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Australia: The Land Where Time Began |
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The Origin of the
Tetrapod Stem Group – Climate Change in The Devonian and breathing The stem group of
tetrapods diversified in
the Middle to Late
Devonian (Givetian - Famennian stages) about 285-365 Ma. According
to Clack1 a reasonable consensus has been reached concerning
the relationships between the known taxa that represent this radiation
which allows the interpretation of the order of appearance of characters
of the tetrapods to be possible. What is interpreted as a progressive
increase in the spiracular chamber and its opening to the outside world
is seen in the fish that are immediately related to the earliest limbed
tetrapods, which is inferred here to be an increased capacity for
breathing air. According to Clack most early bony
fish are believed to have had
lungs which most likely were ventilated by gulping air which is
suggested to have possibly brought about the facultative capacity for
breathing air from the atmosphere, the tetrapod stem group then
exploiting it its greatest degree. Not only freshwater forms have these
adaptations, it has been shown that they were also present in estuarine
and marginal marine forms. It is suggested by estimates of atmospheric
oxygen levels during the Givetian and Frasnian periods oxygen levels
were unprecedentedly low, while at the same time the diversification of
plants was at its most rapid level, which was contributing to the
changes of the character of the landscape and, via soils, soluble
nutrients and plant matter that was decaying, to the development of
anoxia in all water systems. In at least 2 lobed-finned groups, the
co-occurrence of these events on a global scale may explain the
evolution of the adaptations for breathing air that contributed directly
to the rise of the tetrapod stem group. Low levels of atmospheric oxygen
is not considered to be a causal factor in the lack of fossils that
document the early evolution of tetrapods in the Early
Carboniferous,
in contrast to recent studies.
Introduction Knowledge of the “fish
- tetrapod transition” – the diversification of the tetrapod stem
group, the sequence and timing of the events have been augmented in
recent years by finds of tetrapods from the Devonian (Ahlberg, 1991a,
1995; Ahlberg et al., 1994;
Daeschler et al., 1994;
Ahlberg & Clack, 1998; Lebedev, 2004) and their immediate stem group
(Daeschler et al., 2006;
Shubin et al., 2006). A
result of this is that attempts can now be made to answer the questions
such as where and when tetrapods (vertebrates with legs) evolved, and
the order in which tetrapod characters arose, and how their evolution
took place (Clack, 2002a, 2005). Knowledge of the environments they
inhabited has also increased (Lebedev, 2004; Clack, 2006). A question
that is more difficult to answer, though is commonly asked, is why the
transitional forms made the first move towards life on land. Attempts to
answer this question have recently involved reconstructions of the
composition of the atmosphere in the Devonian, and the global
temperatures and the distribution of continents at that time (Berner,
1999, 2006; Scotese, 2002; Berner et
al., 2003). Combination of
these with recent assessments of vegetation cover, the effect of this
cover on rates of sedimentation, the production and erosion of soils
(Algeo et al., 1995, 2001;
Algeo & Scheckler, 1998), and when the invertebrate fauna are taken into
account (Shear & Selden, 2001), has led to the generation of some new
suggestions that might explain some of the events leading to the
evolution of air breathing and ultimately to tetrapods that lived on the
land. There are 2 aspects of the morphology of tetrapods
[i.e., stem plus crown groups of tetrapods (Ahlberg, 1991b)] that
includes the early limbed forms, are of particular significance, the
region of the spiracle and the hyoid arch, and the pectoral limb. It is
also possible that changes seen in these regions among the
tetrapodomorphs could be related. The spiracular region is interpreted
as being associated intimately with breathing; It is known that the
spiracular chamber became enlarged in the tetrapod stem group, and it
has been suggested that the increasing size and complexity of the
pectoral fin/limb skeleton function as support for the head and body
anterior end as the animals emerged partly from the water (Shubin et
al., 2004, 2006). In this article Clack1 reviews the
spiracular region and the hyoid arch across the basal tetrapodomorph
fish to limbed tetrapod transition with the aim of showing the
increasing capacity of the spiracular chamber and increasing breadth of
the spiracular notch apparent among them, suggesting the best
explanation could be as an elaboration of an air-breathing mechanism.
Information from other studies of environments, atmospheric composition,
soils, sediments, plants, and invertebrates which suggests a new
scenario for the evolution of air-breathing in the tetrapod stem group
and related forms, such as the dipnoans, in the Devonian. The subsequent
tetrapod evolution in the Carboniferous, which was a time when
air-breathing became the major mode of breathing, is also considered,
though to a lesser extent.
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Fish to Amphibians |
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Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |