What is the primary distinction of a tetrapod?

The primary distinction of a tetrapod is the presence of two pairs of limbs. This anatomical feature defines the group, which includes amphibians, reptiles, birds, and mammals. The evolution of limbs adapted for life on land was a significant development in vertebrate history, enabling these organisms to move efficiently in terrestrial environments.

While the other options may apply to certain groups of animals, they do not universally define tetrapods. For instance, not all tetrapods have wings, as wings are specific to birds and some insects. Aquatic adaptations are characteristic of some tetrapods like amphibians, but they do not apply to all members of the group, particularly those that are fully terrestrial. The ability to fly is restricted to birds and some bats, thus not representing the entire tetrapod classification. Therefore, the defining characteristic remains the presence of two pairs of limbs, which is fundamental to the definition of tetrapods.