Prehistoric animals

The fossil record indicates that eubacteria evolved as long as 3,800 million years ago, whereas a comparison of ribosomal ribonucleic acid (RNA) suggests that cells with nuclei probably diverged from archaebacteria as long as 2,800 million years ago. Although cells with respiratory and photosynthetic organelles (mitochondria and chloroplasts) evolved much later, about 1,500 million years ago, no trace of animals has been found in the fossil record before 1,000 million years ago. The fossil record of microscopic animals is nearly nonexistent between 1,000 and 600 million years ago. All knowledge of prehistoric animals comes from the fossil record that formed during the last 600 million years.

Near the end of the Precambrian period (540 million years ago), a few sponges, jellyfish, colonial filter feeders, wormlike animals, early mollusks, and primitive arthropods are found in the fossil record. One or more minor extinction events that eliminated up to 50 percent of all species brought the Precambrian period to an end. Animals whose body plan was based on segmentation quickly diversified into a myriad of forms.

How Did So Many Animals Develop So Quickly?

The Cambrian period (540 to 500 million years ago) is characterized by the “sudden” appearance (in 1 to 10 million years) of a large number of morphologically different animals. One of the most innovative body plans that evolved near the end of the Precambrian period was that associated with the many wormlike animals that developed segmented bodies. Segmentation allowed for rapid evolution of all sorts of animals simply by changing the number of segments, their size and shape, as well as the associated appendages. Evolution of homeotic genes and the genes they regulate altered not only segment number and segment characteristics but also appendages. For example, legs could be easily changed into preening, mating, and sensory appendages. During the Cambrian period, all kinds of marine worms, caterpillar-like animals, arthropods, and chordates made their appearance. Some of these caterpillar-like animals had broad legs (Aysheaia) and resembled modern velvet worms, whereas others had long, sharp spikes for legs (Hallucigenia) and were unrelated to any other known animals.

Many of the early arthropods vaguely resemble modern sow bugs, shrimp, centipedes, and horseshoe crabs. A very successful group of marine arthropods that superficially resemble sow bugs were the trilobites. A trilobite had a prominent, centrally located, segmented, concave, dorsal ridge running the full length of its body. The trilobites differentiated and evolved for over 300 million years until the few remaining forms went extinct during the Permian-Triassic mass extinction (250 million years ago) that wiped out nearly 90 percent of all species. A trilobite had a head, thorax-abdomen, and tail (pygidium). The segments of the head were fused, but the thoracic-abdominal and tail segments were varied. Depending upon the trilobite, thoracic-abdominal segments ranged from nine (low of six) to thirteen (high of twenty-two) in number, whereas tail segments varied from six to sixteen (high of twenty-two). Legs and other specialized appendages protruded ventrally from the segments. Gills were also associated with a number of the segments. Although some of the first trilobites had “eyes,” their compound nature is not clear because of their size. Most of the later trilobites developed compound eyes that closely resemble the compound eyes seen in extant insects.

One of the most unusual arthropod-like creatures of the Cambrian period was the enormous predator known as Anomalocaris, which ranged up to three feet in length. It had a long, oval, dorsoventrally flattened head with compound eyes on short stalks near the back of the head. Protruding from the ventral region at the front of the head were two grasping feeding appendages that, for many years, were mistaken for separate shrimplike organisms. Just behind the feeding appendages was a circular mouth resembling a cored pineapple slice. This mouth had been misinterpreted as a jellyfish by early discoverers. Saw-shaped “teeth” lined the inside of the mouth and most likely contracted around animals that were positioned in the mouth by the feeding appendages. A number of fossil trilobites have been found with missing chunks that appear to have been torn away by Anamalocaris. No modern arthropods have any sort of mouth like that of Anamalocaris. This creature was a specialized swimmer, propelled by winglike flaps that appear to consist of numerous overlapping flaps originating from most of the body segments.

The Evolution of Fish

The oldest fossils of jawless fish, referred to as the Agnatha, are about 470 million years old. Modern jawless lampreys and hagfish are most closely related to the ancient Agnatha of the Ordovician period (500 to 440 million years ago). These prehistoric Agnatha attached to larger animals through their whorls of “teeth” and rasped the flesh of the host. Lamprey teeth are horny, sharp structures devoid of calcium, derived from the skin. Because the teeth contained enamel-like proteins, they are considered precursors to modern teeth, which consist of enamel over dentine.

The more advanced Agnatha that developed in the Silurian and Devonian periods did not survive the Devonian-Carboniferous mass extinction. Yet they are important because they suggest when the evolution of mineralized teeth, bone, body armor, eyes, and paired limbs occurred. The jawed fish first evolved in the Devonian period (410 to 360 million years ago) but did not dominate until after the Devonian-Carboniferous mass extinction. The earliest jawed fishes were sharks. The discovery of fossilized shark teeth dated to the Devonian period suggests that they evolved during this period. The earliest fossil skeletons of sharks discovered to date, however, are from the Carboniferous period (360 to 290 million years ago).

The First Terrestrial Vertebrates

The first land-dwelling vertebrates were amphibians that evolved from freshwater, lobe-finned fishes closely related to Eusthenopteron, a crossopterygian that lived midway through the Devonian period (410 to 360 million years ago). Most amphibians are tetrapods that begin life in a watery environment but turn to terrestrial life as adults. Extant amphibians include frogs, toads, salamanders, and the legless, snakelike caecilians.

An unusual prehistoric amphibian was Ichthyostega, a three-foot-long crocodile-like animal that lived somewhere between 350 and 370 million years ago. Its jaws were lined with two rows of teeth. Along the front was a row of large teeth, and behind them was a row of densely packed, small, sharp teeth bounded by a couple of very large canine teeth. The skull was similar in some respects to the lobe-finned fish that lived during the Devonian. Its neck was only two to three vertebrae long and the pectoral bones were attached to the skull and backbone by muscle.

External gills were a prominent feature of the juvenile or larva. The adult, however, lost the external gills when it underwent metamorphosis and developed primitive lungs. The aquatic juvenile amphibians depended on gills to breathe, but when they metamorphosed into terrestrial adults, they developed primitive lungs out of part of the swim-bladder. Air was swallowed into the intestine-swim bladder tract. A high concentration of capillaries in the wall of these simple lungs carries oxygen to the heart.

The pelvic bones were attached to the spinal cord through muscles. Ichthyostega had a heavy rib cage to support its weight out of water, in contrast to other contemporaneous vertebrates. In general, the early amphibians had very short leg bones compared to the primitive reptiles that evolved from them. Ichthyostega had seven digits on both its front and back feet, whereas Acanthostega had eight digits. Other ancient amphibians had variations in the number of digits. Living amphibians generally have four to five digits on the front feet and five digits on the back feet. Ichthyostega‘s diet was mainly fish, but on land, it may have consumed arthropods and smaller amphibians.

Mastodonsaurus giganteus, another crocodile-like animal, lived 230 million years ago, during the early Triassic period (250 to 205 million years ago). It was the largest amphibian that ever existed, growing to nearly thirteen feet in length. Like Ichthyostega, two rows of pointed teeth lined the jaws of this amphibian, making it a formidable match for the early reptiles.

A small group of amphibians lost their legs and took on the appearance of snakelike animals. These legless amphibians are highly segmented, making them look more like giant earthworms than snakes. One of the largest extant caecilians is Caecilia thompsoni, which lives in Colombia and grows to a length of five feet.

Reptile Diversity

The first reptiles evolved from amphibians during the early Carboniferous period (360 to 290 million years ago). Early reptile skeletons can be distinguished from amphibian remains by the skull and jaw characteristics, the number of neck vertebrae, the absence of external gills, the length of the leg bones, and the number of foot and toe bones.

Prehistoric amphibians had dorsoventrally flattened skulls and skull bones similar to their crossopterygian fish relatives. Amphibians had no nasal holes in their skulls because they breathed through their gills or through their mouths. Reptiles, however, had anterior nasal openings in the snout. The diapsid reptiles had two additional holes behind the eye openings for muscle attachment, whereas synapsid reptiles had one hole, and anapsid reptiles had none. The early amphibians had necks with two to three vertebrae, whereas reptiles generally had necks with more than six vertebrae. The early amphibians generally had very short front and rear leg bones and feet with six to eight digits, whereas early reptiles had front and rear feet with only five digits. Later amphibians reduced the number of digits to four in the front and five in the rear, similar to what is found in extant frogs. Early amphibians had digits that varied considerably, whereas early reptiles usually had digits with two, three, four, five, and three bones (moving from thumb to little finger).

The evolution of reptiles required the development of a new type of egg. The new egg provided a watery environment in which the embryo and juvenile could develop. An amniotic sac evolved to contain the developing reptile and its watery environment, the yolk sac developed to provide the nutrients needed for growth, and the allantoic sac was acquired to help eliminate wastes. The evolution of a tough, membranous egg covering allowed oxygen and carbon dioxide exchange but prevented water loss. Fish and amphibians must lay their eggs in water because they never evolved the adaptations needed to lay their eggs on land. Reptiles, dinosaurs, birds, and mammals are all amniotes and can lay their eggs on land because their eggs have amniotic sacs, yolk sacs, or allantoic sacs.

Although amphibians’ skin was protected by thick layers of secreted mucus, it was always in danger of drying or burning under the sun if the animal strayed too far from water. Reptiles and dinosaurs evolved thick scales to prevent drying and burning, whereas birds and mammals evolved feathers and hair to protect their skin.

Flying Reptiles

The oldest flying reptiles are dated to the late Triassic period (250 to 205 million years ago). The first of these pterosaurs (flying lizards) had long, thin tails, often more than half the length of their bodies. The wings consisted of a leathery skin that was supported along its anterior margin by the pterosaurs’ front legs and a highly elongated fourth digit. Digits one, two, and three were tipped with claws. The wings were attached to the body and hind legs, and the wingspan ranged from one to six feet. Their elongated jaws were lined with numerous sharp teeth that they used to grab surface fish, like some present-day birds. In the late Jurassic and Cretaceous period, pterosaurs lost their long tails. Some became quite large. Pteranodon had a wingspan of up to twenty-five feet, whereas the largest Quetzalcoatlus had a wingspan of nearly forty feet. Toothless Pteranodon fished on the move, scooping up surface fish, whereas Quetzalcoatlus hunted much like modern vultures, but swooping down on a dead dinosaur instead of the carcass of a wildebeest. Both these large flying reptiles were consummate soaring reptiles, able to take advantage of the slightest updraft of air because of their long, narrow wings.

Reptiles Returned to the Sea

Some reptiles became adapted to living most of their lives in the sea. Nothosaurus lived near the beginning of the Triassic period (250 to 205 million years ago) and grew up to ten feet long. Its neck and tail each accounted for a third of its length. Nothosaurus used its legs and webbed feet to swim and to drag itself onto beaches, where it laid its eggs.

By the end of the Triassic period, marine reptiles such as Liopleurodon and Plesiosaurus had flippers instead of legs and feet. Liopleurodon sometimes grew to lengths of seventy-five feet and achieved weights of nearly one hundred tons. It was twenty times heavier than Tyrannosaurus rex and had teeth twice as long. Long necks and tails were characteristic of the plesiosaurs, which had anywhere from twenty-eight to seventy neck vertebrae, depending upon the genus. The small mouth and teeth of Plesiosaurus were adapted to capturing small fish and squid.

Over a period of millions of years, ichthyosaurs evolved an extremely long, thin snout. The leg bones of these reptiles shortened, and the digits were remodeled to form flippers. A dorsal and tail fin appeared. Some of these reptiles have the distinction of having the largest eyes of any animal that ever lived. Fully grown Temnodontosaurus had eyes with diameters of nearly eleven inches. (In comparison, adult elephants have eyes that are two inches in diameter, and those of blue whales are six inches wide.) These reptiles became so adapted to their marine lives that they could not return to land to lay eggs. Ichthyosaurs solved the problem of producing the next generation by giving birth to live babies. A fossil of Stenopterygius illustrates a baby exiting the reptile’s birth canal, much like modern marine mammals.

All these marine reptiles had nasal openings and lungs that had to be filled with air from time to time. This need for oxygen required them to surface, just like extant marine mammals. Even though the marine reptiles were highly adapted to the sea, none survived the Cretaceous-Tertiary extinction, 65 million years ago.

Dinosaurs

Dinosaurs evolved from reptiles sometime after the Permian extinction, 250 million years ago. Dinosaurs developed legs and a skeleton that supported them from beneath, in contrast to most reptiles, which supported themselves on legs that protruded from the sides of their bodies. Dinosaurs were slow to diversify during the Triassic (250 to 205 million years ago) and early Jurassic (205 to 145 million years ago) periods. Even so, many distinctive animals evolved from four major groups present near the beginning of the Jurassic.

Evolution of animals in the Eurypoda led to tetrapods such as Stegosaurus. These huge vegetarians up to thirty feet in length had large, diamond-shaped plates that protected their necks and backs from predators. It is believed that these large dinosaurs were cold-blooded and also used the plates to rapidly cool or heat their bodies.

Animals in the Euornithopoda were mostly tetrapods. Iguanosaurus was a large vegetarian with big, hornlike claws on its thumb and toes that ended in “hoofs.” Hadrosaurus was a large vegetarian with a long, hollow, head crest along the top of its skull that protruded up to 1.5 feet behind the head. Parasaurolophus grew up to thirty feet in length and had a head crest over 5 feet long. These animals could blow air through the head crest to create various trumpeting sounds. Triceratops was another large vegetarian that grew up to nine feet long. It had a sharp, horny beak, a rhinoceros-like horn above and somewhat posterior to its nostrils, two long horns above its eyes, and a bony neck frill protruding from the back of its skull to protect its neck from predators. Styracosaurus, a relative of Triceratops, grew to lengths of seventeen feet. Several smaller horns on either side of two long horns extended the neck frill to make Styracosaurus appear larger and to protect its neck.

Certain Sauropoda evolved into some of the largest land animals that ever existed. The vegetarian tetrapods Titanosaurus weighed up to seventy tons, while Supersaurus ranged up to fifty-five tons, Brachiosaurus forty-five tons, and Seismosaurus thirty tons. Some scientists argue that these reptiles must have been cold-blooded because they could not possibly process enough vegetation to maintain a constant temperature. In comparison, warm-blood mammals such as elephants often reach weights of six to eight tons, whereas warm-blooded blue whales may attain weights of up to ninety tons.

Archaeopteryx, the Dinosaur That Flew

Theropod dinosaurs were warm-blooded, bipedal animals that came in a myriad of sizes and superficially resembled Tyrannosaurus rex. Approximately 155 million years ago, a number of therapod dinosaurs were developing feathered coats. Most of the feathers were similar to down feathers that superficially resemble a tuft of hair. A tuft of hair, however, consists of hairs derived from many hair follicles, whereas a down feather is a single unit in which each hairlike barb attaches to the base of the feather. Some theropod dinosaurs also produced vaned feathers having a distinct shaft and barbs attached along the length of the shaft. Like scales, feathers develop from the differentiation of certain epithelial cells. The earliest stages of feather development resemble the first stages of reptilian scale formation. This suggests that one or more genes evolved that blocked scale development in favor of feather development.

During the late Jurassic period, many of the small, bipedal theropod dinosaurs related to the ancestor of birds evolved long arms, three-fingered hands with claws, fused clavicles (the beginnings of a wishbone), feet with three toes for running, first toes protruding from the back of the foot, and hollow bones.

The first primitive bird, Archaeopteryx, is dated to the late Jurassic, about 150 million years ago. Fossil remains suggest that Archaeopteryx had primary, secondary, and tertiary flight feathers originating from its palm, forearm, and upper arm, respectively. These flight feathers were covered at their base by other feathers known as main coverts and lesser coverts. The three clawed digits of the hand appear not to have supported primary feathers. Instead, they protruded three-quarters of the way along the wing.

In modern birds, the palm and first digit bones have fused and greatly diminished so that a single, small thumb bone protrudes near the wrist. This first finger supports feathers that run parallel to the front of the wing. The palm bones associated with the second and third digits have partially fused at their ends to become a bone that roughly resembles a fused miniature radius and ulna. The second digit that supports the primary feathers that extend the wing and compose the wing tip has shortened and thickened. The third finger has been reduced to a single, tiny bone.

Modern flying birds have large, keel-like sterna for attaching the muscles that move the wings up and down. Although Archaeopteryx had wings that suggest it could perform intricate maneuvers in flight, its tiny sternum indicates it probably lacked the power to take off from the ground. It probably launched itself by running rapidly until it developed sufficient lift. Its feet are similar to those on fast-running dinosaurs. The first digit is high on the foot, suggesting that Archaeopteryx did not depend upon trees or high bushes to launch itself.

Archaeopteryx inherited a long, bony tail from its dinosaur ancestors. The tail supported numerous vaned feathers along its length. In modern birds, the bony tail has been greatly reduced by the loss of numerous segments at the end and the fusion of segments at the base. The short, bony tail in modern birds is called the pygostyle. Most of the tail in modern birds consists of long feathers that extend toward the rear from the pygostyle.

Archaeopteryx also inherited a head that closely resembled that of its dinosaur ancestors. The skull and mandible are light because of cavities in the bones, whereas its elongated snout, not yet a true beak, has a mouth lined with teeth. Although Archaeopteryx and many primitive birds of the Cretaceous period did not survive the Cretaceous-Tertiary extinction, 65 million years ago, a few did. These survivors gave rise to the myriad of modern bird species.

Principal Terms

Arthropods: The phylum of invertebrate animals having clearly segmented bodies and appendages (legs, antennae, and mouth parts) with many segments and joints

Chordates: The phylum of animals that have a stiff, rodlike structure called the notochord running their length; chordates include the boneless fishlike animals similar to amphioxus, as well as fish, amphibians, reptiles, birds, and mammals

Evolution: The science that studies the changes in astronomical bodies, the earth, and living organisms and creates explanations of how changes occur and how entities and events are related

Geological Periods: The twelve divisions in successive layers of sedimentary and volcanic rocks, which are differentiated by the distinctive fossils present within each division; because of many recently discovered fossils and careful dating, the beginning and ending dates for these periods tend to vary somewhat among different references

Paleontology: The branch of geology that deals with prehistoric forms of life through the study of fossil animals, plants, and microorganisms

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