One of the best-known examples for supposed evolutionary transitions is the step from water to land. From the evolutionary point of view this step must have occurred several times, to plants, to invertebrates (such as arthropods) and to vertebrates (animals with an endoskeleton and a spine). The latter group is the one of interest. To simplify matters, this step is known as the transition from fish to amphibian.
During this transition serious changes must have occurred: Main concerns are the ability to bear the body (moving forward on firm ground is completely different to moving in water, affecting spine and extremities), the method of locomotion (new function of extremities: moving forward instead of steering), the development of a neck to improve the head’s mobility (having consequences for the pectoral girdle, the linkage of head and spine and the muscular system), a more solid head (in contrast, flexible connections of the parts of the skull are essential for gill breathing), ingestion, respiration, water supply, sensory organs (e.g. sound transmission, eyes: different refractive index) and reproduction. According to evolutionary theory such a process can only happen step by step. Thus, the question arises whether it is possible for those substantial changes to take place in small steps at all, while the former functions are being maintained.
The terrestrial vertebrates are called tetrapods with their characteristic feature being expressed in their name: They are vertebrates having replaced their former fins by four legs. In addition the legs show a characteristic skeleton structure (fig. 2).
As a result of their skeleton structure the lobe-finned fish are the most eligible ancestors of tetrapods. Lobe-finned fish are fish with strong fleshy fins with a skeleton structure similar to that of tetrapods (fig. 16). Though fringe-finned fish or crossopterygians as the famous Latimeria belong to this class, Latimeria is to be ruled out as direct ancestor of tetrapods due to a number of unsuitable features. At present, Eusthenopteron (fig. 3) and Panderichthys (fig. 9) are regarded as the best suitable species. Ichthyostega (fig. 12) found in the Late Devonian which is said to have derived from lobe-finned fish such as Eusthenopteron is considered the first primitive amphibian.
Apart from the species stated earlier Devonian Acanthostega (fig. 15) is to be especially mentioned. This fossil had eight fingers, it was definitely a tetrapod but it was also adapted to a permanent aquatic life in many respects and did not walk on land. Moreover, there are further fossils being considered as Devonian tetrapods, although no extremities have been found (in most cases only skull parts are known). They are classified as tetrapods with regard to the similarities in the structure of the skull with definite tetrapods or due to other indirect hints. Strictly speaking, the classification must therefore remain uncertain. This species will only be discussed in advanced part.
Since the geographic distribution was substantial, a multiple, independent development from various fish ancestors is discussed.