How is the organism like both a fish and four-legged vertebrate such as an ampibian?

Fish have fins, and four-legged vertebrates (tetrapods) have limbs. The organism that is both a fish and a tetrapod is called a lobe-finned fish. Lobefins are a group of bony fish that have fleshy, lobed fins that can be used to support their weight and move on land. Some lobefinned fishes also have lungs, which allow them to breathe air.

Lungfish

Lungfish are a type of lobefinned fish that have both lungs and gills. They can breathe air with their lungs or water with their gills. Lungfish are found in both fresh and saltwater habitats, and they are native to Africa, South America, and Australia.

Tiktaalik

Tiktaalik roseae is a species of lobe-finned fish that lived in the late Devonian period, about 375 million years ago. It is considered to be a transitional fossil between fish and tetrapods. Tiktaalik had many features that are similar to tetrapods, such as lungs, a neck, and limbs that could be used to support its weight on land. However, it also had many features that are similar to fish, such as gills and a tail fin.

The discovery of Tiktaalik and other lobe-finned fish has helped scientists to better understand the evolution of tetrapods. These animals provide evidence that the first tetrapods evolved from lobe-finned fish that were able to move onto land.