Evolution: Palaeontology

Basic Ideas: Origin of tetrapods

Contents

This chapter deals with fossil findings within the evolutionary transition ranging from fish to tetrapods. Though new findings reduce the gap between fish and tetrapods, the appropriate transition forms are still missing.

A famous transition

A new view of the matter

Panderichthys

Ichthyostega

Acanthostega

The tale of shrinking waterholes

Outlook on Carboniferous tetrapods

A famous transition

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.

A new view of the matter

Until recently the transition to tetrapods was regarded equivalent to the transition from aquatic to a (partial) terrestrial life. The leap forward from Eusthenopteron to Ichthyostega was therefore considerable. Based on findings of the last 25 years the picture has changed in some aspects. Becoming a tetrapod is no longer put on the same level with the transition to terrestrial life. On the contrary, there are strong arguments based on prerequisites of the evolutionary theory that quadrupedality had been attained in the aquatic environment. In other words: some Devonian tetrapods may have lived in an aquatic environment, presumably along the water’s edge, where there is a dense vegetation and where the possession of a polydactylic limb was suitable. The transition from aquatic to terrestrial life is therefore subdivided into two, meanwhile into three partial steps (fig. 7): 1. acquiring of fingers (respectively the typical tetrapod extremity), 2. ability to walk, 3. step onto land.

Panderichthys

This species, which is about 1 m long, with a flattened skull is regarded as a fish resembling tetrapods (fig. 9). In some characteristics this finding reduces the gap between Eusthenopteron and Ichthyostega. Characteristics of tetrapods are mainly some features in the skull area, e.g. the cranium is more stable than that of the more primitive species. In many aspects the construction of the skull resembles the one of Eusthenopteron, it is therefore of fish kind. In the skull structure there is a significant difference to former tetrapods.

The missing of dorsal and anal fins is striking. These fins disappeared out of a sudden. The paired fins also do not show any characteristics of terrestrial vertebrates. The skeleton parts of the fins are broad and flat. Their resemblance to earlier tetrapods is even less than the one of Eusthenopteron (fig. 3). The tail fin on the other hand is similar to those of early tetrapods.

Since Panderichthys was found in older strata than Eusthenopteron it does not fit within the series of strata into the sequence Eusthenopteron - tetrapods.

Due to the significant differences between the species most similar to tetrapods, i.e. Panderichthys, and the first real tetrapods Panderichthys can hardly be regarded as acceptable transition form between fish and tetrapods.

First tetrapods. The late Devonian species Elginerpeton, Obruchevichthys and Livoniana are classified as first tetrapods. However, there are only indirect hints for this classification since the decisive feature ﹣the structure of the extremities ﹣is unknown, mainly fragments of the skulls have been handed down as fossils. Based on strongly specialized features of the head region the three species are not included in a line leading to the later tetrapods, instead they are interpreted as an earlier side branch. It is still remarkable, that these species of the early Devonian show a greater variety in their skulls as the species occurring in the subsequent strata of the latest late Devonian.

Ichthyostega

Ichthyostega from the late Devonian is the best known Devonian tetrapod and was introduced to the history of paleontology as the “first terrestrial vertebrate”. This species, which measured 1,5 m, was composed of a mosaic of features both of fish and tetrapods. The typical extremity of a tetrapod showing 7 instead of 5 fingers (which is not without controversy) is striking. However, the hind legs resemble more a paddle than a leg and they show remarkable similarities to the skeleton of a seal or a river dolphin. The tail is built like a fish tail and is obviously suited for swimming. It seems that Ichthyostega did not perform a real shore leave.

Some striking unique features do not fit into a transition position of Ichthyostega such as the solid inflexible chest with overlapping ribs (whose function is not clear) and the extremely narrow construction of the skull. The front leg was much longer than the hind leg, which is rather untypical for tetrapods, the conditions being quite opposite, since the hind legs give the strength while walking. With regard to all known features Ichthyostega cannot be interpreted as transition form between fish and amphibians.

Acanthostega

Though the smaller Acanthostega (fig. 15) was found in the same strata as Ichthyostega and has the typical extremity of a tetrapod (with 8 fingers) there are still differences in many features. Despite having fingers Acanthostega was (just as Ichthyostega) almost unsuited for a terrestrial life and therefore it must have lived entirely in an aquatic environment. Hints for a permanent aquatic life are e.g. an oustanding colateral line organ on the head, the gill skeleton and spine of a fish. The front leg appears to be laterally directed. Proportions and shapes of the legs are unique among the tetrapods, but they also show the proportions of lobe-finned fish as Eusthenopteron; they were more suited for paddling than for carrying the weight of the body.

Presumabely Acanthostega lived in aquatic regions near the water-edge with a dense vegetation where a tetrapod extremity was suitable although the animal lived in the water. As in the case of Ichthyostega one may conclude that the total combination of features of Acanthostega does not fit well into a transition position between fish and amphibians. Still some of the singular features fit into a transition position.

There are more species of the same age which are only partially known (Hynerpeton, Tulerpeton, Ventastega, Densignathus, Metaxygnathus, Sinostega), they are shortly discussed in advanced part.
 They do not contribute essential information to the issue of the origination of tetrapods.

General classification of late Devonian tetrapods. All in all late Devonian tetrapods show various mosaics of features demanding the supposition of many convergences (independent development of the same features) and reversions (return to an earlier form of a feature) for genealogical descriptions.

It is striking that all late Devonian forms have been deposited along with fish. These forms may not even account for the conquering of land. The tetrapod-like features may also be explained by a specialized life near a coastline covered with dense vegetation.

The tale of shrinking waterholes

Many textbooks present the idea that the tetrapod leg and the conquering of land happened because periods of draughts in the Devonian led to the draining of waterholes. Thus, a selection pressure occurred to make sure that the next life saving waterhole was reached—a contradictable scenario: The extremity required for terrestrial life is supposed to have developed to grant an aquatic life. Apart from this paradoxon the scenario could hardly be verified. Nowadays it is regarded not plausible, due to the supposed mode of life of Acanthostega (an exclusively aquatic life with a tetrapod extremity).

In addition most environments were not threatened by draught during the Devonian, the selection pressure mentioned before didn’t play a particular role worldwide.

According to recent scenarios the tetrapod extremity is said to have developed in the aquatic environment, the earliest species with fingers had a permanent aquatic life as far as we know today. But why should legs have developed in an aquatic environment? Isn’t that development to be prevented by selection? The following dilemma arises: There is strong evidence that the tetrapod legs have developed in an aquatic environment but the conditions for selection are contradicting.

However, a return to an aquatic life as it has been discussed repeatedly is not logical regarding the conditions for selection. Having shortly escaped from competitors in water, taking efforts to adapt to a semi-terrestrial life, thus losing the equipment for aquatic life: in this situation early tetrapods are supposed to manage new competition in the aquatic environment anew?

An alternative scenario: Alternatively the Devonian and Carboniferous fossil series might reveal an ecological series or ecological bordering. To support this alternative geological findings must be taken into consideration, e.g. it should be proven that the relevant Devonian strata were or could be sedimented within short intervals. In many cases this is plausible. Species such as Panderichthys, Ichthyostega and Acanthostega might fit into the transition area, but there may also be alternative interpretations: Their mosaic of characteristics corresponds to a specific way of life in shallow water along a coastline with dense vegetation and thus can be ecologically explained.

Outlook on Carboniferous tetrapods

Finally let’s have an outlook on the tetrapods of the Carboniferous. Following the late Devonian there is a remarkable gap of 20 million years in fossil documentation of tetrapods, called “Romer’s gap” after the famous vertebrate paleontologist Alfred. S. Romer (fig. 220). Only recently the species Pederpes and some other fossils has been described as the first fossils to enter this gap. (According to the combination of characteristics Pederpes fits only partially into the ideas of the evolution theory.)

It is striking that, starting from Visé (middle Earlier Carboniferous) suddenly a large variety of tetrapods is found (fig. 220), among them explicitly derived species such as Aistopoda (fig. 8). The variety of feature combinations does not allow for the reconstruction of a genealogic line. And the link between late Devonian and early Carboniferous species is not clear so far, therefore it is discussed whether the Late Devonian species might lead to a dead end. This would mean that from the evolution theoretic point of view the famous Ichthyostega and other Late Devonian species are not on their way to the later amphibians.

Translator: Isa Sefzick, 17.11.2012

Author: Reinhard Junker

 
© 2012