The earliest tetrapods were found in the sediments from the Evonian Period (374 million to 354 million years ago); except genus Salamandridae that was found in the ocean outside of Russia , most sediments are located in fresh water bodies. The most well-known tetrapod are genera Ichthyostega and genus Acanthostega; the two were found in eastern Greenland and can be traced back to 365 million years ago. Other species are slightly older and are more widely spread to other regions, including genus Elginerpeton from Scotland , genus Hynerpeton from northeastern USA , genus Metaxygnathus from southeastern Australia , and genus Obruchevichthys from eastern Europe. The distribution at that time crossed both sides of the equator within the tropical region which ranged from present time Australia to northeastern US through Asia that was moist and warm at the time. Greenland may not seem to be a place that the early tetrapod flourished, but its location and the climate has changed significantly since the Evonian Period (417 million to 354 million years ago). It was when the evidence of Pangaea formed in the early Jurassic Period (approximately 417 million years ago) (when all continents were joined) was found that the rapid expansion of the earliest tetrapods was supported. Records even show that they spread to Antarctica during the early Triassic period (approximately 230 million years ago).

The ancestors of the early tetrapod were bony fish (class Osteichthyes) from class Sarcopterygii (fish with fleshy fin). Unlike most bony fish that has cartilaginous supported fins (such as class Actinopterygii, ray-finned fish, and including most of the fish today), the fins of Sarcopterygii have portions of bone that are similar to the appendages of terrestrial animals.

In addition, the Sarcopterygii have developed the “lung” for tetrapod (although they have little relevance to a group of lung fish in the class Dipneusti); some may even have internal nares (nostrils), which allow them to bring air to the lungs when they close their mouths or expose their external nares above water. Internal nares are the characteristic of terrestrial vertebrates; most external nares on fish provide sensual functions only.

Genus Ichthyostega is very similar to an extinct fish – genus Eusthenopteron (family Osteolepidae); both were found in the sediments in the Devonian Period from Quebec , Canada . The two fishes have both lungs and internal nares, and have the features found in some of the Osteolepiformes and early tetrapods: 1) a skull that is horizontally divided into anterior and posterior portions; 2) teeth with enamel coatings that curves inwards which forms a complex appearance looked at from a cross-section. This extinct fish was the closest in relationship to tetrapods, but not known to most people is that there were two genera categorized as family Osteolepiformes: genus Elpistostege in Quebec , Canada , and genus Panderichthys (family Panderichthyidae) in Eastern Europe .

There is no doubt that genus Ichthyostega is a tetrapod but has retained some of the same characteristics of fish. Among these features, the operculum – the residual bony flap connecting the gill flap and the cheek, and the caudal fin, are supported by radial bones. However, the appendages and girdle structure in the Ichthyostega has completely evolved to the state of early tetrapod animals. No records of the earliest terrestrial vertebrates – the Ichthyostegas, including the early relatives of genus Ichthyostega, were found; they would have to be searched in earlier sediments. Genus Panderichthys is the closest relative of tetrapods that proliferated between 380 million to 375 million years ago; but, the earliest tetrapods appeared within 5 to 10 million years afterwards. Due to their proximity to the transitional period between the fish and the tetrapods, genera Elginerpeton and Obruchevichthys were mistakenly identified as the major branch of the tetrapods: they belonged to earlier tetrapods and not amphibians.