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19.1: Overview of Vertebrates

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Lesson Objectives

  • List the characteristics of vertebrates.
  • Explain how vertebrates reproduce.
  • Identify the nine classes of vertebrates.
  • Give an overview of vertebrate evolution.

Vocabulary

bone
hard tissue in most vertebrates that consists of a collagen matrix, or framework, filled in with minerals such a calcium
cartilage
dense connective tissue that provides a smooth surface for the movement of bones at joints
cranium
part of a vertebrate endoskeleton that encloses and protects the brain; also called the skull
ectothermy
regulation of body temperature from the outside through behavioral changes such as basking in the sun
endothermy
regulation of body temperature from the inside through metabolic or other physical changes
immune system
body system that consists of skin, mucous, membranes, and other tissues and organs that defends the body from pathogens and cancer
kidney
main organ of the excretory system that filters blood and forms urine
ovipary
type of reproduction in which an embryo develops within an egg outside the mother’s body
ovovivipary
type of reproduction in which an embryo develops inside an egg within the mother’s body but in which the mother provides no nourishment to the developing embryo in the egg
vertebrae (singular, vertebra)
repeating bony units that make up the vertebral column of vertebrates
vivipary
type of reproduction in which an embryo develops within, and is nourished by, the mother’s body

Introduction

Vertebrates are a subphylum of the phylum Chordata. Like all chordates, vertebrates have a notochord, a dorsal hollow nerve cord, pharyngeal slits, and a post-anal tail. What other characteristics do vertebrates have? What traits set them apart from invertebrate chordates?

Characteristics of Vertebrates

The main distinguishing feature of vertebrates is their vertebral column, or backbone (see Figure below). The backbone runs from the head to the tail along the dorsal (top) side of the body. The vertebral column is the core of the endoskeleton. It allows a vertebrate to hold its shape. It also houses and protects the spinal (nerve) cord that passes through it. The vertebral column is made up of repeating units called vertebrae (singular, vertebra). In many species, there are shock-absorbing discs between the vertebrae to cushion them during movement.

Human Vertebral Column and Vertebrae. The human vertebral column consists of 33 vertebrae. Two vertebrae are shown here enlarged.

Vertebrate Endoskeleton

Another distinguishing feature of vertebrates is an endoskeleton made of bone or cartilage. Cartilage is a tough tissue that contains a protein called collagen. Bone is a hard tissue that consists of a collagen matrix, or framework, filled in with minerals such as calcium. Bone is less flexible than cartilage but stronger. An endoskeleton made of bone rather than cartilage allows animals to grow larger and heavier. Bone also provides more protection for soft tissues and internal organs. As shown in Figure below, the vertebrate endoskeleton includes a cranium, or skull, to enclose and protect the brain. It also generally includes two pairs of limbs. Limb girdles (such as the human hips and shoulders) connect the limbs to the rest of the endoskeleton.

Vertebrate Endoskeletons. The vertebrate endoskeleton includes a vertebral column, cranium, limbs, and limb girdles. Can you find these parts in each endoskeleton shown here?

Other Vertebrate Traits

There are several additional traits found in virtually all vertebrates.

  • Vertebrates have a system of muscles attached to the endoskeleton to enable movement. Muscles control movement by alternately contracting (shortening) and relaxing (lengthening). Generally, muscles work together in opposing pairs.
  • Vertebrates have a closed circulatory system with a heart. Blood is completely contained within blood vessels that carry the blood throughout the body. The heart is divided into chambers that work together to pump blood. There are between two and four chambers in the vertebrate heart. With more chambers, there is more oxygen in the blood and more vigorous pumping action.
  • Most vertebrates have skin covered with scales, feathers, fur, or hair. These features serve a variety of functions, such as waterproofing and insulating the body.
  • Vertebrates have an excretory system that includes a pair of kidneys. Kidneys are organs that filter wastes from blood so they can be excreted from the body.
  • Vertebrates have an endocrine system of glands that secrete hormones. Hormones are chemical messengers that control many body functions.
  • Vertebrates have an adaptive immune system. The immune system is the organ system that defends the body from pathogens and other causes of disease. Being adaptive means that the immune system can learn to recognize specific pathogens. Then it can produce tailor-made chemicals called antibodies to attack them. This allows the immune system to launch a rapid attack whenever the pathogens invade the body again.
  • Vertebrates have a centralized nervous system. As shown in Figure below, the nervous system consists of a brain in the head region. It also includes a long spinal cord that runs from the brain to the tail end of the backbone. Long nerve fibers extend from the spinal cord to muscles and organs throughout the body.

Nervous System (Human). The vertebrate nervous system includes a brain and spinal cord. It also includes a body-wide network of nerves, called peripheral nerves. They connect the spinal cord with the rest of the body.

Vertebrate Reproduction

Vertebrates reproduce sexually, and almost all of them have separate male and female sexes. Generally, aquatic species have external fertilization, whereas terrestrial species have internal fertilization. Can you think of a reason why aquatic and terrestrial vertebrates differ in this way? Vertebrates have one of the following three reproductive strategies: ovipary, ovovivipary, or vivipary.

  • Ovipary refers to the development of an embryo within an egg outside the mother’s body. This occurs in most amphibians and reptiles and in all birds.
  • Ovovivipary refers to the development of an embryo inside an egg within the mother’s body until it hatches. The mother provides no nourishment to the developing embryo inside the egg. This occurs in some species of fish and reptiles.
  • Vivipary refers to the development and nourishment of an embryo within the mother’s body. Birth may be followed by a period of parental care of the offspring. This reproductive strategy occurs in almost all mammals.

Vertebrate Classification

There are about 50,000 vertebrate species, and they are placed in nine different classes. Five of the classes are fish. The other classes are amphibians, reptiles, birds, and mammals. Table below lists some of the distinguishing traits of each class.

Class Distinguishing Traits Example
Hagfish They have a cranium but no backbone; they do not have jaws; their endoskeleton is made of cartilage; they are ectothermic.

hagfish

Lampreys They have a partial backbone; they do not have jaws; their endoskeleton is made of cartilage; they are ectothermic.

lamprey

Cartilaginous Fish They have a complete backbone; they have jaws; their endoskeleton is made of cartilage; they are ectothermic.

shark

Ray-Finned Fish They have a backbone and jaws; their endoskeleton is made of bones; they have thin, bony fins; they are ectothermic.

perch

Lobe-Finned Fish They have a backbone and jaws; their endoskeleton is made of bones; they have thick, fleshy fins; they are ectothermic.

coelacanth

Amphibians They have a bony endoskeleton with a backbone and jaws; they have gills as larvae and lungs as adults; they have four limbs; they are ectothermic

frog

Reptiles They have a bony endoskeleton with a backbone and jaws; they breathe only with lungs; they have four limbs; their skin is covered with scales; they have amniotic eggs; they are ectothermic.

alligator

Birds They have a bony endoskeleton with a backbone but no jaws; they breathe only with lungs; they have four limbs, with the two front limbs modified as wings; their skin is covered with feathers; they have amniotic eggs; they are endothermic.

bird

Mammals They have a bony endoskeleton with a backbone and jaws; they breathe only with lungs; they have four limbs; their skin is covered with hair or fur; they have amniotic eggs; they have mammary (milk-producing) glands; they are endothermic.

bear

Vertebrate Evolution

The earliest vertebrates were jawless fish, similar to living hagfish. They lived between 500 and 600 million years ago. They had a cranium but no vertebral column. The phylogenetic tree in Figure below gives an overview of vertebrate evolution. As more data become available, new ideas about vertebrate evolution emerge.

Phylogenetic Tree of Vertebrate Evolution. The earliest vertebrates evolved almost 550 million years ago. Which class of vertebrates evolved last?

Evolution of Fish

Not too long after hagfish first appeared, fish similar to lampreys evolved a partial vertebral column. The first fish with a complete vertebral column evolved about 450 million years ago. These fish also had jaws and may have been similar to living sharks. Up to this point, all early vertebrates had an endoskeleton made of cartilage rather than bone. About 400 million years ago, the first bony fish appeared. A bony skeletal could support a larger body. Early bony fish evolved into modern ray-finned and lobe-finned fish.

Evolution of Other Vertebrate Classes

Amphibians, reptiles, mammals, and birds evolved after fish.

  • The first amphibians evolved from a lobe-finned fish ancestor about 365 million years ago. They were the first vertebrates to live on land, but they had to return to water to reproduce. This meant they had to live near bodies of water.
  • The first reptiles evolved from an amphibian ancestor at least 300 million years ago. They laid amniotic eggs and had internal fertilization. They were the first vertebrates that no longer had to return to water to reproduce. They could live just about anywhere.
  • Mammals and birds both evolved from reptile-like ancestors. The first mammals appeared about 200 million years ago and the earliest birds about 150 million years ago.

Evolution of Endothermy

Until mammals and birds evolved, all vertebrates were ectothermic. Ectothermy means regulating body temperature from the outside through behavioral changes. For example, an ectotherm might stay under a rock in the shade in order to keep cool on a hot, sunny day. Almost all living fish, amphibians, and reptiles are ectothermic. Their metabolic rate and level of activity depend mainly on the outside temperature. They can raise or lower their own temperature only slightly through behavior alone.

Both mammals and birds evolved endothermy. Endothermy means regulating body temperature from the inside through metabolic or other physical changes. On a cold day, for example, an endotherm may produce more heat by raising its metabolic rate. On a hot day, it may give off more heat by increasing blood flow to the surface of the body. Keeping body temperature stable allows cells to function at peak efficiency at all times. The metabolic rate and activity level can also remain high regardless of the outside temperature. On the other hand, maintaining a stable body temperature requires more energy—and more food.

Lesson Summary

  • Vertebrates are a subphylum of chordates that have a vertebral column and an endoskeleton made of cartilage or bone. Vertebrates also have complex organ systems, including a closed circulatory system with a heart, an excretory system with a pair of kidneys, and an adaptive immune system.
  • Vertebrates reproduce sexually, and almost all have separate male and female sexes. Aquatic species generally have external fertilization, whereas terrestrial species usually have internal fertilization. Vertebrates have one of three reproductive strategies, known as ovipary, ovovivipary, or vivipary.
  • The 50,000 species of living vertebrates are placed in nine classes: hagfish, lampreys, cartilaginous fish, ray-finned fish, lobe-finned fish, amphibians, reptiles, birds, and mammals.
  • The earliest vertebrates resembled hagfish and lived more than 500 million years ago. As other classes of fish appeared, they evolved traits such as a complete vertebral column, jaws, and a bony endoskeleton. Amphibians were the first tetrapod vertebrates as well as the first vertebrates to live on land. Reptiles were the first amniotic vertebrates. Mammals and birds, which both descended from reptile-like ancestors, evolved endothermy, or the ability to regulate body temperature from the inside.

Lesson Review Questions

Recall

1. Describe the vertebrate vertebral column, and list its functions.

2. Contrast cartilage and bone, and state the advantages of a bony endoskeleton relative to a cartilaginous endoskeleton.

3. Identify the components of the vertebrate nervous system.

4. What is an adaptive immune system?

5. Define ovipary, ovovivipary, and vivipary. Which vertebrates use each type of reproductive strategy?

Apply Concepts

6. Create a time line of vertebrate evolution that shows how and when important vertebrate traits evolved.

Think Critically

7. Explain the significance of changes in the number of heart chambers during the course of vertebrate evolution.

8. Compare and contrast ectothermy and endothermy, including their pros and cons.

Points to Consider

The earliest and simplest vertebrates are fish. Fish also have the greatest number of vertebrate classes. Think about some of the fish you are familiar with, such as fish you eat or fish you may have seen in aquariums.

  • Using the Classes of Vertebrates Table above, which fish class or classes should these fish be placed in?
  • How are all of the fish the same? In what ways do they differ?

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