6 Basic Animal Classes
Animals, the most complex and advanced organisms on the planet, are thought to have evolved between 620 and 550 million years ago. Since then, they have spread to all parts of the world and diversified into millions of species – 8.7 million, to be exact (although there are probably many more living on Earth). While there are millions of animals on Earth, we can divide them into six main groups or classes: invertebrates, fish, amphibians, reptiles, birds and mammals. Each group has its own set of distinct characteristics and behaviors, so let’s check out all 6 classes of animals in detail down below.
When it comes to bare basics animal classifications, there are vertebrates and invertebrates, or animals with backbones and those without. The word “invertebrate” itself comes from the Latin word vertebra, which refers to a joint – either in general or a joint from the spinal column of a vertebrate. The prefix “in” simply means not or without.
Invertebrates, or animals without a bony skeleton, were the first to evolve. Their evolution happened about a billion years ago, so they’ve been on the Earth for the longest of all. Very old, this group of animals is also quite simple in its anatomy and behavior, at least if we’re comparing them to vertebrates. However, this is also by far the largest group of animals – 97% of all animal kingdom are, in fact, invertebrates. They range from microscopic mites and tiny flies to giant squids and octopuses. While there are countless families within the invertebrate class, the largest number belongs to insects.
One of the reasons for a large number of invertebrates is how quickly they’re able to reproduce. Corals, for instance, produce both eggs and sperm, while social insects like ants lay eggs that can develop without fertilization. Another reason is their adaptability. Take insects for example – they feed on animals, plants and decaying matter, can survive in extreme environments, and many can both crawl and fly.
Some of the animals within the invertebrates class include insects, worms, mollusks, crustaceans, corals, arachnids, sponges, and many other families.
Fish have evolved from invertebrate ancestors about 500 million years ago, which makes them the first true vertebrates on the planet. While there are thousands of fish species, all fish share two traits: they have a backbone (which is why they belong to the vertebrates) and they live in water. There are other traits that most fish share as well, with two most interesting being breathing through gills and communicating through the use of acoustic communication. But when it comes to the main types of fish, there are three of them: bony fish, cartilaginous fish and jawless fish. So, what are the main differences between each species?
The bony fish, also known as Osteichthyes, is a diverse group of fish, all of whom share one characteristic – they have skeletons primarily made of bone tissue. This is the largest class of vertebrates on Earth, so it’s not surprising it’s divided into two subclasses: ray-finned fish (tuna, swordfish, salmon, cod, catfish, goldfish and many other) and lobe-finned fish (coelacanths and lungfish species).
Next, we have cartilaginous fish species, also known as Chondrichthyes, which are jawed vertebrates with skeletons composed of cartilage. This species too is divided into two subclasses: Elasmobranchi (these would be sharks, sawfish, rays, skates) and Holocephali (chimeras).
The third and final type of fish species is a superclass of jawless fish, also known as Agnatha. This type consists of both present and extinct species, but like all fish, they both belong to the class of vertebrates. The modern jaw fish have bodies covered in skin without scales. They all have lots of slime glands too, as slime is their mechanism of defense. There were various subgroups of jawless fish, but today, the family is made up entirely of hagfish and lamprey.
Amphibians are tetrapod vertebrates, which is just a fancy word for four-legged vertebrates (or animals with a bony skeleton), who need water to survive. There are various subclasses of amphibians and they inhabit a variety of environments, with most species living near bodies of water. Some of the animals belonging to this class include frogs, toads, salamanders and caecilians (limbless, serpentine amphibians). All amphibians can breathe and absorb water through their thin skin, which is their secondary respiratory system. In fact, some small amphibians like frogs and salamanders rely entirely on their skin to breathe as they have no lungs.
While many amphibians look similar to lizards, they are actually quite different – unlike amphibians, reptiles don’t need bodies of water to survive and breed, and unlike reptiles, amphibians can die from extreme sun and wind exposure. Because of their specific living conditions, this class of animals is the first to die off when their habitats are either contaminated or disturbed. Today, amphibians are amongst the most endangered animals on earth, which should tell you a lot about how we’re treating our planet, and with it, all living things.
Like amphibians, reptiles are tetrapod animals. Although they ruled the Earth in the form of dinosaurs for over 160 million years, today, reptiles make up a relatively small proportion of terrestrial animals. Some of the animals in the reptile class include crocodiles (who, ironically, are more closely related to birds than lizards!), snakes, turtles, lizards and other. That said, there are four main types of reptiles: Crocodilia (crocodiles and alligators), Testudines (turtles and tortoises), Squamata (lizards and snakes) and Sphenodontia (tuataras). While this is a diverse class of animals, all reptiles share at least two traits – they shed the outer layer of their skin and they’re cold-blooded.
Skin shedding, also known as ecdysis, is present in lots of reptiles, but snakes are perhaps the most famous skin-shedders. But why is this a thing even? Snakes, lizards and other reptiles slough their skin on a regular basis to allow further growth and also to get rid of any parasites residing in their old skin. Before they shed their old skin though, reptiles grow a new layer underneath it, so when the old layer is gone, they already have a new one protecting them. As for being cold-blooded, all reptiles (as well as amphibians and fish) rely on accumulating and dissipating heat from their environment as they’re unable to efficiently do it on their own. This is because they have no fur or feathers to keep their warm, or sweat glands to keep them cool. Instead, to maintain proper internal body temperature, reptiles move into the sun to get warm, or into the shade to cool off. This is also why they’re inactive during cooler parts of the year.
Did you know that birds are also known as Avian Dinosaurs? Yep, and the reason for the name is obvious – they evolved from (feathered) dinosaurs, probably several times. Fossil records show that our modern birds evolved alongside dinosaurs during the Jurassic period, so around 160 million years ago. By far the most prolific flying vertebrates, birds come in thousands of species. But no matter their order or species, all birds are characterized by a coat of feathers and warm-blooded metabolisms. Most birds can fly, of course, but many can also run, jump, swim and dive as well. Some birds have even lost their ability to fly despite having wings (penguins). There is another characteristic that all birds share – they’re highly adaptive. Birds live and breed in all corners of the planet – literally on all seven continents. The largest bird is a 9-foot tall ostrich, and the smallest one is a 2-inch long bee hummingbird. Clearly, birds come in all sizes, shapes and with different abilities.
Birds have a specific digestive system that allows them to eat on the go – while they walk, run or fly – and digest their food later. And unlike other animals, they use their beaks to grab and swallow (whole) food, which is varied –fruit and plants, seeds, nectar, small animals and carrion.
One of the most complex and adaptable vertebrate animal groups are the mammals. Found on every continent and in every ocean, ranging from tiny Etruscan shrew to giant Blue whale, mammals come in all shapes and sizes. While it’s natural to think of mammals as the most advanced animals in the world – after all, humans are mammals – they aren’t that different from the rest of the species. Like the rest 4 animal groups, mammals belong to vertebrates as they (we) have a bony skeleton. They’re also highly adaptive – there are mammals that can walk, run, jump, climb, swing, burrow, swim, dive and fly. Similarly, their diet is also extremely diverse – there are carnivore mammals that live solitary lives (tigers, polar bears), carnivores who live in family groups (lions), highly social herbivores (deer, zebra), and of course highly intelligent omnivores (primates).
Despite this, mammals are among the least diverse animal classes – there are a little over 5000 species in total. What unifies all these species though are four things: they produce milk for nursing their young, they have fur or hair, they have a neocortex and three middle ear bones.
- 6 Basic Animal Classes – Thought Co