Vertebrates

A vertebrate is an animal with a backbone. They also have internal skeletons and a system of muscles and bones that allows them to move about easily. All vertebrates have bilateral symmetry, well-developed body systems, and a brain that controls many functions. Vertebrates are divided into five groups, all of which vary in structure, life cycle, and behavior. These five groups are further divided according to whether or not they are cold-blooded or warm-blooded.

CHARCTERISTICS OF VERTEBRATES

All five major classes of vertebrates—fish, amphibians, reptiles, birds, and mammals—belong to the phylum Chordata. The animals in these five classes all have a true backbone, meaning that it is made of bone and not cartilage. A backbone, or spinal column, is made up of individual vertebra (plural, vertebrae) that first form as cartilage in the embryonic stage and then ossify, or harden, into bone. The vertebrae all lock together and give the entire column rigidity and support as well as flexibility. The vertebrae are separated and cushioned from each other by soft, flexible structures known as discs. Inside the hollow center of each vertebra runs a column of soft nervous tissue called the spinal cord that is connected to the brain and is an essential part of the animal's nervous system. The hard, bony, yet flexible vertebrae protect the delicate cord.

Vertebrates also have several other things in common. All have what might be called a "head end" containing a control center and sensory organs. As vertebrates are forward-moving animals, it is important that their sense organs be located in the front-most part of their bodies. Vertebrates also have bilateral symmetry, which means that a line drawn lengthwise (top-to-bottom) through the center of the body divides it into halves that are mirror images of each other. This could only be the case for an animal that has a head end, a tail end, and a right and left side.

ALFRED SHERWOOD ROMER

American paleontologist (a person who studies animals, plants, and other organisms that lived in prehistoric times) Alfred Romer (1894–1973) was one of the most influential evolutionary biologists of the twentieth century. Romer's detailed studies of the evolution of extinct fishes, amphibians, and reptiles enabled him to be able to trace the basic changes in their structure and function that came about as they evolved into more complex land animals. As the ultimate authority on the evolution of vertebrates, Romer can be said to have demonstrated Darwin's ideas.

Alfred Romer was born in White Plains, New York, the son of a newspaper man. Since his family moved many times, he was eventually sent back to White Plains at the age of fifteen to live with his grandmother. At age eighteen, he spent a year doing odd jobs to earn money for college, and entered Amherst College entirely on his own initiative. He was able to pay for his schooling through scholarships, jobs, and loans, and finally graduated in 1917 knowing that he wanted to become a paleontologist (one who studies the life of the past by examining its fossil remains). During World War I (1914–18), the army sent Romer to France, and on his return home in 1919, he entered graduate school at Columbia University and earned his Ph.D. in just two years. His dissertation on myology (the scientific study of muscles) became a classic in that field. In 1923 Romer joined the University of Chicago, and in 1934 moved to Harvard University where he remained until his retirement.

Romer spent nearly all of his career investigating vertebrate evolution (the process by which all living things change over generations). His comprehensive books, Vertebrate Paleontology and The Vertebrate Body among others shaped much of the thinking of his subject for decades. Romer was able to focus attention on the importance of form and function as it related to an animal's environment as being the key to how it evolved. His painstaking studies of fossils enabled him to document the slow changes that occurred over long periods of time. Romer's work would bring English naturalist Charles Darwin's theory of evolution to life as Romer used evidence taken from comparative anatomy and even embryology (the study of the early development of organisms). His work also contained scores of examples of specific anatomical adaptations that had taken place in organisms as a result of or in response to environmental changes. Romer not only shaped and defined his field, but was able to give tangible evidence of Darwin's ideas of adaptation through natural selection.

All vertebrates have an endoskeleton, which is a hard bone and cartilage framework located inside their bodies and which supports and maintains their body shape. An endoskeleton linked to a system of muscles enables vertebrates to move. Vertebrates also have a brain and highly developed nervous systems. Finally, vertebrates have a closed circulatory system and a well-developed heart that pumps blood to all parts of the body. Although there are some 40,000 species of vertebrates in the world today, vertebrates actually make up only a very small percentage of the total number of animals.

FISH

Fish are the most successful group of vertebrates and belong to the class, Osteichthyes which means "bony fishes." As the name implies, these fish have skeletons made of real bone and cartilage, like eels, sharks, and rays, which are not true fish. A fish is a vertebrate animal that lives in the water and breathes through gills. The bones of a fish are usually thin and light since they can use the natural buoyancy of water to support their bodies. Fish are also ectotherms ("cold-blooded") which means that their body temperature changes with that of their surroundings. Fish spend their entire lives in water and, therefore, have a respiratory system with gills that uses the oxygen that is dissolved in water. They cannot take oxygen directly from the air. Fish are streamlined in body design and made to move forward efficiently. Fish also have fins that are winglike structures used for balance and control. Most fish have overlapping scales that are covered with mucus to help them glide through the water. They have a heart, but their circulatory system is the simplest of the vertebrates. Fish reproduce sexually through the union of male sperm and female eggs, and they lay these eggs.

AMPHIBIANS

Amphibians are the second group of vertebrates and they, like fish, are also cold-blooded. Amphibians are considered to be the first vertebrates to develop legs and move from water to the land. Their name is taken from Greek words meaning "to have two lives." This is so because the natural life cycle of an amphibian requires that it live partly on land and partly in the water. An amphibian, like a frog, spends part of its life underwater as a tadpole having the characteristics of a fish. The tadpole then undergoes a metamorphosis, or total change in body shape and function, and develops into a land animal. Amphibians, therefore, have "two lives." Most amphibians, like a salamander, frog, toad, and newt, lack a waterproof layer to the skin and, therefore, must keep their skin moist. They usually have two pairs of limbs as adults. Amphibians reproduce sexually and the female lays her eggs in the water. When the eggs hatch they look like little fish and even breathe with gills. Soon, however, with the release of a certain hormone (chemical messenger), they will begin to grow legs as their tails shrink and the body enlarges, and they will develop air-breathing lungs and lose their gills.

REPTILES

The third group of cold-blooded amphibians is made up of reptiles. Although people immediately think of giant dinosaurs that dominated Earth when they think of reptiles, the name now includes snakes, lizards, turtles, and alligators and crocodiles. A reptile is a vertebrate with dry, scaly skin and sealed eggs. Reptiles are not to be confused with amphibians because reptiles have fully adapted to life on land. Although many live in and around water, they always breathe air through lungs. The complete transition to land was made possible when reptiles began laying eggs with a thick, leathery shell that contained water and food. This allowed the egg to be laid on dry land where it would remain safe until it eventually hatched. With the exception of snakes, reptiles usually have four limbs. Snakes have long, thin bodies, no legs, and a jaw that can unhinge and open wide to swallow large prey. Lizards are much like snakes but have two pairs of legs. Turtles have hard protective shells above and below their bodies, but they also have an endoskeleton. Alligators and crocodiles are similar and have very tough armored skin and a powerful tail for swimming. Reptiles reproduce sexually and most lay eggs.

BIRDS AND MAMMALS

Birds and mammals make up the warm-blooded vertebrates. A warm-blooded animal is able to maintain a constant body temperature no matter how hot or cold its surroundings are. Unlike cold-blooded animals, which slow down in the cold, warm-blooded animals can remain active because they convert their food into body heat. They also have a covering of feathers or hair that keeps them warm.

Birds are the only animals with feathers, and along with bats, are the only vertebrates able to fly. A bird is a warm-blooded animal with wings and feathers. All birds have a beak, two legs, and reproduce by laying eggs. They are found in nearly every environment. The act of flying consumes a high amount of energy, and birds have very efficient respiratory and circulatory systems. They also have powerful breast muscles to push their wings back against the air, and hollow bones that are extremely light. Birds' beaks and feet are especially adapted to their habitat and diet, and the shape of a bird's beak and the type of feet it has can usually give clues to what the bird eats. Birds reproduce sexually and lay eggs that have a hard shell. Being able to fly gives a bird a definite competitive advantage since it allows them to escape quickly and easily from danger, as well as leave, or migrate, to another place where food is more plentiful.

A mammal is a warm-blooded vertebrate with hair that feeds milk to its young. Mammals are named after the mammary glands of females that secrete milk and provide food for their young. There are about 4,500 species of mammals, which include humans. Rats and elephants are mammals as are bats and horses. All mammals have some hair or fur on at least part of their body that helps to keep them warm. The mammal brain is also larger than other vertebrates and allows for greater learning. Mammals breathe air through lungs and have specialized teeth and a four-chambered heart. Mammals also exhibit complex behavior that is often directed by instinct, or an inborn pattern, of doing things a certain way. In most mammal species, the unborn young remain inside the mother's body until they are fully developed. Female mammals that mate and are fertilized develop a special organ called a placenta that carries food and oxygen to the embryo and also takes waste away. Most mammals live on land, although whales, dolphins, seals, and manatees live in water. The current geological era is called the Age of Mammals because mammals have successfully spread throughout the world. Mammals are considered by humans (who themselves are mammals) to be among the "higher" vertebrate animals. Although mammals may earn this distinction because they demonstrate evidence of obvious learning and exhibit complex behavior, mammals may also be classed as "higher" vertebrate animals since humans are the ones making up the rules of classification.

[See alsoAmphibians; Birds; Fish; Invertebrates; Mammals; Reptiles ]

Vertebrata

The vertebrates are commonly called "animals with backbones," but this is a simplified description of a group of animals who are the most anatomically and functionally diverse on Earth. As with most major groups of animals, their beginnings are not known. However, scientists have constructed a theory of the origins of vertebrates that is generally well accepted by the academic community.

Scientists base their model of a hypothetical vertebrate ancestor on several primitive living vertebrates. At the forefront is Amphioxus, whose body shape is close to what scientists believe resembles that of the ancestral vertebrate. Many researchers have successfully examined the body of this small marine animal and it has become a popular organism for study in biology classes.

Based on Amphioxus, scientists believe that the first vertebrates had a fishlike body with individual segmented muscles along its entire length. They were small ocean dwellers who lived close to the bottom and used their muscles to contract and move their bodies and tail in a side-to-side motion. This action propelled them through the water. They had no distinct head. The brain of early vertebrates is a somewhat controversial topic among scientists, but most researchers agree that the early forms had a brain that was more complex than the simple brain of Amphioxus. However, the brain was not the highly complicated structure we see in most vertebrates today. The early brain probably carried out only the most basic of body and sensory functions. As with many animals with cerebral head ganglia, nerve tracts emerged from the brain and ran along the length of the body. Scientists assume the nerve tracts responded to sensory stimuli. The lateral line system, present in most fish, was most likely present in early vertebrates.

Just as in Amphioxus, a dorsal (top of body) hollow nerve cord ran the entire length of the body. This nerve cord was supported by an important evolutionary structure made of cartilage called the notocord. The notocord, and its role in the evolution of vertebrates, is one of the most important characteristics distinguishing vertebrates from nonvertebrates. Although there are many other characteristics that help to classify the group, the notocord is the structure from which the backbone is believed to have evolved and is the structure from which vertebrates get their name.

As vertebrates became more specialized and increased their ability to move and sense their environment, the brain and spine became more complex. In looking at the growth of fetal vertebrates it has been shown that the developing muscles place a strain on the cartilaginous notocord. As the strain on the notocord increases with growth of the muscles, deposits of bone replace the cartilage, giving the rod greater strength. This process of replacement eventually produces the bones known as the vertebrae. Each vertebra in a primitive vertebrate corresponds to an individual set of muscles. This pattern is harder to recognize in more derived vertebrates, like mammals and birds, but it is there nonetheless.

In primitive vertebrates, the mouth is a simple oral opening that leads to the gill slits and digestive system. As the vertebrates continued to evolve, the oral cavity was replaced by a more specialized mouth and gill apparatus. Although sharks and their relatives have a primitive type mouth and gills, bony fishes such as salmon and perch have developed complicated gills with a bony covering called an operculum. Sharks are primitive vertebrates in that they do not develop bony skeletons, but even so, the cartilage structure of vertebrates is easy to see.

As vertebrates become complicated in body structure, the mouth becomes a very characteristic structure. Many have teeth that are actually modified body scales. The increasing specialization of the teeth, such as the pointed, socket-bound teeth of the reptile or the many cusped teeth of the mammals, is a major trait on which groups of vertebrates are identified. Birds have no teeth whatsoever.

Early vertebrates had a simple mouth opening through which they gulped food like a frog or fish. This structure was not only poorly adapted for capturing and holding on to active prey but also prevented the animal from breathing while trying to feed. Hunting and swallowing quick prey, like some flying or hopping insects, was problematic to the ill-equipped vertebrates. As a response to increasingly swift food sources that were adapting to life on land, the vertebrates became swifter and more dangerous. The increased specialization of the mouth proved to be an advantage for the group in capturing food.

As they struggled with larger or stronger prey, it was necessary to bite and hold on to wriggling and unwieldy insects. It was hard for them to breathe and many primitive vertebrates were unable to capture these more agile animals.

A major evolutionary trend in the vertebrates was the development of the secondary palate in the mouth, a platform of bone that separates the nasal cavity from the mouth. Mammals, including humans, have a secondary palate that allows for breathing while feeding. This means that the hunter can bite and hold onto its prey and still breathe. It can chew or tear at its food instead of gulping like a crocodile. Lions are an excellent example of how the secondary palate helps the lion to bite and hold onto its intended victim until it is dead and then tear off portions for eating.

It is difficult to provide a generalized summary of the characters of all vertebrates. The group is extremely diverse and includes fish, sharks and rays, amphibians, reptiles, birds, and mammals. However, there are several characteristics that are common to all vertebrates and four that are completely exclusive to the group.

All vertebrates are bilaterally symmetrical—they have two sides which are identical to each other in one plane only. A vertebrate can be divided down the middle, or sagital plane , to produce two equal halves. However, if it is divided down the side, or transverse plane , the sides will not be identical.

The notocord, or skeletal rod, and the dorsal hollow nerve cord are present in all vertebrates. These two characters are unique to vertebrates.

Other characteristics unique to the vertebrates are pharyngeal (at the sides of the pharynx, or throat) gill slits and a tail behind the anal opening. The presence of the tail may seem an obvious trait, but no other group of animals has a structure that can be identified as an actual tail behind the anal opening. Some insects have bodies that extend beyond the anus, but they do not have tails. Vertebrate tails can move and provide locomotion, or balance and support, as in birds and dinosaurs. In many vertebrate groups, such as monkeys, which use their prehensile tails for swinging through trees, the tail can act as an extra appendage.

The vertebrate circulatory system is always closed, but this is not unique to the group. However, the vertebrate heart is always located in a ventral position and the digestive system is complete. Surprising to many, certain vertebrates, especially extinct forms, have an exoskeleton . Early fishes, called ostracoderms and placoderms, had bony exoskeletons that protected their head and sensory areas.

The vertebrates have become a highly successful group of animals with an interesting and exciting evolutionary story. Because humans are vertebrates, they have a natural and continuing curiosity about their predecessors who, most likely, had their beginnings about 500 million years ago in the seas of Earth.

Brook Ellen Hall

Vertebrates

Vertebrates are animals classified in the subphylum Vertebrata, phylum Chordata. Vertebrates share a number of features. They all have an internal skeleton of bone and/or cartilage, which includes a bony cranium surrounding the brain and a bony vertebral column enclosing the spinal cord. Vertebrates are all covered by a skin composed of dermal and superficial epidermal layers of scales, feathers or fur, a ventral heart, formed red and white blood cells, a liver, pancreas, kidney, and a number of other internal organs. Many vertebrates also have jaws, teeth, limbs or fins, and an internal skeletal structure with pelvic and pectoral girdles, and thoracic lungs.

Classification among vertebrates is somewhat under debate. There are between 8 and 15 classes, with most of the discussion surrounding those groups that are only known from the fossil record. The most well known classes are listed below, in the order of their first appearance in the fossil record:

(1) The class Agnatha is a group of jawless, fish-like animals with poorly developed fins, which first appeared more than 500 million years ago, during the late Cambrian. The roughly 75 surviving species include the jawless lampreys and hagfishes.

(2) The class Placodermi is an extinct group of bony-plated aquatic animals. The placoderms were primitive, jawless, fish like creatures, whose head was heavily armored by an external shield of bony plates. These creatures were most abundant during the Devonian period, some 413-365 million years ago.

(3) The class Chondrichthyes includes about 800 living species of sharks, rays, and rat fishes, all of which have a cartilaginous skeleton, true jaws, and a number of other distinctive characters.

(4) The class Osteichthyes includes some 20,000 species of true fishes, with a bony skeleton, a sutured skull, teeth fused to the jaws, lobed or rayed fins, and a number of other distinguishing features.

(5) The class Amphibia includes some 3,500 living species of frogs, toads, salamanders, newts, and caecilians, all of which have four limbs (making them tetrapods), a moist glandular skin, external fertilization, and a complex life cycle.

(6) The class Reptilia are four-legged, tailed animals, with dermal scales, internal fertilization, amniotic eggs, and direct development. Living reptiles include about 6,200 species of crocodiles, turtles, lizards, snakes, and tuataras. Important extinct groups of reptiles include the dinosaurs, hadrosaurs, ichthyosaurs, pterosaurs, and plesiosaurs.

(7) The class Aves, the birds, is a diverse group of about 8,800 species of warm-blooded (or homoiothermic) tetrapods whose forelimbs are specialized for flight (although some species are secondarily flightless). Birds have a characteristic covering of feathers, a beak which lacks teeth, and reproduce by laying eggs.

(8) The class Mammalia includes more than 4,000 species of homoeothermic tetrapods, with epidermal hair and female mammary glands for suckling the young. All give birth to young, although a very few, primitive species reproduce by laying eggs.

Vertebrates are the most complex of Earth’s animal life forms. The earliest vertebrates were marine, jawless, fish-like creatures that probably fed on algae, small animals, and decaying organic matter. The evolution of jaws allowed a more complex exploitation of ecological opportunities, including the pursuit of a predatory life style. The evolution of limbs and the complex life cycle of amphibians allowed the adults to exploit moist terrestrial habitats as well as aquatic habits. The subsequent evolution of internal fertilization and the self-contained, amniotic eggs of reptiles, birds, and mammals allowed reproduction on land, and led to fully terrestrial forms. Birds and mammals further advanced vertebrate adaptations to terrestrial environments through their complex anatomical, physiological, and behavioral adaptations, and this has allowed them to extensively exploit all of Earth’s habitable environments.

See also Cartilaginous fish; Chordates; Fish; Invertebrates.

Bill Freedman

Vertebrates

Vertebrates are animals classified in the subphylum Vertebrata, phylum Chordata. Vertebrates share a number of features. They all have an internal skeleton of bone and/or cartilage, which includes a bony cranium surrounding the brain and a bony vertebral column enclosing the spinal cord. Vertebrates are all covered by a skin composed of dermal and superficial epidermal layers of scales, feathers or fur, a ventral heart , formed red and white blood cells, a liver, pancreas, kidney, and a number of other internal organs. The most advanced vertebrates also have jaws, teeth, limbs or fins, and an internal skeletal structure with pelvic and pectoral girdles, and thoracic lungs.

Eight classes of vertebrates are recognized. These are listed below, in the order of their first appearance in the fossil record:

1. The class Agnatha is a group of jawless, fish-like animals with poorly developed fins, which first appeared more than 500 million years ago, during the late Cambrian. The 75 surviving species include the jawless lampreys and hagfishes .
2. The class Placodermi is an extinct group of bony-plated aquatic animals. The placoderms were primitive, jawless, fish-like creatures, whose head was heavily armored by an external shield of bony plates. These creatures were most abundant during the Devonian period, some 413-365 million years ago.
3. The class Chondrichthyes includes about 800 living species of sharks , rays , and rat fishes, all of which have a cartilaginous skeleton, true jaws, and a number of other distinctive characters.
4. The class Osteichthyes includes some 20,000 species of true fishes, with a bony skeleton, a sutured skull, teeth fused to the jaws, lobed or rayed fins, and a number of other distinguishing features.
5. The class Amphibia includes some 3,500 living species of frogs , toads , salamanders , newts , and caecilians , all of which have four limbs (making them tetrapods), a moist glandular skin, external fertilization , and a complex life cycle.
6. The class Reptilia are four-legged, tailed animals, with dermal scales, internal fertilization, amniotic eggs, and direct development. Living reptiles include about 6,200 species of crocodiles , turtles , lizards, snakes , and tuataras. Important extinct groups of reptiles include the dinosaurs, hadrosaurs, ichthyosaurs, pterosaurs, and plesiosaurs.
7. The class Aves, the birds , is a diverse group of about 8,800 species of warm-blooded (or homoiothermic) tetrapods whose forelimbs are specialized for flight (although some species are secondarily flightless). Birds have a characteristic covering of feathers, a beak which lacks teeth, and reproduce by laying eggs.
8. The class Mammalia includes more than 4,000 species of homoiothermic tetrapods, with epidermal hair and female mammary glands for suckling the young. All give birth to young, although a very few, primitive species reproduce by laying eggs.

Vertebrates are the most complex of Earth's animal life forms. The earliest vertebrates were marine, jawless, fish-like creatures that probably fed on algae , small animals, and decaying organic matter . The evolution of jaws allowed a more complex exploitation of ecological opportunities, including the pursuit of a predatory life style. The evolution of limbs and the complex life cycle of amphibians allowed the adults to exploit moist terrestrial habitats as well as aquatic habits. The subsequent evolution of internal fertilization and the self-contained, amniotic eggs of reptiles, birds, and mammals allowed reproduction on land, and led to fully terrestrial forms. Birds and mammals further advanced vertebrate adaptations to terrestrial environments through their complex anatomical, physiological, and behavioral adaptations, and this has allowed them to extensively exploit all of Earth's habitable environments.

See also Cartilaginous fish; Chordates; Fish; Invertebrates.

Bill Freedman

Vertebrates

Vertebrates are any animals that have a backbone or spinal column. These animals are so named because nearly all adults have vertebrae, bone or segments of cartilage forming the spinal column. The five main classes of vertebrates are fish, amphibians, birds, reptiles, and mammals.

Vertebrates are the most complex of Earth's animal life-forms. The earliest vertebrates were marine, jawless, fishlike creatures with poorly developed fins. First appearing on Earth more than 500 million years ago, they probably fed on algae (single-celled or multicellular plants and plant-like animals), small animals, and decaying organic matter. The evolution of jaws, limbs, internal reproduction organs, and other anatomical changes over millions of years allowed vertebrates to move from ocean habitats to those on land.

All vertebrates have an internal skeleton of bone and cartilage or just cartilage alone. In addition to a bony spinal column, all have a bony cranium surrounding the brain. Vertebrates have a heart with two to four chambers, a liver, pancreas, kidneys, and a number of other internal organs. Most have two pairs of appendages that have formed as either fins, limbs, or wings.

[See also Amphibians; Birds; Fish; Invertebrates; Mammals; Reptiles ]