the cooler, drier climates.
The development of placentals – most modern placentals probably evolved at the beginning of the Cenozoic era. These creatures had small bodies. For example, Miacis, a forerunner of all meat-eating mammals, grew about as large as a weasel. The ancient horse Hyracotherium, also called Eohippus, stood no taller than a dog. Primates started out as small creatures but later evolved into monkeys, apes, and human beings. One of the earliest whales, pakicetus, grew only about 1.8 meters long. Its descendants, however, became some of the world’s largest animals.
By the middle of the Cenozoic era, many forests had disappeared. Grasslands spread across many parts of the world, and climates grew cooler and drier. Such hoofed mammals as deer and horses developed teeth suitable for feeding on grass. Many hoofed mammals also had long, slender legs, enabling them to run fast to escape from predators in open country.
The great increase in plant-eating mammals provided abundant prey for meat-eating mammals. These meat-eaters included the earliest dogs and the saber-toothed cats, large, lion like predators that could kill their prey with large fanglike teeth. In Africa and Asia, the first apes evolved. Many new kinds of rodents appeared, and they quickly became the most common small mammals.
The distribution of mammals – about 250 million years before, all the continents drifted together to form one huge land mass, the super continent Pangaea. About 200 million years before, Pangaea began to break up again into separate continents, which slowly moved to their present positions.
Placental and marsupial mammals may have developed on the northern continents. When Australia separated from South America and Antarctica at the beginning of the Cenozoic, many marsupials and probably only a few placentals had spread there. With few placentals to compete with for food and nesting sites, many kinds of marsupials evolved only in Australia. Some marsupials resembled placental mammals in appearance and habits. They included diprotodon, which resembled a hippopotamus, and the doglike Tasmanian wolf. Procoptodon and other early kangaroos developed a plant-eating way of life similar to that of hoofed placentals.
Both marsupials and placental mammals spread to South America during the cretaceous period. But at the end of that period, South America became separated from the rest of the world for about 60 million years. As a result, many unique kinds of mammals developed on that continent. Prehistoric South American marsupials included the thylacosmilus, a predator that looked like a saber-toothed cat, and Borhyaena, which resembled a wolf. Placental mammals in South America included armadillos and huge ground sloths.
About 3 million years before, a land connection formed between north and South America. Many North American mammals moved to South America, causing the extinction of numerous South American marsupials and placentals. But some South American mammals migrated in the opposite direction and settled on North America. These included the opossum, a marsupial, and the porcupine, a placental.
The development of other animals – all major groups of modern amphibians and reptiles had evolved by the cretaceous period and survived the great extinction 65 million years before. Among reptiles, lizards and snakes became common. Many birds developed during the Cenozoic era. They included the flightless diatryma, which grew about 2 meters tall.
The Ice Ages
A few million years before, the climate grew colder on the northern continents. This led to a series of ice ages during the northern continents. This led to a series of ice ages during the Pleistocene epoch, which occurred about 2 million to 11,500 years before. During these ice ages, glaciers repeatedly spread the retreated over large areas of North America, Europe, and Asia.
One of the most prominent mammals of this period, the elephant like mammoth, roamed the frozen plains of the Northern Hemisphere. Its thick coat of dense, long hair helped keep it warm.
Many large placental mammals died out before the last glaciers retreated about 11,500 years before. In North America, these included giant ground sloths, saber-toothed cats, and mammoths. Some scientists believe many large mammals were hunted to extinction by prehistoric people, but others argue that the animals disappeared because of changes in climate and plant life.
Prehistoric human beings lived only a short part of earth’s long history. Most scientists now think that prehistoric people evolved from human like apes between about 5 million to 10 million years before.