Scientists estimate that there may be between 3 and 30 million species of animals on our planet. If you're just starting to learn about animals and wildlife, it would be an overwhelming task to try and memorize species, one by one. Instead, an understanding of the main groups of animals provides a good foundation for further learning. In this article, we'll take a look at six groups of animals, their characteristics and the types of organisms that belong to each group. Once you have a understanding of these six groups, you'll be on your way to grasping the basic characteristics of virtually every animal on the planet.
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Invertebrates were the first animals to evolve. Fossil evidence of invertebrates dates back to the late Precambrian, 600 million years ago. Invertebrates evolved from single-celled microorganisms. Since then, invertebrates have diversified into countless forms. An estimated 97 percent of all species are alive today are invertebrates. Invertebrates are united more by what they lack (a backbone) than by shared characteristics. Invertebrates include animal groups such as sponges, cnidarians, flatworms, molluscs, arthropods, insects, segmented worms, and echinoderms as well as many other lesser-known groups of animals.
The key characteristics of invertebrates include:
- no backbone
- earliest animals were invertebrates
- more than 97 percent of all living species are invertebrates
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Fishes were among the first vertebrates to evolve. The earliest known fishes were the ostracoderms, a now-extinct group of jawless fishes that appeared in the Cambrian Period, about 510 million years ago. Other early fish include the conodonts and the agnanthans (the hagfish and the lamprey). Fishes later evolved jaws and diversified into a number of lineages including cartilaginous fishes, ray-finned fishes and lobe-finned fishes. The ray-finned fishes are the most diverse of all vertebrate groups, with some 24,000 species. There are about 810 species of cartilaginous fishes and 8 species of lobe-finned fishes.
The key characteristics of fishes include:
- first animals to evolve jaws
- breathe using gills
- no limbs
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Amphibians were the first vertebrates to make the move from life in water to life on land. Despite their early colonization of terrestrial habitats, most lineages of amphibians have never fully severed their ties with aquatic habitats. The first amphibians evolved from lobe-finned fishes approximately 370 million years ago during the Devonian Period. Amphibians include newts and salamanders, frogs and toads, and caecilians. There are between 5000 and 6000 species of known amphibians alive today. Amphibian species are in decline around the world due to a variety of threats including of invasive species, habitat destruction, disease, climate change and toxins.
The key characteristics of amphibians include:
- aquatic larvae
- first vertebrates to colonize terrestrial habitats
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Reptiles are cold-blooded vertebrates that diverged from ancestral amphibians about 340 million years ago during the Carboniferous Period. Early reptiles included organisms such as Hylonomus, Petrolacosaurus, Archaeothyris and Paleothyris. The oldest evidence of reptiles is a set of footprints found in Nova Scotia. Two characteristics distinguish early reptiles from amphibians: scales and the ability to lay hard-shelled amniotic eggs. Reptiles include turtles, squamates, crocodiles and tuatara. There are about 8,000 species of reptiles alive today.
The key characteristics of reptiles include:
- scales and scutes
- amniotic eggs
Mammals are vertebrates that evolved from therapsid reptiles during the Jurassic Period about 200 million years ago. The first mammals, known as morganucodontids, were nocturnal insect eaters that resembled modern-day shrews. For the better part of 130 million years, mammals remained small and lived in a world dominated by the dinosaurs. But around 65 million years ago, a drastic shift in climate caused the extinction of more than two-thirds of the animal species on the planet, including the dinosaurs. Today, mammals are remarkably varied, with some 5,400 species occupying every continent on the globe. Some of the better-known mammal groups include carnivores, rodents, elephants, marsupials, rabbits, bats, primates, seals, anteaters, cetaceans, odd-toed ungulates and even-toed ungulates.
The key characteristics of mammals include:
- mammary glands
- lower jaw consists of a single bone
- three middle ear bones
- three-chambered heart
Birds evolved from reptiles during the Mesozoic Era about 150 million years ago. Today, more than 9,000 species inhabit virtually every terrestrial habitat on the planet. Birds have a number of characteristics that sets them apart from other vertebrates such as feathers, bills and a furcula. Birds, best known for their ability to fly, are unmatched in their command of the skies. Albatrosses glide over the vast open sea, hummingbirds hover motionless in mid-air, and birds of prey capture prey with pinpoint accuracy. But not all birds are aerobatic experts. Some species such as ostriches, kiwis and penguins, lost their ability to fly long ago in favor of lifestyles suited more for land or water.
The key characteristics of birds include:
- lightweight bones (bones that are either hollow or spongy/strutted)
- fused bones in pelvis, feet, hands, and head
- no teeth or maxillary bones of the jaw (reduces anterior weight)
- four-chambered heart and high metabolic rates
- produce large, richly provisioned external eggs
Hickman C, Roberts L, Keen S, Larson A, l'Anson H, Eisenhour D. Integrated Principles of Zoology. 14th ed. Boston MA: McGraw-Hill; 2006. 910 p.
Ruppert E, Fox R, Barnes R. Invertebrates Zoology: A Functional Evolutionary Approach. 7th ed. Belmont CA: Brooks/Cole; 2004. 963 p.