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Australia: The Land Where Time Began |
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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.
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Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |