Australia: The Land Where Time Began

A biography of the Australian continent 

Stygobites

These animals, a subsection of the fauna of the hyporheic zone, have evolved to live exclusively in groundwater. Named after the River Styx, which in Ancient Greek mythology, was the river across which the souls of the dead were ferried to Hades. Stygobites are the small creatures, crustaceans and insects (water beetles), that live in underground water in limestone areas which are riddled with holes, caves, lakes and rivers. Stygobites have been known about in karst areas of North America and Europe for a long time, but have only recently been found in many limestone deposits, isolated from each other, under the West Australian desert. 

In Europe and North America there are extensive areas of karst, limestone deposits that are been carved out by rainwater seeping down from the surface and the result is limestone that is riddled with holes, lakes and rivers. It is believed that the ancestors of the present-day stygobites moved down to the underground water about 1.9 million years ago to escape the rigours of the Ice Ages. 

In Australia, the formation of the limestone deposits resulted from a different mechanism from those in karst areas in North America and Europe. In Australia the limestone deposits formed along the courses of dried-up river beds between 37 and 30 million years ago. Underground water flowing beneath these dry river beds came close enough to the surface in some places, usually upstream of salt lakes, to evaporate, leaving behind any dissolved minerals they carried, in this case limestone, to form calcretes, limestone deposits on a much smaller scale than those in the Northern Hemisphere karst areas. A later wet phase brought the return of the rivers to the ancient river beds which allowed water to again seep down, this time to carve out the hollows and crevices in the limestone that still hold water under the now sun-baked deserts above. There are more than 200 known major calcretes in Western Australia and many more smaller one, with concentrations in the Yilgarn and Pilbara regions, 2 of the first blocks of land in the formation of the Australian continent. 

Each calcrete has its own unique fauna. A result of this isolation of the faunas of the different calcretes is that there are potentially huge numbers of the species still to be discovered. 2 species of a primitive group of crustaceans, Spelaeogriphacea, have been found in the Pilbara region. The only other species of this group are 1 found in a cave in Table Mountain, South Africa, and a species in caves in the Matto Grosso, western Brazil. Their distribution indicates that they were separated when Gondwana broke up and the southern continents drifted apart. The earliest known ancestors of the Spelaeogriphacea were marine organisms in the Tethys Sea.

Bathynellids are another group of primitive groundwater crustaceans that have survived in the calcretes. This group originated in Pangaea and they still have a world-wide distribution of organisms that live among sand grains in freshwater aquifers. The characteristic that make the Australian members of the group unique is their size. All others are about 1/3 mm long, whereas the Australian Bathynellids are 8 mm long and live free in highly saline groundwater. Their appearance is also very close to that of the hypothetical ancestral form.

Links

  1. Effects of spates on the vertical distribution of the interstitial community

Sources & Further reading

Mary E. White, Earth Alive, From Microbes to a Living Planet, Rosenberg Publishing Pty. Ltd., 2003
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                                                                                           Author: M.H.Monroe  Email: admin@austhrutime.com     Sources & Further reading