Science, Tech, Math › Animals & Nature Tetrapods: the Four-By-Fours of the Vertebrate World Share Flipboard Email Print Photo © Kevin Schafer / Getty Images. Science, Tech, Math Amphibians Birds Habitat Profiles Mammals Reptiles Insects Marine Life Forestry Dinosaurs Evolution View More By Laura Klappenbach Ecology Expert M.S., Applied Ecology, Indiana University Bloomington B.S., Biology and Chemistry, University of Illinois at Urbana-Champaign Laura Klappenbach, M.S., is a science writer specializing in ecology, biology, and wildlife. our editorial process Laura Klappenbach Updated July 29, 2019 Tetrapods are a group of vertebrates that includes amphibians, reptiles, birds, and mammals. Tetrapods include all living land vertebrates as well as some former land vertebrates that have since adopted an aquatic lifestyle (such as whales, dolphins, seals, sea lions, sea turtles, and sea snakes). One of the key characteristics of tetrapods is that they have four limbs or, if they lack four limbs, their ancestors had four limbs. Tetrapods Are Different Sizes Tetrapods vary greatly in size. The smallest living tetrapod is the Paedophyrine frog, which measures just 8 millimeters long. The largest living tetrapod is the blue whale, which can grow to lengths of up to 30 meters. Tetrapods occupy a wide variety of terrestrial habitats including forests, grasslands, deserts, scrublands, mountains, and polar regions. Although most tetrapods are terrestrial, there are numerous groups that have evolved to live in aquatic habitats. For example, whales, dolphins, seals, walrus, otters, sea snakes, sea turtles, frogs, and salamanders, are all examples of tetrapods that depend on aquatic habitats for some or all of their life cycle. Several groups of tetrapods have also adopted an arboreal or aerial lifestyle. Such groups include birds, bats, flying squirrels, and flying lemurs. Tetrapods First Appeared During the Devonian Period Tetrapods first appeared about 370 million years ago during the Devonian Period. Early tetrapods evolved from a group of vertebrates known as the tetrapodomorph fishes. These ancient fishes were a lineage of lobe-finned fishes whose paired, fleshy fins evolved into limbs with digits. Examples of tetrapodomorph fishes include Tiktaalik and Panderichthys. The tetrapods that arose from the tetrapodomorph fishes became the first vertebrates to leave the water and embark on a life on land. Some early tetrapods that have been described in the fossil record include Acanthostega, Ichthyostega, and Nectridea. Key Characteristics Four limbs (or descended from ancestors with four limbs)Various adaptations of the skeleton and muscles that enable proper support and movement on landAdaptations to the cranial bones that allows the head to remain stable while the animal movesA layer of dead cells that reduces evaporation and water loss across the surface of the bodyWell-developed muscular tongueThe parathyroid gland that in part controls calcium levels in the bloodA gland that lubricates the eyes (Harderian gland)An olfactory organ (vomeronasal organ) that enables the detection of pheromones and plays a role in taste and smellAn absence of internal gills Classification Tetrapods are classified within the following taxonomic hierarchy: Animals > Chordates > Vertebrates > Tetrapods Tetrapods are divided into the following taxonomic groups: Amphibians (Lissamphibia): There are about 5,000 species of amphibians alive today. Members of this group include frogs, toads, caecilians, newts, and salamanders. Amphibians begin their life cycle as aquatic larvae that go through a complex metamorphosis as they grow to adulthood.Amniotes (Aminota): There are about 25,000 species of amniotes alive today. Members of this group include birds, reptiles, and mammals. Amniotes reproduce using an egg that is protected by a set of membranes that shelter it from the harsh conditions of a terrestrial environment. References Hickman C, Roberts L, Keen S. Animal Diversity. 6th ed. New York: McGraw Hill; 2012. 479 p.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.