Lesson Explainer: Kingdom Animalia: Vertebrates Biology

In this explainer, we will learn how to recall the characteristics of different vertebrates within the animal kingdom.

You will most likely be very familiar with examples of vertebrates; pet dogs and cats are vertebrates, the meat used to make burgers and sausages comes from vertebrates, and you, as a human, are a vertebrate! Although vertebrates of the animal kingdom surround us in all environments and in every day of our lives, vertebrate species are estimated to make up less than 5% of animal species alive on Earth today! However, vertebrates are still highly varied and diverse, and in this explainer, we will be taking a look at the key characteristics of some of the animals that belong to this group.

When studying living organisms, scientists will often classify them into distinct groups, in a process known as taxonomy. Organisms within the same group will share some key characteristics.

Definition: Taxonomy

Taxonomy is the study of classifying and naming organisms.

An example of a taxonomic group is a kingdom, which is a group consisting of many different organisms that all share some key characteristics. An outline of the 5-kingdom system of classification is provided in Figure 1.

Figure 1: A diagram demonstrating the 5-kingdom classification system.

Almost all the organisms belonging to kingdom Animalia share some key characteristics. They are usually very mobile, they are multicellular, and they are heterotrophic, meaning they need to consume other organisms to obtain their nutrition.

Definition: Heterotroph

A heterotroph is an organism that obtains food from consuming other organisms or organic matter.

If we take a closer look at kingdom Animalia, we can see that biologists will classify this kingdom into two different groups: vertebrates and invertebrates. The basic outline of this grouping and some key characteristics of each group are shown in Figure 2.

Figure 2: A diagram to show the division of kingdom Animalia into vertebrates and invertebrates.

As we can see from Figure 2, vertebrates include all animals that possess a vertebral column, or backbone. This vertebral column acts as the core of the endoskeleton (internal skeleton) of vertebrates, which is made of bone or cartilage. The endoskeleton includes a skull that encloses and protects the brain of the organism. All vertebrates (subphylum vertebrata) belong to the phylum Chordata, which includes any organism that possesses a notochord at any stage of its development. They will also all have a closed circulatory system, containing a heart with multiple chambers.

Definition: Vertebrate

A vertebrate is an organism that possesses a vertebral column (backbone).

Definition: Chordate

A chordate is an organism that, at some stage during its development, possesses a notochord.

You may recall that the different groups of vertebrates are referred to as fish, reptiles, amphibians, birds, and mammals. Organisms will be classified into these groups based on particular characteristics. Let’s take a look at the different classes of vertebrates (subphylum vertebrata) in more detail.

There are three large classes of vertebrates that we commonly call “fish”: class Agnatha (the jawless fish), class Chondrichthyes (the cartilaginous fish), and class Osteichthyes (the bony fish). Figure 3 outlines their key characteristics and makes some comparisons between the classes.

Figure 3: A comparison of the characteristics of classes Agnatha, Chondrichthyes, and Osteichthyes. These three classes make up a group we commonly refer to as “fish.”

These classes of fish have some things in common. The vast majority of fish lay eggs that are then fertilized externally to the body. Fish are commonly referred to as cold blooded, but this does not mean they have cold blood! It refers to their thermoregulation; unlike humans, fish rely on the external environment to heat their bodies. The appropriate term for these animals is ectotherms.

Definition: Ectotherm

An ectotherm is an organism that relies on the external environment to regulate its body temperature.

Example 1: Classifying Organisms into Classes Based On Given Characteristics

An unknown marine organism is discovered. The organism found has a long, thin body that is not covered in scales, and it has no paired fins. What class of vertebrates is this organism most likely to belong to?

  1. Osteichthyes
  2. Agnatha
  3. Chondrichthyes


When asked to classify organisms into certain groups, we should start by recalling the defining features of those groups. Here, we have three options for this organism: class Osteichthyes, class Agnatha, and class Chondrichthyes. Let’s recap the defining characteristics of each class in turn to find the correct match.

Organisms belonging to class Osteichthyes have bony skeletons, and their bodies are covered with scales. The question states that the organism has a long, thin body that is not covered in scales. This suggests that class Osteichthyes is not the correct choice.

Organisms belonging to class Agnatha are commonly known as the jawless fish. They have long, thin bodies, a funnel-like mouth, and no paired fins. You can see an example of a jawless fish, a lamprey, in the photo below:

The European brook lamprey

Looking back at our question, this choice seems like it could be the correct one; it matches the description of the organism’s body and the absence of the paired fins. To be certain, we will check that the third option is incorrect.

Class Chondrichthyes includes organisms like rays and sharks, which have bodies covered in scales and have paired fins. This is the direct opposite to the organism described in the question, so this option is definitely incorrect.

Therefore, the class of vertebrates that the organism in question is most likely to belong to is class Agnatha.

Another class of vertebrates is class Reptilia. Organisms belonging to this class are known as reptiles, and examples include lizards, crocodiles, turtles, and snakes. Like fish, reptiles are commonly referred to as cold blooded, but we now know the correct term for this is ectotherm. If you live in, or have visited, a hot country, you may have seen lizards sunbathing in the heat of the day, like the one in the picture below; it is how they warm their bodies up!

Tenerife lizard-edit
Figure 4

The bodies of reptiles are divided into four regions: head, neck, trunk, and tail. Their skin is dry to touch and is covered with thick, horny scales. The majority of reptiles also have four pentadactyl limbs that end with horny claws to support and move their body, but in some (like snakes), these may be absent. Reptiles, unlike fish, obtain oxygen by breathing through their lungs. They are unisexual organisms, and fertilization occurs internally and results in eggs being laid. These eggs can have tough, calcified shells or relatively soft ones, in which case the reptiles may build protective structures like nests around the eggs to prevent them from getting damaged.

Key Term: Pentadactyl Limb

A pentadactyl limb is a limb with 5 digits, found in reptiles, amphibians, and mammals.

Class Amphibia is a separate class of vertebrates, but one that shares some characteristics with class Reptilia. Amphibians are also ectotherms and so rely on the environment for their temperature regulation. They also have four pentadactyl limbs to support their movement. However, their bodies are covered with moist, smooth skin which secretes mucus to keep it moist, rather than hard scales. This adaptation of amphibians means that some can even exchange oxygen with the environment through their skin! While, like reptiles, they are unisexual organisms, they fertilize eggs externally to the body. These eggs will be laid in water. An example of an amphibian, a fire salamander, is pictured in water below.

lizard Fire salamander
Figure 5

Amphibians have very interesting methods of obtaining oxygen: the embryos and very young forms of most amphibians live in water and obtain oxygen through gas exchange from water in the gills. However, when the amphibians grow, they will develop lungs and change to breathing atmospheric oxygen! This helps them develop some interesting adaptations; for instance, the males of the European tree frog (pictured below) have large vocal sacs that help the frog make loud croaking sounds to attract a mate!

Croaking European tree frog
Figure 6

Another class of vertebrates is class Aves, and organisms in this class are more commonly referred to as birds. In contrast to reptiles and amphibians, birds are “warm blooded”; the more correct scientific term for this is endothermic, and it means that birds have specialized systems in their body to regulate their own temperatures and are less reliant on the environment.

Definition: Endotherm

An endotherm is an organism that can regulate its own body temperature independently of the environment.

Organisms belonging to class Aves have bodies largely covered with feathers and bones that are hollow and incredibly light, and this is to aid their main mechanism of movement: flying. They have four limbs: two legs that contain four digits and horny claws and two wings. Their hind limbs allow the birds to walk while on land and while climbing or moving along trees and in some cases to catch prey! To help them move their wings during flight, they have strong thoracic muscles attached to their sternum. Birds are very active and so require a large supply of oxygen to allow their cells to respire and release energy. To do this, birds have air sacs within their hollow bones that increase the available oxygen for respiration during flying. Like reptiles, they breath atmospheric oxygen using lungs, and fertilization takes place internally and results in eggs being laid. Also similarly to reptiles, their legs are covered with scales. Examples of birds include pigeons, hawks, eagles, and sparrows. Some species of bird can also be flightless, where their bodies have adapted to environments and resulted in them losing the ability to fly. These include penguins and ostriches, an example of which is shown in the picture below.

Figure 7

Example 2: Recalling Characteristics Shared by Classes Amphibia, Reptilia, and Aves

Which of the following statements describes a characteristic shared by amphibians, reptiles, and birds?

  1. The females of these organisms produce milk to feed their young.
  2. These organisms are all covered by scales rather than skin.
  3. These organisms all give birth to live young.
  4. These organisms all lay eggs rather than giving birth to live young.


Classes Amphibia, Reptilia, and Aves all belong to the Chordata phylum. This means that they will share certain characteristics and differ in others.

The anatomy of the three classes is very different. Reptiles are often found in hot, dry environments and are covered in hard scales. Amphibians live in moist environments and are covered with soft skin. Birds are found in a wide variety of habitats, but their distinctive feature is that they are covered with feathers.

When it comes to reproduction, however, the organisms have more in common. None of these classes give birth to live young, and instead they all lay eggs. Birds will usually create nests to do this; reptiles will often find safe areas of land and dig holes or burrows for their eggs, while amphibians lay theirs in water. Because they do not give birth to live young, they also do not produce milk to feed their young—these are both defining characteristics of class Mammalia.

Therefore, the characteristic shared by amphibians, reptiles, and birds is that they all lay eggs rather than giving birth to live young.

Class Mammalia is a class of vertebrates that most will be very familiar with; it is the class that cats, dogs, horses, sheep, and humans belong to! All mammals are endothermic and breath atmospheric oxygen using lungs. The bodies of mammals are divided into a head, neck, thorax, and abdomen. The skin of mammals is covered with hair, rather than scales like reptiles or feathers like birds. Mammals typically have four pentadactyl limbs, and depending on the species, these limbs may possess nails, claws, or hooves. Mammals have distinct teeth, including incisors, canines, and molars, the number and size of which will vary depending on the diet of the organism. Fertilization in mammals happens internally, and in the majority of cases the young will develop inside the body of the mother. Organisms that have their young develop inside the mothers are often called viviparous. When the live young are born, the mothers will produce milk to feed them via mammary glands.

Example 3: Classifying Organisms into Classes Based On Given Characteristics

A scientist makes the following notes on an organism:

  1. Has a body covered in scales
  2. Uses the environment to regulate body temperature
  3. Breathes through lungs

Which class of vertebrates does this organism most likely belong to?


To answer this question, we need to recall some defining features of the different classes of vertebrates and see which class this organism would fit into best.

There are three classes of vertebrate fish: class Osteichthyes, class Agnatha, and class Chondrichthyes. As these organisms have aquatic habitats, they have gills or gill slits to allow gas exchange. The organism in question, however, breathes through lungs, so we can rule out the possibility of it being any class of fish.

Class Amphibia contains organisms that live in water during their larval stages but on land in adult stages. By the time amphibians develop into their adult form, they have usually developed lungs. Amphibians are also ectothermic, which means they have to rely on the environment to regulate their temperature. However, amphibians are not covered by scales, so we can rule out class Amphibian as well.

Class Aves includes all species of birds. The defining feature of birds is that their bodies are largely covered by feathers. They are also endothermic, which means that they regulate their own body temperature and rely far less on their environment to do so.

Class Mammalia includes all mammals, including humans! Although we breathe through lungs, we definitely do not have bodies covered in scales! Like birds, we are endothermic and so do not use the environment to regulate our core body temperature.

The final option is class Reptilia. Reptiles, like lizards and snakes, are often found in hot environments; they are ectothermic, so it is easier for them to keep their body temperature constant there. Reptiles largely live on land, so they breathe through lungs. They do have a body that is covered in scales, which vary in their degrees of thickness and size.

Therefore, the class of vertebrates that this organism is most likely to belong to is class Reptilia.

Class Mammalia is a very diverse group; let’s take a look at three subclasses of class Mammalia in Table 1, which have been categorized according to embryo development and method of birth.

SubclassLay Eggs?Give Birth to Live Young?Young Feed on Milk?Where Do Embryos Complete Their Developmment?Example Organism(s)
PrototheriaYesNoYesIncubated in an eggDuck-billed platypus
MetatheriaNoYesYesNursed in a pouchKangaroo, possum
EutheriaNoYesYesIn a uterus and fed through placentaPrimates, carnivores, majority of mammals

Example 4: Recognizing the Defining Features of the Subclasses of Class Mammalia

Name the subclass of Mammalia that is being described: These are mammals that often give birth to immature young that must be kept in their mothers’ pouches for weeks after birth.


Class Mammalia is a large and diverse class of organisms. Class Mammalia can be split into subclasses based on the way in which their embryos develop and their method of giving birth. The largest of these subclasses is Eutheria, and this includes the majority of mammals, including humans. Eutherian mammals will allow their young to develop fully in the uterus, providing nutrients for growth through the placenta until a viable baby is ready to be born. Metatherian mammals will have their young develop in their uterus for a short time, then give birth to what is known as immature young. These young will then be kept in a specialized pouch to complete their development. Prototherians, however, lay eggs. They then incubate these eggs until the young are ready to hatch. Although the methods of embryonic development and birth are different, all of these classes of mammals will feed their young on milk.

Therefore, the subclass that can be described as giving birth to immature young that must be kept in their mother’s pouches for weeks after birth is subclass Metatheria.

Subclass Eutheria is the largest subclass of class Mammalia. It includes a wide range of mammals, including ones used in livestock, those kept as pets, and all primates, including humans! Subclass Eutheria can be divided into multiple orders. Organisms within the same order will share certain characteristics. Figure 8 provides a simple phylogenetic tree of subclass Eutheria, with additional information about the characteristics of each order.

Figure 8: A simple phylogenetic tree of the orders belonging to subclass Eutheria. The key characteristics of each order are given, along with some example animals.

Key Term: Phylogenetic Tree

A phylogenetic tree is a diagram that represents evolutionary relationships among organisms.

Example 5: Recognizing Defining Features of the Mammalian Orders

The following is a list of mammals and some of their key features:

  1. A domestic sheep, with two toes on its feet
  2. A capybara, with clawed toes
  3. A gibbon, with opposable thumbs
  4. A zebra, with a single toe covered by a hoof
  5. A rhino, with three toes on its feet

State the letters corresponding to the two mammals that belong to the same order.


Class Mammalia can be divided into many different orders. Although all organisms in class Mammalia share some characteristics, when they are placed into orders, the features they share become more specific. To find the answer to this question, we need to determine which two organisms share a specific feature found in one order that is not possessed by any of the other organisms belonging to different orders. Domestic sheep have two toes on their feet; this means that they are even toed and are classified into order Artiodactyla. A capybara has clawed toes and long chisel-shaped teeth; this means it is classified into order Rodentia. Only order Primates includes organisms that have opposable thumbs, so this must be the order that gibbons belong to. Order Perissodactyla includes herbivorous animals that have an odd number of toes. A zebra with a single toe, as well as a rhino with three toes, would fall into this category.

Therefore, the two organisms that belong to the same order are D (a zebra) and E (a rhino).

Let’s review some of the key points we have learned about vertebrates in the animal kingdom.

Key Points

  • Kingdom Animalia can be split into two groups: vertebrates and invertebrates.
  • Vertebrates are organisms that possess a vertebral column, or backbone.
  • Classes of vertebrates include classes Agnatha, Chondrichthyes, Osteichthyes, Amphibia, Reptilia, Aves, and Mammalia.
  • Class Mammalia can be divided into subclasses Prototheria, Metatheria, and Eutheria.
  • Subclass Eutheria can be further divided into the orders Edentata, Insectivora, Carnivora, Perissodactyla, Artiodactyla, Cetacea, Rodentia, Lagomorpha, Chiroptera, Proboscidea, and Primates.

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