Earliest Tetrapods, Origin from Marine Environment

Australia: The Land Where Time Began

Earliest Tetrapods, Origin from Marine Environment

The earliest known fossil tracks of tetrapods date from the Middle Devonian, the Eifelian, about 397 Ma, and their earliest known body fossils date to the Upper Devonian, the Frasnian, 385-375 Ma. It is generally considered that tetrapods colonised the land, one of the major events in the history of life, in the Carboniferous after 359 Ma. The authors¹ carried out an analysis of tetrapod evolution by using molecular data which consisted of 13 proteins from 17 species and different palaeontological data. They performed the analysis on the molecular data with the TreeSAAP program, the results being analysed to determine any implications of the palaeontological data that had been collected. Tetrapods have been shown by the results to have evolved from marine environments at times of higher atmospheric oxygen levels. The authors¹ suggest changes in the environmental conditions had a major role in the evolution of tetrapods. The results of the analysis showed that this evolution took place at about 416-397 Ma during the Early Devonian, which is at odds with what was previous believed. Various environmental factors, such as sea levels, oxygen rate [levels?], and biotic factors, such as biodiversity of arthropods and coral reefs, also support this idea. The molecular data also strongly support lungfish as the closest living relative of tetrapods.

Geobiological data

The conclusion that they originated before the Middle Devonian, probably in the Early Devonian (Young, 2006; Niedzwiedzki et al., 2010; Warren, Jupp & Bolton, 1986), is arrived at by most modern interpretations concerning the origin of tetrapods and their representatives from the Devonian. This is the reason the trackway that was found in the courtyard of the Glenisla Homestead in the Grampian Mountains, western Victoria, Australia (Young, 2006; Niedzwiedzki et al., 2010; Warren, Jupp & Bolton, 1986), is important (Young, 2006; Blieck, Clément & Streel, 2010). The authors¹ say that because of its age it presents a physical argument for an origin in the Early Devonian for tetrapods. Therefore the higher atmospheric oxygen levels from the GEOCARBSULF model and the revised model (Berner, 2009) at about 416-397 Ma in the Early Devonian appear to coincide with the “elpistosetgid-tetrapod changeover” (sensu (Niedzwiedzki et al., 2010)). There also appears to be an increase in the number of orders of terrestrial arthropods, biodiversity of autotrophic reefs, the size and genera of marine invertebrates at this same time (Ward, Labandeira, Laurin & Berner, 2006; Alroy, 2008; Novack-Gottshall, 2008; Joachimski, 2009). The authors¹ concede that coincidence does not necessarily imply a correlation; though they suggest that these events were indeed related. Another important finding resulting from this analysis is the re-confirmation of the earliest Carboniferous Romer’s Gap as an interval of low atmospheric oxygen levels (Ward, Labandeira, Laurin & Berner, 2006; Alroy, 2008; Novack-Gottshall, 2008; Joachimski, 2009), though a higher atmospheric oxygen level is suggested by the revised GEOCARBSULF model (Berner, 209) than the original does. The Early Carboniferous genus-level biodiversity of marine invertebrates is below 400 genera (Alroy et al., 2008), compared to the genus-level biodiversity of invertebrates in the Early Devonian of 585 genera. This lower biodiversity in the Early Carboniferous fits with the lower oxygen levels at that time, though better insights and improved clarity to the problem would be provided by further analysis.

The authors¹ stress an important point is the influence the fossil record has, the likelihood of preservation, differences in palaeoenvironments, the abundance of field studies, etc. that has an influence on the diversity of fossils. Also, according to the auithors¹ there are apparently many palaeontologists, and the lead author of the paper includes himself, who had wrong impressions of the fossil diversity through time. The example given here is the number of general of reef builders which is higher in the Early Devonian than in the remainder of the Devonian. The authors¹ say it is often claimed that the Givetian-Frasnian of the late Middle Devonian to the early Late Devonian is the most important period of development of coral reefs, a time when huge systems of reefs, comparable to the Barrier Reef of Australia at the present, were developed in, for example, the Canadian Arctic or Western Australia (Copper, 1994; Copper, 2002; Kiessling, Flügel & Golonka, 1999). It is actually the case that most of the global recent evaluations of the diversity of reefs have found that the highest mean reef thickness and diversity was reached in the Early Devonian (Joachimski et al., 2009; Flügel & Kiessling, 2002). If such a reappraisal for a single group of organisms was generalised to all aquatic and terrestrial taxa of the Palaeozoic, a picture that was very different from the classical one would be depicted by, e.g., the ‘Sepkoski Curve’ {(Sepkoski Jr, 1981), and later critical re-evaluations such as, e.g., (Alroy et al., 2008; McGowan & Smith, 2008)}.

Conclusions

The authors¹ conclude that the co-occurrence of a series of bio-events and physical properties of the oceans during the Early Devonian is more than a coincidence, rather reflecting a global rearrangement of the biosphere. It is suggested that it is likely there was an increase in atmospheric oxygen in the Early Devonian. The emergence of tetrapods in shallow marine environments would have been triggered by this, as “walking” on the bottom, or in the water would have been an advantage in terms of energy expenditure and predation over other fish. An ideal environment for the growth of young tetrapods in the shallow water would have been provided as a result of fewer predators being present. Another conclusion of the molecular studies was that lungfish are much more closely related to tetrapods than they are to coelacanths, such a result not being in contradiction with most phylogenetic analyses that are morphology-based, though the authors¹ point out that it would be difficult to pinpoint and show that these changes suggested by the phylogenetic analysis actually occurred in the Early Devonian, it remains very much a possibility that some of the changes occurred in the Early Devonian. Scenarios such as those described above (Dahl et al., 2008; Payne et al., 2009; Webby, Paris, Droser & Percival. 2004; Servais et al., 2010; Klug et al., 2010) represent possible solutions to the relation of global environmental factors and the development of life on Earth. The authors¹ say this conclusion is applicable to most, if not all, geobiological scenarios throughout the history of the Earth.

See Amphibians

See Grampian Ranges Footprints

Sources & Further reading