CHARACTERISTICS
In nearly all vertebrates, bone gives the skeleton its strength. Bone is a living tissue composed of hard mineral salts produced by specialized cells. Unlike an oyster’s shell or a grasshopper’s body case, bone can strengthen after it has reached full size, and it can be repaired if it breaks. The only vertebrates that do not have this kind of skeleton are cartilaginous fish, a group that includes sharks, skates, and rays. As their name suggests, the skeletons of these species are made of cartilage, a rubbery tissue that other vertebrates have mainly in their joints.
A vertebrate's spinal column is held together by strong ligaments, but the faces of adjoining vertebrae are separated by elastic pads called intervertebral disks. These disks allow a small amount of movement at each joint, and as a result the entire spine can bend. How far the spine bends depends on the number of vertebrae that compose it and how they are shaped. Frogs, for example, can have as few as nine vertebrae, and their backbones hardly bend at all. Humans have 33 vertebrae, making us fairly flexible, and some snakes have more than 400, enabling them to shape their bodies into coils.
Besides the backbone, vertebrates share many other physical features. Their bodies are more or less bilaterally symmetrical (divisible into two equal halves), with sense organs concentrated in the head. Most vertebrates have jaws, and their brains are usually protected by a bony case called the cranium. Most also have limbs, but the shapes and uses of vertebrate limbs vary enormously. Fish typically have several paired fins and a large finned tail, but all other vertebrates either have four limbs or are descended from ancestors that had four. Four-limbed animals, known as tetrapods, use their limbs to swim, walk, run, and fly.
Although vertebrates do not have external skeletons, they often have other anatomical features that protect the surface of their bodies. Most fish and reptiles have a covering of hard scales, while birds and mammals have feathers or hair. Feathers and hair are not as tough as scales, but they have other functions apart from physical protection. One of the most important is insulation. By regulating the heat generated inside the body, such coverings allow birds and mammals to remain active in a wide range of temperatures.
Nearly all vertebrates breed by sexual reproduction, either laying eggs or giving birth to live young. The few exceptions to this rule include animals such as North American whiptail lizards, which can breed without mating in a process known as parthenogenesis. In several species of these lizards, males have never been found.
TYPES OF VERTEBRATES
There are over 40,000 species of vertebrates, which scientists classify into five groups: (1) fish, (2) amphibians, (3) reptiles, (4) birds, and (5) mammals. Scientists divide fish into three groups based on their anatomy: jawless fish, cartilaginous fish, and bony fish. The other vertebrate groups are made up of tetrapods, which have lungs and generally live on land.
Jawless Fish
Jawless fish are the only living vertebrates that have never evolved jaws. There are about 50 species—a tiny fraction of the world's total fish—and they are instantly recognizable by their suckerlike mouths. Eels, lampreys, and hagfish are examples of jawless fish.
Cartilaginous Fish
Cartilaginous fish do have jaws and use them to deadly effect. Numbering about 1,000 species, they include sharks, skates, and rays, as well as chimaeras, also known as ratfish. Cartilaginous fish are widespread throughout the world's oceans. Most skates and rays feed on or near the seabed, but sharks typically hunt in open water.
Bony Fish
Bony fish are some of the most successful vertebrates alive today. These animals can be found in a vast variety of habitats, from coral reefs and the deep-sea bed to lakes hidden away in caves. As their name indicates, bony fish have a skeleton made of bone, and most also have an air-filled sac called a swim bladder that keeps them buoyant. At least 24,000 species of bony fish have been identified, and many more probably await discovery. Common bony fish include salmon, sturgeon, and cod.
Amphibians
Amphibians make up the smallest of the four groups of tetrapods, with about 4,000 species. Most amphibians, such as frogs and toads, live in damp habitats. Like fish, the majority of amphibians reproduce by laying eggs. Amphibians usually lay their eggs in water, because they dry out quickly in air. The eggs produce swimming, fishlike young called tadpoles, which develop limbs and lungs as they mature.
Reptiles
Compared to amphibians, reptiles are much more fully adapted to life on land. They have scaly, waterproof skin, and they either give birth to live young or lay eggs with waterproof shells. There are about 7,000 species alive today, including snakes, alligators, and turtles. During the age of the dinosaurs, about 230 million to 65 million years ago, reptiles outnumbered all other land vertebrates put together.
Birds
Birds evolved from flightless reptiles but underwent some major changes in body form during their evolution. Of the roughly 10,000 species alive today, most have lightweight, air-filled bones, and all have a unique and highly efficient respiratory system that is found in no other group of vertebrates.
MammalsMammals
are the only vertebrates that raise their young by feeding them on milk produced by the mother’s body, and the only ones that have teeth that are individually specialized for particular functions. Mammal species number about 4,600, and they include the largest animals on land and in the sea. Dogs, bears, monkeys, whales, and humans are all mammals.
THE ORIGIN OF VERTEBRATES
Biologists believe that vertebrates evolved over millions of years from animals similar to today’s lancelets, which burrow in sand on the seabed and filter food from the water. Lancelets possess certain traits similar to vertebrates, including a reinforcing structure called a notochord that runs the length of the body. In a lancelet the notochord is the only hard part of the body, and it allows the animal to wriggle without losing its shape. In most vertebrates, the notochord is lost during early development, and its role is taken over by bone. The characteristics shared by lancelets and vertebrates cause scientists to classify them together in the chordate phylum.
Scientists do not know exactly how the transition from lancelet to vertebrate occurred. Fossils of fishlike animals found in China indicate that vertebrates evolved at the start of the Cambrian Period, an interval of geologic time that began about 570 million years ago. These fish lacked a bony skeleton and teeth (scientists propose that their skeletal structures were made of cartilage), but they did have gill slits and a muscle arrangement similar to today’s fish. Once vertebrates evolved hard body parts, they began to leave more fossilized remains. Fish called ostracoderms, which had bony plates covering their bodies, first appeared in the late Cambrian Period, about 500 million years ago. Like present-day lampreys and hagfish, ostracoderms had no jaws. They probably fed by sucking water into their mouths and then swallowing any food it contained.
With the evolution of jaws, vertebrates acquired a valuable new asset in the struggle for survival, one that enabled them to collect food in a variety of different ways. Jaws first appeared in fish about 420 million years ago, during the mid-Silurian Period. Unlike earlier vertebrates, jawed fish developed complex internal skeletons and paired fins, which helped them maneuver as they pursued their food or escaped from their enemies.
Over time, evolution has produced vertebrates with many different body types and behaviors. As a result, vertebrates can now be found in almost every part of the world.
In nearly all vertebrates, bone gives the skeleton its strength. Bone is a living tissue composed of hard mineral salts produced by specialized cells. Unlike an oyster’s shell or a grasshopper’s body case, bone can strengthen after it has reached full size, and it can be repaired if it breaks. The only vertebrates that do not have this kind of skeleton are cartilaginous fish, a group that includes sharks, skates, and rays. As their name suggests, the skeletons of these species are made of cartilage, a rubbery tissue that other vertebrates have mainly in their joints.
A vertebrate's spinal column is held together by strong ligaments, but the faces of adjoining vertebrae are separated by elastic pads called intervertebral disks. These disks allow a small amount of movement at each joint, and as a result the entire spine can bend. How far the spine bends depends on the number of vertebrae that compose it and how they are shaped. Frogs, for example, can have as few as nine vertebrae, and their backbones hardly bend at all. Humans have 33 vertebrae, making us fairly flexible, and some snakes have more than 400, enabling them to shape their bodies into coils.
Besides the backbone, vertebrates share many other physical features. Their bodies are more or less bilaterally symmetrical (divisible into two equal halves), with sense organs concentrated in the head. Most vertebrates have jaws, and their brains are usually protected by a bony case called the cranium. Most also have limbs, but the shapes and uses of vertebrate limbs vary enormously. Fish typically have several paired fins and a large finned tail, but all other vertebrates either have four limbs or are descended from ancestors that had four. Four-limbed animals, known as tetrapods, use their limbs to swim, walk, run, and fly.
Although vertebrates do not have external skeletons, they often have other anatomical features that protect the surface of their bodies. Most fish and reptiles have a covering of hard scales, while birds and mammals have feathers or hair. Feathers and hair are not as tough as scales, but they have other functions apart from physical protection. One of the most important is insulation. By regulating the heat generated inside the body, such coverings allow birds and mammals to remain active in a wide range of temperatures.
Nearly all vertebrates breed by sexual reproduction, either laying eggs or giving birth to live young. The few exceptions to this rule include animals such as North American whiptail lizards, which can breed without mating in a process known as parthenogenesis. In several species of these lizards, males have never been found.
TYPES OF VERTEBRATES
There are over 40,000 species of vertebrates, which scientists classify into five groups: (1) fish, (2) amphibians, (3) reptiles, (4) birds, and (5) mammals. Scientists divide fish into three groups based on their anatomy: jawless fish, cartilaginous fish, and bony fish. The other vertebrate groups are made up of tetrapods, which have lungs and generally live on land.
Jawless Fish
Jawless fish are the only living vertebrates that have never evolved jaws. There are about 50 species—a tiny fraction of the world's total fish—and they are instantly recognizable by their suckerlike mouths. Eels, lampreys, and hagfish are examples of jawless fish.
Cartilaginous Fish
Cartilaginous fish do have jaws and use them to deadly effect. Numbering about 1,000 species, they include sharks, skates, and rays, as well as chimaeras, also known as ratfish. Cartilaginous fish are widespread throughout the world's oceans. Most skates and rays feed on or near the seabed, but sharks typically hunt in open water.
Bony Fish
Bony fish are some of the most successful vertebrates alive today. These animals can be found in a vast variety of habitats, from coral reefs and the deep-sea bed to lakes hidden away in caves. As their name indicates, bony fish have a skeleton made of bone, and most also have an air-filled sac called a swim bladder that keeps them buoyant. At least 24,000 species of bony fish have been identified, and many more probably await discovery. Common bony fish include salmon, sturgeon, and cod.
Amphibians
Amphibians make up the smallest of the four groups of tetrapods, with about 4,000 species. Most amphibians, such as frogs and toads, live in damp habitats. Like fish, the majority of amphibians reproduce by laying eggs. Amphibians usually lay their eggs in water, because they dry out quickly in air. The eggs produce swimming, fishlike young called tadpoles, which develop limbs and lungs as they mature.
Reptiles
Compared to amphibians, reptiles are much more fully adapted to life on land. They have scaly, waterproof skin, and they either give birth to live young or lay eggs with waterproof shells. There are about 7,000 species alive today, including snakes, alligators, and turtles. During the age of the dinosaurs, about 230 million to 65 million years ago, reptiles outnumbered all other land vertebrates put together.
Birds
Birds evolved from flightless reptiles but underwent some major changes in body form during their evolution. Of the roughly 10,000 species alive today, most have lightweight, air-filled bones, and all have a unique and highly efficient respiratory system that is found in no other group of vertebrates.
MammalsMammals
are the only vertebrates that raise their young by feeding them on milk produced by the mother’s body, and the only ones that have teeth that are individually specialized for particular functions. Mammal species number about 4,600, and they include the largest animals on land and in the sea. Dogs, bears, monkeys, whales, and humans are all mammals.
THE ORIGIN OF VERTEBRATES
Biologists believe that vertebrates evolved over millions of years from animals similar to today’s lancelets, which burrow in sand on the seabed and filter food from the water. Lancelets possess certain traits similar to vertebrates, including a reinforcing structure called a notochord that runs the length of the body. In a lancelet the notochord is the only hard part of the body, and it allows the animal to wriggle without losing its shape. In most vertebrates, the notochord is lost during early development, and its role is taken over by bone. The characteristics shared by lancelets and vertebrates cause scientists to classify them together in the chordate phylum.
Scientists do not know exactly how the transition from lancelet to vertebrate occurred. Fossils of fishlike animals found in China indicate that vertebrates evolved at the start of the Cambrian Period, an interval of geologic time that began about 570 million years ago. These fish lacked a bony skeleton and teeth (scientists propose that their skeletal structures were made of cartilage), but they did have gill slits and a muscle arrangement similar to today’s fish. Once vertebrates evolved hard body parts, they began to leave more fossilized remains. Fish called ostracoderms, which had bony plates covering their bodies, first appeared in the late Cambrian Period, about 500 million years ago. Like present-day lampreys and hagfish, ostracoderms had no jaws. They probably fed by sucking water into their mouths and then swallowing any food it contained.
With the evolution of jaws, vertebrates acquired a valuable new asset in the struggle for survival, one that enabled them to collect food in a variety of different ways. Jaws first appeared in fish about 420 million years ago, during the mid-Silurian Period. Unlike earlier vertebrates, jawed fish developed complex internal skeletons and paired fins, which helped them maneuver as they pursued their food or escaped from their enemies.
Over time, evolution has produced vertebrates with many different body types and behaviors. As a result, vertebrates can now be found in almost every part of the world.
