Australia: The Land Where Time Began

A biography of the Australian continent 

Tetrapods – the Environments of Early Tetrapods

There was a period of great upheaval in the landscape of Laurussia in the middle Late Devonian, around the Frasnian-Famennian time (Blieck et al., 2007), a time when major crustal blocks of Laurussia, Gondwana, Kazakhstan, and Siberia were in the process of colliding. Around the world mountains were rising, with wide areas of continents being uplifted and oceans being closed. Both continental and marine environments were affected by these events, and these events may also have contributed to the continuing global cooling and in addition increased plant productivity resulted in drawdown of atmospheric CO₂. This is believed to be the background to the first major radiation of tetrapods, adding new perspectives to the consideration of the environments inhabited by the early tetrapods.

It the past it had been assumed that the first lobe-finned fishes, and hence the first tetrapods, had inhabited freshwater environments, the tetrapods emerging on to the land from rivers and swamps. Clack¹ says this belief arose because the body forms of the first examples to be found were those of the best-known genera from the locality in East Greenland. The limbs and digits of Acanthostega are believed to have been adapted for use in the waters of swamps where fins would probably been a disadvantage, and this animal appears to have been adapted to be more or less permanently aquatic. There are other lines of evidence that indicate that tetrapods arose in fresh water, such as the fact that at the present all known amphibians can live only in fresh water. A temnospondyl from the Triassic is the only known amphibian in the fossil record that has been indisputably known to have lived in a marine environment. Also, it was always assumed that the ancestral lobe-fined fishes were also adapted to live in fresh water that had not evolved the physiological adaptations that would have allowed them to live in saltwater.

Many early scenarios that were developed to explain how the evolution of tetrapods might have taken place were based on the assumption of a freshwater origin for tetrapods, the main such hypothesis was the drying of pools hypothesis. Over the past 20 years these questions have been re-examined with the result that the assumption of a freshwater origin for tetrapods has been challenged strongly. Many of the more recent finds of early tetrapods are indicated to have been from tidal, marginal marine or brackish water localities. According to Clack¹ these findings militate strongly against the suggestion by some authors (e.g., by Graham & Lee, 2004) that the intertidal environment would have been have been quite unsuitable for tetrapods making it unlikely for the origin of tetrapods to have occurred in such an environment, based on the intertidal fishes of the present.

It now seems likely that some of the earliest lobe-finned fishes, such as lungfish, were actually marine species (Campbell & Barwick, 1987) that moved to freshwater localities only in the Late Devonian. It seems that many of the early lobe-finned fishes may have been euryhaline, or possibly anadromous or catadromous, i.e., they could move between freshwater and saltwater at different stages in their lives, though the direction of this movement and the life stage at which it occurred is difficult to establish, as some species are found in both kinds of palaeoenvironments, such as Rhabdoderma, a coelacanth (Forey, 1981). Clack¹ suggests that as several Palaeozoic sharks have been shown to have occurred in fresh water, they are unreliable as evidence of salt concentration. Evidence from modern fish has been increasing that suggests they adjust their salt tolerance from marine to freshwater environments, and vice versa, and it is actually not difficult for many vertebrate types, so maybe tetrapods could also do it.

According to Clack¹ it is very difficult to determine if a deposit was laid down under freshwater or marine conditions, evidence used to identify freshwater deposits is partly negative, consisting of the absence of fossil marine organisms. Examples of fossils that would be looked for are echinoderms, none of which are known from freshwater deposits or freshwater environments at the present; conodonts, which are usually found as very small toothlike elements that previously were believed to be useful only in relation with strata correlation, though they are now regarded as the remains of some primitive vertebrates; acritarchs, which are microfossils that are formed from the shells of very small marine organisms; or sedentary polychaete worm shells. Inferences from vertebrates are now regarded as the most likely to lead to circular reasoning, though previously they were widely used as indicators of saline conditions. Geochemical analysis, using isotopic ratios of elements such as oxygen and carbon, and the amounts of boron, that have been shown to be characteristically different in freshwater and marine deposits,  has provided more independent evidence. The fossilised bones of some vertebrates have also been found to contain isotopic ratios of strontium that can be used in the same way (Schmitz et al., 1991). Clues to the environmental origins of fossils can also be added to by an understanding of the sedimentary sequence in the deposits the fossils were recovered from.

Miguasha, a deposit dating from the Devonian that has been well studied, is a locality that can be suggested as a locality that had previously been assumed to have originated as a freshwater environment that is now regarded as being either brackish or marginal marine. The original assumption had been based on the high proportion of lobe-finned fishes in the fauna from this deposit, as well as the lack of evidence of sharks, or any other marine vertebrates, having been present.

Looking at the localities from which early tetrapods have been recovered, such as the East Greenland deposits that have produced the tetrapods from the Devonian that were the first to be discovered, were regarded by almost all as being of freshwater origin. These deposits have been interpreted as laid down in a great river basin that was surrounded by mountains and bounded by faults, and in which evidence of a marine influence has never been found. The result of this first discovery of the East Greenland faunas was that they had a very significant influence on the ideas concerning the tetrapod origins being in freshwater environments, which accorded well with the ideas that were current at that time. The discoveries of the remains of Ichthyostega have also contributed to the idea that tetrapods arose in freshwater environments.

The fragmentary remains of tetrapods that have been found in Australia have also been interpreted as being laid down in freshwater environments, as are other localities from the Devonian. The fossils recovered in Australia were found in localities that are continental – being deposited in small lakes and rivers that were far from the coast and any chance of marine influence. In the USA tetrapods of Famennian age have been recovered from the Catskill Basin associated with sedimentary sequences that represent a large river valley and its meandering channels that are similar in many ways to the environments in which the faunas from East Greenland lived (Daeschler et al., 1994).

The locality of Tula, by contrast, that had a complement of fishes and tetrapods, has also produced stromatolites, which suggest strongly that there was at least some marine influence, and the site is suggested by the geological conditions to have been rather far from the nearest land (Lebedev & Clack, 1993). Estuarine localities around the edges of the Old Red Sandstone Continent are sites where Elpistostege, Tiktaalik and Panderichthys were recovered (Daeschler et al., 2006). Panderichthys, Obruchevichthys and Ventastega were recovered from Baltic sites dating to the Frasnian that have been interpreted as being marginal localities at the edge of a large marine basin on the east side of the Old Red Sandstone Continent (Kurss, 1992; Lukšervičs & Zupins, 2004). Clack¹ suggests that animals living in such estuarine or brackish water would have been well situated to move around coastlines and river deltas.

The most recent evidence concerning animals, such as Tulerpeton and Ventastega, is that they appear to have not lived in habitats that were purely fresh water, but in brackish or lagoonal environments that were influenced by the sea. The intertidal zone in ecosystems of the present is ecologically the richest in terms of niches and diversity of forms, covering the species to the phylum level, of any that exists. According to Clack¹ this results from the constantly changing conditions, and they are exposed to the tides and to the atmosphere, factors that produce environmental stresses which apply great evolutionary pressure in the animals living in that environment. The evolution of air-breathing vertebrates of the present was strongly influenced by these conditions, examples being such teleosts as mudskippers. Clack¹ suggests that therefore it would not be surprising if such diversity existed among Devonian age intertidal animals which evolved into terrestrial air-breathing vertebrates.

One thing is obvious when the sites from which tetrapods have been found around the world are looked at is that they were scattered around the world in places remote from each other, often on different continents, even in  the Devonian. If the early tetrapods could not live in marine environments it is difficult to see how they could have reached such widely dispersed continents (Thompson, 1980). Alternatively, tetrapods could have arisen from lobe-finned fishes that were originally euryhaline, subsequently losing their salt tolerance, though Clack¹ suggest this appears even more unlikely, and is counteracted by the degree of detailed similarity that exists at present in the tetrapods that are now known from around the world. The earliest tetrapods might have been living in the shallow swampy waters of marine lagoons that were populated by emergent plants that were increasing is size and diversity, and might have provided shelter for the larvae of the evolving amphibians.

Sources & Further reading

1.      Clack, JA, (2012). "Gaining Ground: The origin and evolution of tetrapods", Indiana University Press