- Describe traits of vertebrates.
- Identify three reproductive strategies of vertebrates.
- Explain how vertebrates are classified.
- Outline the evolution of vertebrates.
- vertebra (vertebrae, plural)
Vertebrates are animals in Phylum Chordata. Modern vertebrates include fish, amphibians, reptiles, birds, and mammals. You can see examples of all these groups of vertebrates in Figure below.
Examples of Vertebrates: (left to right) Fish, Amphibian, Reptile, Bird, and Mammal.
What Are Vertebrates?
Like all chordates, vertebrates are animals with four defining traits, at least during the embryonic stage. The four traits are:
- a notochord;
- a dorsal hollow nerve cord;
- a post-anal tail; and
- pharyngeal slits.
Some invertebrates also have these traits and are classified as chordates. What traits do vertebrates have that set them apart from invertebrate chordates?
The main trait that sets vertebrates apart from invertebrate chordates is their vertebral column, or backbone. It develops from the notochord after the embryonic stage. As you can see in Figure below the vertebral column runs from head to tail along the dorsal (top) side of the body. The vertebral column is made up of repeating units of bone called vertebrae (vertebra, singular). The vertebral column helps the vertebrate body hold its shape. It also protects the spinal (nerve) cord that runs through it.
This sketch of the vertebral column of a goat shows the groups of vertebrae into which the vertebral column is commonly divided.
The vertebral column is the core of the vertebrate endoskeleton, or internal skeleton. You can see a human skeleton as an example of the vertebrate endoskeleton in Figure below. In addition to the vertebral column, the vertebrate endoskeleton includes:
- a cranium, or bony skull, that encloses and protects the brain;
- two pairs of limbs (in humans, arms and legs);
- limb girdles that connect the limbs to the rest of the endoskeleton (in humans, shoulders and hips).
Bone and Cartilage
The vertebrate endoskeleton is made of bone and cartilage. Cartilage is a tough, flexible tissue that contains a protein called collagen. Bone is a hard tissue consisting of a collagen framework that is filled in with minerals such as calcium. Bone is less flexible than cartilage but stronger. A bony endoskeleton allows an animal to grow larger and heavier than a cartilage endoskeleton would. Bone also provides more protection for soft tissues and internal organs.
Other Traits of Vertebrates
Most vertebrates share several other traits. The majority of vertebrates have:
- scales, feathers, fur, or hair covering their skin;
- muscles attached to the endoskeleton to allow movement;
- a circulatory system with a heart that pumps blood through a closed network of blood vessels;
- an excretory system that includes a pair of kidneys for filtering wastes out of the blood;
- a central nervous system with a brain, spinal cord, and nerve fibers throughout the body;
- an adaptive immune system that learns to recognize specific pathogens and launch tailor-made attacks against them; and
- an endocrine system with glands that secrete chemical messenger molecules called hormones.
How Vertebrates Reproduce
Vertebrates reproduce sexually. Most have separate male and female sexes. Vertebrates have one of 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 fish, amphibians, and reptiles. It also occurs in all birds.
- Ovovivipary refers to the development of an embryo inside an egg within the mother’s body. The egg remains inside the mother’s body until it hatches, but 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 but not inside an egg. Birth may be followed by a period of parental care of the offspring. This reproductive strategy occurs in almost all mammals including humans.
There are about 50,000 living species of vertebrates. They are placed in nine different classes. Table below lists these vertebrate classes and some of their traits. Five of the classes are fish. The other four classes are amphibians, reptiles, birds, and mammals.
They have a cranium but no backbone; they do not have jaws; their endoskeleton is made of cartilage; they are ectothermic.
They have a partial backbone; they do not have jaws; their endoskeleton is made of cartilage; they are ectothermic.
They have a complete backbone; they have jaws; their endoskeleton is made of cartilage; they are ectothermic.
They have a backbone and jaws; their endoskeleton is made of bones; they have thin, bony fins; they are ectothermic.
They have a backbone and jaws; their endoskeleton is made of bones; they have thick, fleshy fins; they are ectothermic.
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
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.
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.
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.
The earliest vertebrates were jawless fish. They evolved about 550 million years ago. They were probably similar to modern hagfish (see Table above). The tree diagram in Figure below summarizes how vertebrates evolved from that time forward.
Phylogenetic Tree of Vertebrate Evolution. The earliest vertebrates evolved almost 550 million years ago. Which class of vertebrates evolved last?
How Fish Evolved
The earliest fish had an endoskeleton made of cartilage rather than bone. They also lacked a complete vertebral column. The first fish with a complete vertebral column evolved about 450 million years ago. These fish had jaws. They may have been similar to living sharks. About 400 million years ago, the first fish with a bony endoskeleton evolved. A bony skeleton could support a bigger body. Early bony fish evolved into modern ray-finned fish and lobe-finned fish.
How Other Vertebrate Classes Evolved
The earliest amphibians evolved from a lobe-finned fish ancestor. This occurred about 365 million years ago. Amphibians were the first terrestrial vertebrates. They lived on land as adults, but they had to return to the water to reproduce. The earliest reptiles evolved from an amphibian ancestor. This occurred at least 300 million years ago. Reptiles were the first vertebrates that did not need water to reproduce. That’s because they laid waterproof amniotic eggs. These eggs allowed the embryo inside to breathe without drying out. Mammals and birds both evolved from reptile-like ancestors. The first mammals appeared about 200 million years ago. The earliest birds evolved about 150 million years ago.
Ectothermy and Endothermy
Early vertebrates were ectothermic. Ectothermy means controlling body temperature to just a limited extent from the outside by changing behavior. For example, an ectotherm might stay in the shade to keep cool on a hot, sunny day. On a cold day, an ectotherm might bask in the sun to warm up, like the snake in Figure below. Almost all living fish, amphibians, and reptiles are ectothermic. They can raise or lower their body temperature by their behavior but not by very much. In cold weather, an ectotherm cools down. As its body temperature drops, its metabolism slows down and it becomes inactive.
A water snake climbs onto a rock to bask and warm up in the sun.
Both mammals and birds evolved endothermy. Endothermy means controlling body temperature within a narrow range from the inside through biochemical or physical means. For example, on a cold day, an endotherm may produce more body heat by increasing its rate of metabolism. On a hot day, it may give off more heat by increasing blood flow to the surface of the body. That way, some of the heat can radiate into the air from the body’s surface. Endothermy requires more energy (and food) than ectothermy. However, it allows the animal to stay active regardless of the temperature outside. You can learn more about how vertebrates regulate their temperature by watching this video: https://www.youtube.com/watch?v=TSUCdLkI474
- Vertebrates are chordates with a vertebral column and endoskeleton of cartilage and bone. Vertebrates also have several organ systems.
- Vertebrates reproduce sexually. They have one of three reproductive strategies: ovipary, ovovivipary, or vivipary.
- There are about 50,000 living species of vertebrates. They are placed in nine classes: five classes of fish plus amphibians, reptiles, birds, and mammals.
- The first vertebrates evolved about 550 million years ago. The evolution of amphibians, reptiles, mammals, and birds occurred over the next 400 million years. The first vertebrates were ectotherms; endothermy evolved later. Modern fish, amphibians, and reptiles are ectotherms. Modern birds and mammals are endotherms.
Lesson Review Questions
- What traits set vertebrates apart from invertebrate chordates?
- Describe how living vertebrates are classified.
- Outline vertebrate evolution.
- What can you do as an endotherm that you could not do if you were an ectotherm?
- Compare and contrast the three reproductive strategies of vertebrates.
Points to Consider
Fish were the first vertebrates to evolve. The earliest fish lived in the water, and modern fish are still aquatic.
- What are some examples of modern fish?
- What traits do you think fish have that adapt them for life in the water?