What is a Tetrapod?
Image Citation: Hummel, Heinz (photographer). (2013). Alpine Salamander Amphibian [photography]. Retrieved from https://pixabay.com/photos/alpine-salamander-amphibian-2258279/
Let’s start with the word “tetra”, which means “four”. Combine it with the word “pod”--meaning “foot”--and you’ve got a very basic definition of a tetrapod, which is an animal with four feet.
Most animals you encounter in your daily life probably have four feet, but there was a point in evolutionary history where four-footed body plans were unheard of. Obviously feet of any kind were not needed when all animal life was confined to the oceans. As plants and invertebrates colonized the land, some ancient fishes began to develop foot-like fins that would allow them to take advantage of this new habitat.
The eventual evolutionary result was the first four-footed animal, the first tetrapod. This first tetrapod probably looked like something between a lungfish and a salamander and was still very dependent on proximity to water to reproduce. It was the last common ancestor shared by amphibians and amniotes like reptiles, birds, and mammals.
As a result of their evolutionary history, all amphibians, reptiles, birds, and mammals are considered “tetrapods”. You may be wondering, how are animals like snakes, birds, or whales considered tetrapods when they don’t have four feet? When it comes to deciding which animals are tetrapods, it’s the evolutionary lineage that matters not the actual number of feet.
Snakes, birds, and whales are still evolved from four-footed species, and many of them still have remnants of the four-footed body plan even without having four feet. For example, whales have vestigial hipbones where their hind limbs used to be before they fully adapted to life in the water. Another example is the wings of birds: Under those long, specialized feathers the wings are just modified front legs made from the same basic evolutionary building blocks as a human arm or elephant foot.