What is an evolutionary innovation associated with amphibians?

Answer 1

The earliest forms of amphibians evolved from lobe-finned fish in the Devonian Period, around 350 million years ago. These fish had strong, bony fins that allowed them to drag themselves on land, and some of them developed lungs for breathing air. These two adaptations were very helpful in their freshwater environments that were often stagnant and dried out. Eventually the bony fins of these fish evolved into limbs that became better suited for terrestrial locomotion over time. Early amphibians still spent most of their lives in water and always had to lay their soft eggs in water.

One of the earliest and most well known ancestors of amphibians (and all tetrapods in general) is a lobe-finned fish called Eusthenopteron. Although it never actually went on land, it still had four fins with bone structures homologous to all modern tetrapods, and also had internal nostrils and enfolded enamel found on primitive labyrinthodont amphibians.

Another important transitional form is Tiktaalik, who is an important link in the transition between lobe-finned fish and tetrapods. By the structure of its jointed fins, which contain wrist bones, it was probably one of the first fish to crawl on land. However, it only did so under certain circumstances and still spent almost all of its time in water. It had both gills and primitive lungs, and had a flattened head shaped like that of an amphibian's.

The earliest amphibians include Acanthostega and Icthyostega. Their primitive limbs contained digits, but were still poorly adapted for moving efficiently on land. Yet it was a good start, and they would be succeeded by labyrinthodonts such as Hynerpeton and Eryops, who are the ancestors of true amphibians.

The first modern amphibians emerged in the Carboniferous Period and had all the adaptations necessary to make them suitable for living both in water and on land. Amphibians became the dominant terrestrial vertebrates through most of the Carboniferous until they are replaced by their descendants, the reptiles.

Answer 2

Biologists think they were the first tetrapods (four-legged animals) and evolved from fish. The missing link between them is Tiktaalik roseae.

Answer 3

Amphibians are able to spend time (partly) out of the water for gathering food or mating.

Answer 4

They are tetrapods.