Australia: The Land Where Time Began

A biography of the Australian continent 

Mesozoic Seas – Geographic Extent

A former land to the west of Australia was proposed (Hooker, 1860) to explain the biogeographic patterns surrounding the Indian Ocean. It is now accepted that there was indeed former land to the west, south and east of Australia, a situation which lasted until the breakup of Gondwana. Marine transgressions in the Jurassic-Cretaceous, prior to the breakup of Gondwana, developed over much of central Australia, peaking at the end of the Early Cretaceous, in the Aptian-Albian. The Great Western Plateau, that forms the western half of the continent, was also divided by the sea at sea level maxima (Veevers, 1988). There was land where the sea is now, to the west, south and east of Australia, and sea where there is now land, in central Australia, in the geography of the mid-Cretaceous. Both the modern biota and the modern geography resulted from these landscape revolutions.

Through the Mesozoic large areas of all continents were covered by shallow seas, before and about the time Gondwana was breaking up. The Guarani Seaway in central South America, northern Argentina to southeastern Brazil, and the Sundance Seaway, in the Jurassic, and the Western Interior Seaway, in the Cretaceous, both in North America (Mexico and Alaska). Basins that are known best for trapping large volumes of oil were filled by the seas, as in North America, and in Guarani Aquifer, and in Australia, the Great Australian Basin. Opal fields associated with the Great Australian Basin (GAB), as well as the GAB itself, mark the site of the inland sea, during the Mesozoic in Australia, and the largest aquifer in the world, of 65,000 km³ of fresh water that is held in porous sandstone. The aquifers were capped by sediments deposited on the floor of the inland seas, and the water remained trapped in the GAB after the seas retreated.

The global seas reached highpoints in the Cretaceous that they haven’t achieved since. Sea level increases in the Cretaceous resulted from tectonic changes, and also a bout of global warming. The flooding that occurred in Australia was somewhat out of step with global sea level changes as a result of regional tectonic subsidence in Australia. At this time Australia was moving to the east above slabs that were subducting to the west, and decreasing angles of dip of the slabs led to widespread subsidence in eastern Australia (Mathews et al., 2011). As a result of this, by the Early Cretaceous the interior of Australia was dominated by the seas, before the global sea level peaked in the Late Cretaceous. As global sea levels were reaching a maximum level at about 80 Ma the Australian inland seas were in the process of receding.

At an earlier time foreland sedimentary basins formed by subsidence behind the margin of Gondwana allowed inland seas to flood parts of Australia. As occurred in South America behind the Andes, foreland basins develop in a continent behind a subduction zone at the margin of the continent, and the associated orogen/volcanic arc. The weight of the orogenic pile largely governs the flexure of the foreland basins of a continent. Included among the central Australian basins are the Cooper Basin, Simpson Basin (Permo-Carboniferous-Triassic), the Bowen Basin and the Gunnedah Basin in the Great Artesian Basin (Early Jurassic to Early Cretaceous).

In central and eastern Australia the main tectonic event that occurred in the mid-Cretaceous was a basin inversion phase, that Heads¹ suggests to a biologist might be understood as an ‘eversion’, that occurred at the beginning of the Late Cretaceous (Müller et al., 2012). During phases of uplift in the Late Permian to Middle Triassic (New England Orogen) and the mid-Cretaceous, about 95Ma, the earlier formed basins were inverted. In biogeography this last phase was of direct interest, and New Zealand at this time an import tectonic switch also occurred at the boundary between the Early and Late Cretaceous, 100 Ma.

The basin inversion of the mid-Cretaceous was a reflection of plate tectonics. East Gondwana moved to the east from 135-100 Ma, though between 115 and 110 Ma this movement slowed (Müller et al., 2012). Tectonic inversion occurred in eastern and central Australia when the eastward motion of Gondwana stopped around 100 Ma, which led to the extensional collapse of the Zealandia Cordillera, the large, Andean-style orogen that was built along the Pacific margin of Gondwana. A period of compression landward of the mountain belt resulted from the collapse of this large mountain range. In the foreland basins the switch led to folding and reactivation of reverse (compressional) faults of low dip, uplift, basin inversion erosion, and deep weathering. Formation of hydrocarbon traps in a number of basins, including the Cooper Basin, resulted from some reactivation.

Mesozoic inland seas – key hypothesis of Australian biogeography

An entomologist, K.H.L Key, was the first to propose that the large inland seas that existed in Australia in the past were important in relation to the biogeography of the present, after studying grasshoppers in Africa and Australia for 40 years. He related the distribution and centres of diversity of the present of morabine grasshoppers, Eumastacide: Morabinae, to areas that had been dry land during the marine incursions in the Cretaceous and the Cainozoic (Key, 1976). As this idea was not presented at a symposium on the biota of the Australian arid zone, and when it became more widely known some suggested that the origins of the present-day arid zone biota needed to be sought for in the distant past (Barker & Greenslade, 1982). According to Heads¹ this proposal is still often overlooked in biogeographic research, which mostly continues instead to stress events in the Neogene. Physical geographers, in contrast, now regard the importance of the Cretaceous as a ‘driving point’ in the history of Australian landforms (Wasson, 1982). A diverse set of new habitats was widespread throughout central Australia and the littoral environment; and the coastline was twice as long as its present length.

A paper on centipedes (Giribet & Edgecombe, 2006b) is among the few biogeographic studies that have mentioned the epicontinental seas of the Cretaceous, where they noted a major phylogenetic break, the McPherson-Macleay Overlap, between Queensland and southeastern Australia in Paralamyctes which they attributed to the seaway that was there in the Cretaceous.

In an account of the tenebrionid beetles of arid Australia the Key Hypothesis was also applied (Mathews, 2000), recognising Indo-Malayan, and Tethyan elements in the fauna, writing that at a tribal level Tethyan groups are endemic in the arid zone and there are no known related groups in forests elsewhere (Mathews, 2000, p. 941). Groups from Australian arid regions are suggested to have descended by vicariance from coastal sand dune inhabitants from the Tethys Sea, probably in the Jurassic prior to the desertification of what is now the arid zone, based on their affinities with groups from the Northern Hemisphere, partial presence in coastal dunes, and they apparently have basal phylogenetic positions. This is consistent with the invasion of central Australia by the beetles, as well as their habitat, coastal sand dunes, undergoing vicariance later after they were stranded there, following the retreat of the seas from central Australia in the mid-Cretaceous. It is suggested the seas would have brought an entire biota with them, not only the beetles, and as a result this process could explain the invasion of central Australia by eucalypts and Chenopodiaceae among many others. Heads¹ suggests an analogy with the ring in a bath tub, the groups being left on the former margin of the seas when they retreated.

Knowledge on marine transgressions can also be gained by studying marine and freshwater groups. Most clades in the Atherinidae, a fish family, that are marine, though 1 genus, Craterocephalus, includes 25 species from Australia and New Guinea that are mainly freshwater species. It has been found that the main break in the Craterocephalus is between 2 species that are coastal marine from Australia and the rest (Unmack & Dowling, 2010). It has been suggested that during the marine transgressions that occurred in the mid-Cretaceous there was also an invasion into freshwater, though it has been said that it is not clear how or why marine transgressions would be required for freshwater habitats to be invaded since regardless of the sea level the same interface between marine and freshwater exists (Unmack & Dowling, 2010). Heads ¹ suggests that though this is theoretically correct the marine fish should have remained in the retreating seas, though in reality there is always a chance that some could have been left stranded in deep pools as occurs at the present at low tide. Though many of these stranded populations would eventually die out some would survive.

The clades of Central Australia can be grouped according to where the intersection of their affinities is with the coast of the present, which reflects the distribution of the clades before the marine transgressions. Macquaria ambigua is an example, being a freshwater fish that occurs widely in the eastern half of central Australia and the Murray-Darling Basin, these populations forming a clade, and also the species near the coast are of a basal grade, in the Fitzroy Basin, Queensland (Faulks et al., 2010).

Central Australia – Myoporeae and wattles, 2 Indo-pacific diverse groups

In the deserts of Australia the tribe Myoporeae (Scrophulariaceae) and Acacia, the wattles (Acacia.str; Fabaceae), are the most diverse shrub and small tree groups (Chinnock, 2007), both of which are characteristic of the groups of central Australia and both share a similar intercontinental distribution.

Myoporeae (Scrophulariaceae) (formerly Myporaceae) have a range that is typical Indo-Pacific, and the presence of the Myoporeae in the Caribbean is a standard extension for Pacific groups, which is congruent with the geological suggestion that the Caribbean Plate originated in the Pacific.

The total range of the group is from Madagascar to America, the Myoporeae relatives are allopatric (Oxelman et al., 2005; Chinnock, 2007):

·         Madagascar – Androya.

·         Central America to Peru – Leucophylleae.

·         Mascarenes to Caribbean – Myoporeae.

It is indicated by the allopatry between Myoporeae and related forms that Myoporeae evolved in situ by vicariance instead of having a centre of origin in a restricted part of their range. The ancestral form is suggested by this to have been already widespread in central Australia prior to the group becoming recognisable. The genera within the Myoporeae and subgeneric groups remain based on morphological studies, though Western Australia appears to be the part of their range where they are most diverse. A significant break in central Australia occurs at the MacDonnell Ranges in the sections Sentis, Hygrophanae, Arenariae, and Platycalyx (Chinnock, 2007). It is indicated that there are important localities at the margin of the Western Plateau, as well as important northern centres at the Hamersley Ranges and the Kimberley Region.

Many of the Myoporeae members, being coastal plants, grow in tide-flooded estuaries, more or less as mangroves (Chinnock, 2007), the members of this family also are diverse throughout the arid areas of central Australia, represented by 215 species of Eremophila. Sand dunes and salt lake margins are the habitats that are included in this region, with many species tolerating high levels of disturbance, such as along roadsides. E.g., some members such as Eremophila veronica are ericoid ‘inflorescence plants,’ in which the foliage is composed of inflorescence bracts (cf. the Australian tremands and the New Zealand divaricates). The Myoporeae are hardy plants as a result of their morphological reduction and their production of strong chemical compounds, being the most renowned medicinal plants used by the local people and they have been implicated in cases of stock poisoning. As Myoporum montanum is both ubiquitous and variable, and the vegetation types it inhabits include mangrove margins, woodlands with Eucalyptus, Acacia, Casuarina or Callitris, chenopod shrubland and open grassland, suggests that has retained at least part of its ancestral diverse ecology.

There are about 1,000 species included among the wattles Acacia s.str. (Fabaceae) (Murphy et al., 2010), making them the largest genus of angiosperms in Australia where the genus is mostly confined, with only 19 species being known outside Australia, of which 8 are also present in northern Australia. Overall some have been found in the Mascarenes (an island group to the east of Madagascar in the Indian Ocean Ocean) and some authors also cite Madagascar, tropical Asia, Australia (most species) and the islands in the Pacific Ocean to Hawaii. The distributions for Myoporeae and wattles are similar, with both groups showing breaks in the region of southwest Indian Ocean at the Madagascar/Mascarenes, being widespread between there and the Pacific islands to Hawaii (wattles) or Hawaii – Caribbean (Myoporeae). The trans-Indian Ocean connection is southern, and the trans-Pacific disjunction, which is complete in the Myoporeae, but only to Hawaii is in the northern tropical latitudes.

Acacia is at its most Diverse in the southwest of Australia and in the east, along the Great Dividing Range in eastern Australia, though it is present throughout the continent. According to Heads¹ if the members from the arid zone derived from mesic clades, they would be nested among them. The clade that contains A. victoriae and A. pyrifolia, that arte mostly inhabitants of the arid and semiarid areas in central Australia, is a sister clade to the remainder (Murphy et al., 2010). Central Australia is indicated by this to not have been colonised by modern, mesic clades, but instead by a simple invasion of the centre, an initial break between central Australia and the rest of the Indo-Pacific range with subsequent overlap being suggested by the phylogeny. The local details of the different phases of marine transgressions and regressions that occurred in the Mesozoic would have been complex, given the overall flat nature of the land, and prime conditions for vicariance would have been formed by the changing coastlines. Pre-adapted forms in central Australia would have expanded their range as the aridity increased through the Neogene.

Central Australia – Polygonaceae

Duma (Polygonaceae) is one of the most interesting members of this family, according to Heads¹, and is widespread through inland Australia, though not in Tasmania or northern Australia; it reaches as far as the coast in Western Australia and Kangaroo Island in South Australia. It is a sister of Polygonum, which is native on all continents, instead of being nested in an Asian group, e.g., as is predicted by dispersal theory (Schuster et al., 2011b). Species of Duma are rhizomatous shrubs that are many-branched, and with abortive shoot apices forming thorns, and some species are well known as weeds at the present, and it is suggested Duma ancestors are likely to have been weeds during the Cretaceous, when they invaded the central parts of Australia with the new coastlines which formed prime conditions for vicariance and overlap. Pre-adapted forms in central Australia would have expanded their range as aridity increased through the Neogene.

Central Australia – Chenopodiaceae

It has been observed that in Western Australia typical beach genera, such as Frankenia (Frakeniaceae) also have many inland species (Carlquist, 1974). In Goodeniaceae, Aizoaceae, and Chenopodiaceae, families of plants that are well known in maritime and disturbed, open habitats from around the world, as well as being diverse in arid central Australia, there are other examples of this pattern. The Indian Ocean genus, Zaleya (Aizoaceae) is present in Africa, Pakistan and India, as well as being widespread in central Australia from the Pilbara Region to New South Wales (Venning & Prescott, 1984).

In Australia the Chenopodiaceae are represented by about 300 species, most of which are endemic. Along the coast and in the inland desert diversity is high, where populations can dominate large areas where there are saline soils. The dominant plants in arid Australia have been listed as Chenopodiaceae, Acacia, Casuarinaceae and grasses (Martin, 2006). It was suggested that the Chenopodiaceae of Australia probably originated post-isolation as immigrants, as there is an absence of known fossils prior to that time (Crisp et al., 2004: 1561). It was also estimated that Cainozoic ages of Chenopodiaceae in Australia, the group being attributed to 9 colonisation events that began at 43 Ma (Kadereit et al., 2005). Vicariance was ruled out in both these studies, though only because minimum, fossil-calibrated ages were treated as maximum ages of clades.

The Salicornia that is coastal, and its sister group (Chenopodiaceae: Salicornioideae) in Central Australia

Salicornia (including Sarcocornia) is a leafless succulent from coastlines of the most of the world (Kadereit et al., 2006), where it is often the dominant plant in tidal salt marshes, also occurring in saline and alkaline soils above the high tide level. In some areas, such as South Africa, e.g., S. mossiana is present around inland salt pans that are derived from old lagoons that were cut off from the sea in marine regressions in the Pleistocene (Steffen et al., 2010). Heads¹ suggests analogous populations and species stranded inland and at high elevations in the Andes, such as S. andina (2,300-4,200 m/7,540-13,780 ft) and S. pulvinata (above 3,500 m/11,480 ft) (Alonso & Crespo, 2008) can also be accounted for by a similar ‘ecological lag’. Their alpine ecology of the present has been determined by the rise of the Andes as their previous location around the shores and basins has been uplifted by the formation of the Andes, and a similar model involving the uplift of a population with the rise of the Andes has been proposed to account for the present location of Andean parrots (Ribas et al., 2007).

The sister group of Salicornia comprised on Halosarcia, Tectcornia, Tegicornia, Pachycornia and Sclerostegia, replaced Salicornia in inland Australia, mostly in the southwest and southeast (Shepherd et al., 2004; Kadereit et al.,. 2006). The 5 genera (problems exist with their delimitation) in the sister group are all abundant in central Australia. Of these 5 genera 4 genera are endemic to Australia, while Halosarcia is also found in Pakistan and tropical Africa, in a group typical of the Indian Ocean. Dispersal is not required to or from Australia for Salicornia or its sister group as around the margins of Australia and Africa simple vicariance is indicated. The ancestral is suggested by the distributions to have been global at the time it was broken up into 1 group on the coastlines of the world, and 1 in central Africa, Pakistan and Australia.

Tethys-Australia connections – Australian Camphorosmeae (Chenopodiaceae: Salsoloideae)

Among the Australian chenopods the Camphorosmeae are the tribe with the most species, the Australian members of which have been assigned to a clade of subshrubs (Sclerolaena, Maireana, and others) that are present throughout mainland Australia (with the exception of the Kimberley and Cape York Peninsula), Tasmania and New Caledonia (Cabrera et al, 2011). These plants are abundant in the arid zone of central Australia, the species tolerating saline and alkaline soils high in sodium chloride or gypsum. There is a link between the distribution of several species and salt lakes of the inland areas.

It has been suggested (Kadereit & Freitag, 2011) that the Camphorosmeae evolved in the Late Eocene to the Early Oligocene (minimum age based on fossil age). It was also suggested by these authors that the  tribe originated in Eurasia then dispersed to southern Africa, North America and Australia, though this was only because of the paraphyletic basal grade (‘early branching lineages’) in Eurasia. There were sister groups for all the early-branching lineages of the same age as these lineages and Heads¹ suggests these could possibly have been in Australia before the beginning of differentiation.

The Sclerolaena group from Australia-New Caledonia is a sister of Grubovia of central Asia that is present in steppes around the Tien Shan Mountains and the Altai Mountains (Kadereit & Freitag, 2011; Cabrera et al., 2011), and according to Heads¹ the Tethyan Basins are followed by this connection. The clades in 3 western areas of Australia, ‘southwest’, ‘western desert’ and ‘Pilbara’ form a basal grade which indicates primary centres of differentiation that were spread widely through Western Australia. Heads¹ says that overall the sequence followed by differentiation was: Tethys-Western Australia.

Some earlier authors have invoked the process of ongoing speciation associated with hybridisation in regards to some Australian Camphorosmeae species that have been introduced to other countries where they have become weeds (discussion in Kadereit & Freitag, 2011). The distributions are consistent with the group’s origins being in the marine regression phase in the Middle Cretaceous, though the group has been seen as quite young. Heads¹ suggests the group could represent a hybrid swarm of weedy taxa from the Cretaceous that was stranded in the inland, to some extent frozen in place, though the members are capable of developing renewed mobilism and hybridism when they are introduced elsewhere or cultivated together. In New Zealand Hebe (Plantaginaceae) provides a parallel case.

Sources & Further reading

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