Australia: The Land Where Time Began

A biography of the Australian continent 

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).

Sources & Further reading

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