Australia: The Land Where Time Began

A biography of the Australian continent 

Diversification of angiosperms occurred in the Early to Middle Cretaceous, and subsequently, adaptive radiation of Australian mammals occurred between the Late Cretaceous and the late Palaeogene. As a result of the radiation of the Australian mammalian herbivores that took place following the diversification of the angiosperms, the structure and evolution of these herbivores arose to deal with the requirement of eating the diverse range of angiosperms that had arisen. Based on this relationship, the structure of the dentition is used to infer at least some aspects of the plants they fed upon. There are few experiments that have been carried out on the function of the teeth in extant herbivores in Australia (such as Sanson, 1989). When based on the diets of living species, deductive analysis of the diets of extinct species has resulted in hypotheses concerning the time when major changes in the terrestrial communities occurred in Australia during the mid-late Tertiary.

At the order level, there are 12 distinct endemic mammal groups, 10 of which have living representatives. The various orders were present in the fossil record from:

• Early Cretaceous - Monotremes
• Early Eocene - dasyuromorphians (dasyures, numbats and thylacines) - bandicoots (bilbies & forest bandicoots) - bats
• Early Miocene - marsupial moles
• Oligocene - whales.
• Late Oligocene - diprotodontians (koalas, wombats, possums and kangaroos.
• Early Pliocene - rodents
• late Tertiary - sirenians (dugongs).
• Middle Holocene - dingoes
• Late Pleistocene - seals.

(Archer, 1984; rich, 1991)

The yalkaparidontians (Archer et al., 1988a), an ordinal level group that are believed to have possibly radiated from highly autapomorphic marsupials. They are not known from deposits dated from some time between the Middle Miocene and the Early Pliocene and later. Australian '?condylarthrans' (Godthelp et al., 1992) are believed to have possibly been nonvolant terrestrial placentals. They apparently went extinct in Australia between the early Eocene and the Late Oligocene.

See Murgon Fossil Site

Late Palaeocene-Early Eocene

There are no known sites that are firmly dated to the Palaeocene that contain fossils of mammals. The Tingamarra Local Fauna, found in the deposits of the Murgon Fossil Site, has a minimum age of  of 55 million years ago, based on illite dates (Godthelp et al., 1992), is usually said to be of Eocene age, but may actually date from earlier than the beginning of the Eocene.  At the time of writing, no animals have been found in the Tingamarra Local Fauna that indicate the vegetation in the area of the Tingamarra basin where the deposits were situated, those found being marsupials that were insectivorous and omnivorous, a ?condylarth-like placental, and a small bat that was insectivorous. A similar feature is seen in Patagonia in the Early Eocene, in the Casamayoran mammals. The caroloameghiniids, one of these groups, are thought to have possibly had a representative in the Tingamarra Local Fauna (Godthelp et al., 1992). Assemblages dominated by small omnivores are believed to possibly indicate that the some or most were arboreal frugivores, as is seen in present day Australia, where most comparable small omnivorous marsupials are arboreal possums. It has been suggested that this could indicate that there may have been fruiting angiosperms in the area of the lake that formed the Tingamarra deposit (Archer et al., in Hill, 1994).

It has been concluded that in the Early Eocene the climate of southeastern Queensland was probably mild to warm throughout the year, based on the observation that the vertebrate fauna was dominated by large reptiles.

Late Oligocene - Early Miocene

The Australia continent separated from Antarctica somewhere between 45 - 38 million years ago. There is a gap in the fossil record of Australian mammals that extends on either side of this period, between 55 - 25 million years ago (Veevers, 1984). On all other continents the period covered by the Late Eocene-Early Oligocene was a time when a major decline and turnover of mammals took place. Examples are the Grand Coupure, 'great break', in Europe and the Terminal Eocene Event in North America.

In Australia there are many fossil assemblages dating from the period between the Late Oligocene and Early Miocene (Woodbourne et al., 1985, Archer, et al.,1991; Rich et., 1991). Indications of mammal community structures from the Tertiary in Australia are found in some of these assemblages.

At 2 locations in central Australia, the Tirari Desert in the Lake Eyre Basin and, to the east of the Flinders Ranges, the Lake Frome Embayment, very similar fossil assemblages are found. The difference between the 2 sites are mostly at species level, with the exception of freshwater river dolphins, that have also been found in the Namba Formation at Lake Pinpa, Frome Downs Station (Fordyce, 1991). A number of unique marsupial genera, such as the Ilaria and Pilkipildra, both the only known representatives of these genera, are found at both sites.

Nearly all local faunas from central Australia occur in fluvio-lacustrine deposits, the biotas being dominated by teleost fish, chelid turtles, crocodiles and occasionally water birds. It has been suggested that the mammals were mostly carried into depositional basins by streams (Rich et al., 1991). An example is the Tarkarooloo Local Fauna that apparently accumulated at the edges of lakes, possibly the victims of aquatic predators. An example is the Ditjimanka Local Fauna (Rich et al., 1991). Some larger animals have been found in the form of articulated skeletons, possibly mammals that were trapped in the mud at the edges of water bodies, such as the Pinpa Local Fauna (Tedford et al., 1977).

The vegetation of the area surrounding Etadunna Clays of the Tirari Desert was believed to be scattered rainforest pockets with Brassospora, a subgenus of Nothofagus in grasslands (Harris; Woodburne et al., 1985), based on pollen from the Etadunna Clays, from the Late Oligocene to Early Miocene. This has been challenged. Harris' material has been reanalysed, and it suggested that the grasses represented in the pollen record may have been aquatic grasses, so not indicating grasslands at this time (Martin, 1990). This interpretation has been supported by the lack of grazing species in the local faunas of central Australia, and the large numbers of browsing marsupials and arboreal possums.

Northern Australia appears to have been covered by a species-rich lowland rainforest in the Oligocene-Miocene, based mainly on the diverse assemblages, especially the earlier ones, found at Riversleigh. The evidence has been summarised (Archer et al., 1989, 1991). It showed that there was a very diverse assemblage of sympatric arboreal folivorous marsupials, and many vertebrate groups with modern descendents that are restricted to rainforest. Examples of these are log-runner birds, musky rat-kangaroos and ringtail possums of the Pseudochirops type. More evidence of a rainforest vegetation is seen in the mammal community structures at Riversleigh, where there is a lack of clearly dominant species and high taxonomic diversity, as is found in modern rainforests, but not in mesic communities.

The presence of more open forests in the area surrounding the depositional basin is supported by some geological evidence from the oldest Riversleigh deposits dating from the Oligocene-Miocene (the 'System A' deposits of D. Megirian), though biological evidence for grasslands surrounding the basin is completely lacking, with no known adaptations for grazing, even in the dentitions of the known kangaroos, while many have adaptations for browsing.

A relatively small fragment of a leaf with a margin that is dentate, from the early(?) Late Miocene in the Dunsinane Site at Riversleigh,

s the possibility of a decline in the everwet closed forest, that was apparently present at Riversleigh. This feature of the leaf is more common in temperate rainforests, or possibly seasonal vegetation (R. Hill). 

The rainforest fauna of northern Australia indicate a climate during the Late Oligocene - Middle Miocene that was warm and wet, and possibly seasonal. The climate of central Australia at this time is less well known, though the number of crocodiles and large turtles suggest a warm climate. Pollen of the Nothofagus subgenus Brassospora indicates high rainfall. There are relatively fewer arboreal species in central Australia than are present in the Riversleigh area of northern Australia. An example is the numbers of foliovorous pseudocheirids, with at most 3 in each of the local faunas of central Australia, while the assemblages at Riversleigh have 9. There appears to have been a faunal dominance by the palorchestid Ngapakaldia telfordi at Discovery Bay, Lake Pitikantia. The fossil evidence suggests there were probably large numbers of this sheep-sized marsupial, possibly moving around in herds. Some areas of open woodland or forest, as well as rainforest, is suggested by the presence of such large numbers of this generalised browser.

During the Early Miocene, local faunas of Tasmania, such as the Gilston Bay Local Fauna and the Wynyard Local Fauna, lack mammal fossils that could indicate local climates or habitats at the time, though they contained a subset of the marsupials from that time in central and/or northern Australia.

Middle - Late Miocene

Diverse assemblages from central Australia and northern Australia suggest there were probably wet forests, possibly locally, based on the presence of browsers. It has been suggested that the forests may have been riparian, drier, more open vegetation types being found further from the depocentres. These assemblages have been produced by the Kutjamarpu Local Fauna from the Tirari  Desert, South Australia (Woodburne et al., 1985; Rich et al., 1991), in central Australia and the System C Local Faunas of Archer et al., 1989, 1991, and the Bullock Creek Local Fauna in the Northern Territory (Woodburne0, et al., 1985).

The Bullock Creek Local Fauna in the Northern Territory, of Oligo-Miocene age, as well as a few of the most recent strata of Riversleigh's System C Local Faunas, contain the earliest known evidence of adaptations by mammals to grazing in Australia. A relatively high-crowned molar from a macropodid kangaroo has been found in the Bullock Creek Local Fauna, indicating it had adapted to feed on abrasive plant material, possibly, but not necessarily, grass (B. Cooke). A small number of high-crowned teeth, believed to be from a proto-wombat, have been found in some of the more recent strata of the System C Local Faunas at Riversleigh. This suggests some marsupials were specialising their dentition to eat abrasive plant material, either grass or a pre-adaptation utilising other abrasive plants present at Riversleigh  in the latter part of the Middle Miocene. Modern wombats are all grazers, having high-crowned, rootless teeth, much more specialised than those found in the Riversleigh deposits.

Late Miocene

The Beaumaris Fossil Site, southern Victoria, and the Alcoota Local Fauna, southern Northern Territory, are the only fossil deposits where significant amounts of mammal remains have been found. The site of the Beaumaris Local Fauna deposit was apparently an inshore marine deposit in the Late Miocene, containing a depauperate biota, providing little evidence of the palaeoclimate or palaeohabitats of the area.

The Alcoota Local Fauna has been described as reasonably diverse (M. Archer et al. in Hill, 1994), containing a single known mammal that might possibly have been arboreal. A single dentary of this animal, a species of Pseudochirops-like possum, was found, suggesting that by the Late Miocene the rainforests of the Alcoota, at least, had been replaced by open sclerophyll forest, though it has been suggested it would be more likely to have been woodland. The extant ringtail possum (Petropseudes dahli), occupying rock piles in northern Australia, is a close relative. This animal is only partly arboreal. The herbivores of the Alcoota Local Fauna, such as a diverse range of Diprotodontids and a few kangaroos, were browsers , unlike the modern herbivores in similar habits that are grazers. The is believed to indicate that even though the closed rainforests had been replaced by more open forests or woodlands, abrasive grasses had not yet developed in the area.

Pliocene

An evolutionary change appears to have occurred in Australia between the Late Miocene sites, such as the Alcoota Local Fauna,  and the Early Pliocene assemblages found in northern and southwestern Australia. The later assemblages are characterised by kangaroos with grazing adaptations, though some browsing kangaroos.  continued. Another change is seen in the diprotodontoids, where the body size had increased, but the diversity had decreased. There was also the development of a variety specialised grazing wombats. The gap between continued to increase in the assemblages through the Middle and Late Pliocene.

The situation appears to have been different in New Guinea, where some of the Awe mammals, such as Kolopsis rotundus (Plane. 1967; Rich et al., 1991), from the ?Middle Pliocene, that more closely resemble Australian forms from the Late Miocene. The ecology of New Guinea was more stable than that of Australia during the later Tertiary, and it has been suggested that New Guinea provided a refuge for lineages that had gone extinct in Australia.

Assemblages with a high percentage of animals with extant genera first appear in the fossil record in the Early Pliocene, at places such as Bluff Downs Local Fauna, northwestern Queensland, Bow Local Fauna, Eastern New South Wales, and in South Australia, the Sunlands Local Fauna. A single taxon, the Pseudochirops-like possum, has been found in the Alcoota Local Fauna, from the Late Miocene.  As an example of the Early Pliocene assemblages, the Bluff Downs Local Fauna of northwestern Queensland has produced species of Macropus (Macropus), M. (Osphranter), Petrogale, Pseudochirops, Permeles and Planigale. Of the local faunas from the Early Pliocene, only 1 is believed to have been formed in a rainforest, the Hamilton Local Fauna from southwestern Victoria (Rich et al., 1991). This local fauna has produced generic-level taxa that are now found in the rainforests of New Guinea and northeastern Queensland. Some examples are the Hypsipyrmnodon, Strigocuscus, Thylogale, cf. Dendrolagus and Dorcopsis. The presence of other species that are not usually found in the rainforests of the present, such as Petaurus, gliding possums, suggest open forests were also present in the vicinity of the rainforest.

There are no known diverse mammals assemblages that can be said with confidence to be from the Late Pliocene, But the Kanunka Local Fauna from the Tirari Desert in central South Australia, is believed to be of Late Pliocene age (Tedford et., 1986). Many Pleistocene-like taxa have been found in this deposit, as well as representatives of extant genera, such as Lagorchestes, Bettongia and Vombatus. No definite evidence has been found in this location indicating the presence of desert conditions that exist at the site at the present time. The region is believed to probably have been covered by open sclerophyll forest or woodland, though not grassland and dunes, about 2.5 million years ago.

The only known diverse mammal assemblages from Western Australia from the Pliocene. The only known assemblage, the Quanbun Local Fauna from the Kimberley, is faunistically impoverished (Flannery, 1984).

Quaternary

Many mammal assemblage are known from the Pleistocene and Holocene in Australia (Archer & Hand, 1984; Murray, 1991), most of which have been dated by faunal composition, radiometric dates not having been obtained. Most of the known sites are believed to be of Late Pleistocene age. The assemblages in the Fisherman's Cliff Local Fauna and the Portland Local Fauna, both apparently of Early Pleistocene age, have distinctly different mammal composition.  The ?frugivorous ektopodontid possums, that are found in mid-Tertiary deposits, were also found in the Portland Local Fauna, where they are believed to be the last surviving representatives of their kind. This local fauna also contained many other taxa not present in deposits from the later Quaternary. Not much evidence of Quaternary age has been found in New Guinea, though finds in Papua and Irian Jaya demonstrate that zygomaturine diprotondontids, a megafaunal group, survived in New Guinea until at least the latest part of the Pleistocene, after having gone extinct on the Australian continent about 30,000 years ago.

Changes in the balance of indicator species, such as the western banded bandicoot, Perameles bougainville, an inhabitant of arid/semiarid regions, which became extinct locally in the extreme southwest of Western Australia about 20,000 years ago, has allowed the detection of regional changes in habitat (Merrilees, 1979). On the continental scale, it has been observed that several periods of cooler, more arid conditions correlated with periods of glacial maxima in the Northern Hemisphere. One such period occurred about 17,000 tears ago, when a major contraction of rainforest biotas in northeastern Queensland indicated severe aridity (Kershaw, 1983). Evidence is not available from the fossil record of mammals in well-dated fossil deposits from eastern Australia from the Early-Middle Pleistocene. It is believed the forest mammal biotas of the wet forests of Australia declined significantly, in diversity and range, during these times of extreme aridity. Following the Middle Miocene, there is evidence of gigantism among Australian vertebrates, including mammals, especially obvious in herbivorous marsupial lineages, such as kangaroos, wombats and diprotodontoids, that survived past the Late Miocene. An example of a group increasing in size over time is the diprotodontoids, the average size of which was about the size of a calf during the Oligocene-Miocene, but were about the size of a cow in the Middle Pliocene-Early Pleistocene, and by the Late Pleistocene, all Australian mainland diprotodontoids were large, but in Irian Jaya, zygomaturines of calf size were still present (Plane, 1991). In Papua, slightly larger zygomaturines, that have been described as panda-like, were still present (Flannery & Plane, 1986).

Gigantism also affected other vertebrate members of the megafauna, such as reptiles, Megalania prisca, a giant monitor, Wonambi naracoortensis, a giant madtsoiid python, and birds, Dromornithids, giant flightless birds related to geese (Murray, 1991). There are a number of effects the megafauna are believed to possibly have had on the vegetation of the continent include, foraging of terrestrial browsers at progressively high levels of the vegetation, the transportation of large seeds for greater distances, and effects on surface plants and soil of graviportal trampling.

Graviportal refers to the column-like structure of a leg, as in the weight-bearing leg of an elephant, or a Diprotodon.

In the Late Pleistocene-Holocene a trend towards dwarfism developed among the megafauna groups, those that did not get smaller going extinct (Marshall & Corruccini, 1978). Large Macropus grazing kangaroos are believed to have reduced their size by about 30 %. A number of ecophysiological reasons have been suggested for this decline in size (e.g., Archer, 1984; Murray, 1991), that include the disadvantage resulting from young animals needing to grow to large size before they could be independent. In environments where the food could be scarce and unpredictable, as occurred at times during the Late Quaternary, it would have been difficult to find enough food at the time it was required to sustain an immature large mammal.

By the end of the extinction of the megafauna about 25,000 years ago, most surviving species continued to the present. The causes of the extinction of the megafauna are still being debated, but it is widely believed that climate change and the arrival of humans probably accounted for at least some of the extinctions (Archer et al. in Hill, 1994).

Some of the giant grazing kangaroos survived in downsized forms, the eastern grey kangaroo, red kangaroo and the euro. Following the arrival of Europeans, another 20 small to medium sized mammals have gone extinct. It has been suggested that the extinction of some of the smaller herbivores that lived among the megafauna may have occurred because they depended on the regrowth that resulted from browsing by the megafauna herbivores, following the megafauna into extinction, their food of regrowth no longer being available with no megafauna herbivores to browse the vegetation (Flannery, 1990).

The Aboriginal People arrived in Australia about 60,000 years ago, while at least some of the megafauna were still extant. If the practice of fire-stick farming changed the environment by increasing the fire frequency, the succession in vegetation that followed burning could be expected to simulate the regenerating vegetation following browsing by megafauna, mentioned in Flannery's suggestion. If this was the case, the effects of the megafaunal extinction on the smaller herbivores would have been lessened. This may mean that the small herbivorous mammal extinctions actually resulted from the changes made to the Aboriginal management practices, fire-stick farming, by the European settlers. Ecosystems that were diverse under Aboriginal management, fire-stick farming, were simplified under European management. Conditions in which small mammals may have escaped extinction in the Late Pleistocene following the extinction of the megafauna, possibly serendipitously, were changed and they eventually succumbed under the European-style management of the environment. Examples of these small herbivorous mammals that went extent  are the crescent nailtail wallaby (Onychogalea lunata) and pig-footed banidcoot (Chaeropus ecaudatus) (Archer et al.).

Research on other continents (e.g. Diamond, 1990) has shown that there are so-called 'ghosts' in modern ecosystem. The diversity and distribution of large-seeded trees, that may have previously depended on the megafauna for the distribution, and possibly for their germination, and have now declined in distribution and diversity. Seeds of modern species are sometimes dispersed by large herbivores, such as elephants, then germinate in the animal's dung. Another example that has suggested is some defensive features of plants, such as spines and toxins, when the plant no longer have any modern herbivores to defend themselves against. In this case the defences were evolved to defend against herbivores in the megafauna, but have been retained after the megafauna extinction. In Australia it is believed most plants that depended on the megafauna for seed dispersal would now be extinct. Some possibly survive in places like the north Queensland rainforests because the seeds are dispersed by cassowaries, large birds (Casuarius casuarius). It is also possible that plants such as the stinging trees (Dendrocnide spp.) have retained the toxins originally evolved for defence from megafauna, probably because their retention did not disadvantage the plant. 

Sources & Further reading

1. M. Archer, S.J. Hand & H. Godthelp in Hill, Robert S., (ed.), 1994, History of the Australian Vegetation, Cambridge University Press.

Links

The Evolution and Classification of Marsupials