Australia: The Land Where Time Began

A biography of the Australian continent 

Biogeography of Australia – Northwestern Australia

Among the clades of southern Australia there are many that are endemics, distinctive and isolated, though in northern Australia plants and animal are often allied with groups from Asia, which has often been attributed to recent immigration from Asia to Australia. Camaenidae .lat., a family of land snails with a distribution from Asia to Australia is a typical example. It is the most diverse family of snails in Australia with 80 genera and 400 species. In both rainforest and desert these snails are widespread, though as the group is a northern one, it is absent from southwestern Australia and Tasmania. According to the traditional model for the Australian contingent it is suggested that dispersal occurred in the Miocene from a Laurasian centre of origin (Hugall & Stanisic, 2011), and this model depends on an evolutionary clock and interprets a basal, paraphyletic group’s locality as a centre of origin. Heads¹ suggests it is also possible that rather than invading Australia by dispersal the Camaenidae ‘invaded’ the region by evolving there. If this is indeed the case, the family’s geographic connections with related groups in Southeast Asia and elsewhere would therefore represent ancestral distributions, hence reflecting aspects of former geography. In northwestern Australia there are several groups that have been interpreted in this way and in northwestern Australia examples of camaenid distribution are cited in what follows.

Subterranean endemism - Hamersley Range (Pilbara Region) and Tethyan connections

The discovery of a diverse subterranean fauna in the northwest of the continent is said by Heads¹ to be one of the most interesting developments in Australian biogeography (Eberhard et al., 2005; Humphreys, 2008). This fauna is comprised of a wide range of groups, which includes fish, though the fauna is best known for its wide diversity of unusual Crustacea. Stygobionts make up many of these subterranean clades found in groundwater. Species that breathe air that are found in groundwater are called troglobionts. Cape Range and Barrow Island were the first places where this fauna was found, then further inland at the Hamersley Range, as well as other parts of the Pilbara region. This is the area where most of the iron ore is mined, especially in and around the Hamersley Range. Above and below ground the local flora and fauna are important for understanding the evolutionary history of Australia, and it is the groundwater abstraction and the dewatering of aquifers that takes place before mining begins that is threatening the existence of the stygofauna.

A striking feature of this stygobiont fauna, apart from its unexpected and wide diversity, is the high proportion of species that have affinities with Tethyan or Gondwanan species (Eberhard et al., 2005). Coastal caves of the Mediterranean, the Canary Islands and the Caribbean, are the places where the Tethyan clades that are sister groups of the Australian clades are found. The ‘full Tethyan track’ is the name often given to the distribution that extends from Australia to the Caribbean via the Mediterranean by biologists working on the fauna (Jaume et al., 2001; Jaume, 2008; Humphreys et al., 2009; Karanovich & Eberhard, 2009).

Sandstone or limestone aquifers that are anchialine – having a subterranean connection to the sea – are the habitats of the stygobionts of the northwest. These aquifers have been described as underground estuaries; they are salinity-stratified and affected by marine tides. The origin of the cave fauna have been explained (Page et al., 2008) as being by a process by which a local marine species is stranded in a new, terrestrial habitat as it emerges, the process occurring either as a result of uplift or marine regression, and for stygobiotic faunas is one of the most significant causes of vicariance (Craw et al., 1999; Culver & Pipan, 2009).

Heads, Michael, 2014, Biogeography of Australasia: A Molecular Analysis, Cambridge University Press