Acanthostega

In 1938 a remarkable discovery was made of a living lobe-finned fish thought to have been extinct for about 70 million years. This fish named Latimeria, was found in deep water near the coast of Madagascar, Africa. The crossopterygian lobe finned fishes, having fleshy lobed fins with all the necessary bony elements for feet as well as primitive pectoral and pelvic girdles, were thought to be ancestral to ancient primitive land dwelling amphibians. However, Latimeria lives in deep water and would not be a logical candidate for ancestry to the land dwelling vertebrates.

Eusthenopteron (CLASS: Bony fishes, ORDER: Crossopterygii, SUBORDER: Rhipidistia)

In the fossil record, there exists a group of lobe-finned fishes that were the dominant predators between the Upper Devonian (about 370 million years ago) and the Lower Permian (about 280 million years ago). These fishes also had all the necessary bony elements for feet as well as primitive pectoral and pelvic girdles. They lived in shallow fresh waters in the Carboniferous swamps and used their fleshy lobed fins to crawl on the muddy bottoms of fresh water ponds and perhaps esturaries. One of the best known fossils was Eusthenopteron, and this animal represented an advanced fish that had all the necessary structures as well as living in shallow waters near land to be the ancestor to land vertebrates. , Eusthenopteron was a Rhipidistian lobe-finned fish, a different suborder from that of the living coelocanth Latimeria. The rhipidistian lobe-finned fishes became extinct about 280 million years ago.

As shown above is an ancient amphibian-like fish Acanthostega. It is shown here surfacing for air in a swamp that has low oxygen concentrations dur to high bacterial action. Although the animal had four legs, they would not ahve been able to support the animal on land. Thus, rather than limbs evolving as an adaptation for life on land, they may have initially functioned to help the animal to lift its head out of the oxygen-poor water to breathe and to crawl on the muddy bottoms of swamps (notice the number of toes). Only later did the limbs evolved facilitating walking on land.

Ichthyostega (CLASS: Amphibian, ORDER: Ichthyostegalia)

Ichthyostega
(Ancient fish-like amphibian)

Ichthyostega was an ancient extinct amphibian with four legs and a fish-like tail fin. It existed at the same general time as Eusthenopteron (advanced lobe-fined fish) and Acanthostega, another fish-like amphibian. At that time, about 360 million years ago, was the Upper Devonian Period as shown below. There are many skeletal similarities between the fossils of these two animals, which strongly suggests that they have evolved from the same ancestoral forms. Different from Acanthostega, Ichthyostega had pelvic bones that connected the linbs to the spinal column (backbone), and the spinal column was able to support the body weight on land.

DEVONIAN PERIOD
(360 million years ago)

First land vertebrates probably evolved from lobe-finned fishes some 360 million years ago, living in shallow estuaries that may have had fluctuating water levels. With abundant food on land, the lobe-finned fishes evolved supporting endoskeletal systems having feet, legs and other supportive bony structures. Terrestrial environments have no buoyancy for support as it is in water, so evolutionary advancements in skeletal structures and locomotion made it possible for early amphibians to explore the abundant food sources on land involving plants and early land arthropods.

In addition to skeletal and locomotion advancements, gas exchange surfaces evolved that would utilize oxygen without dehydrating in air. Early lobe finned fishes that lived in shallow estuaries swallowed air and oxygen that was extracted in the gut. Lungs evolved from outpocketing of the gut in the lobe finned fishes as well as utilizing highly vascular moist outer skin surfaces. From these lobe finned fishes evolved the amphibians. Since the skin was necessarily a moist surface (as all gas exchange surfaces), these early and present day amphibians were restricted to living in or near water.