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Australia: The Land Where Time Began |
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Tetrapods – Their
Place of Origin According to Clack1 it is even more
difficult to determine where tetrapods evolved than it is for the time
period in which they arose, with 2 problems needing to be solved, the
geographical location and the type of environment in which they arose.
Geographical location
What geographical evidence there is shows that
tetrapods of the Frasnian were all present in central Laurasia, but they
had expanded much further eastwards by the Famennian, reaching into what
is now Australia and China. In the Late Devonian those continents were
located, according to most estimates, Australia near the equator and the
northern China block to the north, though this continental arrangement
is still subject to dispute. It remains uncertain whether either of
those blocks was attached to other continental landmasses. It has been
possible to infer, based on biogeographical evidence, in spite of the
poor state of the fossil record of tetrapods in the Frasnian, that in
the Middle to early Late Devonian of Laurasia there was a major
radiation of tetrapodomorphs (Ahlberg, 1995; Ahlberg et
al., 2000). The kind of
transitional form that is being found in the Laurasian region is
exemplified by
Livoniana from Latvia
(Ahlberg et al., 2000). Of
the new finds from this area most of the significant ones appear to slot
fairly easily into a stem lineage for tetrapods that is reasonably well
resolved, the near-tetrapods
Panderichthys, Tiktaalik
and
Obruchevichthys being
present in the Baltic sites and Russia (Ahlberg et
al., 2008).
The discovery in Poland of
trackways that appear to have been made by tetrapods does not conflict
with this suggestion. Most of the tetrapodomorphs found in Australia are
either far removed from the stem, such as rhizodonts like
Goologonia, or are
trisctichopterids that are rather derived. Reinvestigation of the
phylogenetic position of
Gogonasus has shown that
it is a more primitive form than may be suggested by some of its
characters that have been discovered recently (Snitting, 2008). In spite
of this it has been suggested (Young. 2006), based on redating of a
locality within Australia that contained the remains of tetrapods from
the Devonian, that it might possibly be considered that tetrapods arose
in Gondwana. Clack1 suggests this hypothesis might stand, if
these localities in Australia, as well as those in China, that contained
tetrapod remains were of Frasnian age instead of Famennian as they are
usually considered to be. A problem with this hypothesis that has been
pointed out by Clack1 is that it doesn’t take into account
that by the Frasnian of Scotland
Elginerpeton already had
a pelvic girdle that was robust. Also the Frasnian date of the
Australian locality has been disputed (Blieck et
al., 2010).
1.
Clack, JA,
(2012). "Gaining Ground: The
origin and evolution of tetrapods", Indiana University Press
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Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |