Australia: The Land Where Time Began
Palaeohabitats as Inferred from Patterns of Mammal
At the order level there are 12 known endemic Australian mammal groups, 10 of which have extant representatives - monotremes (Early Cretaceous to present) marsupial dasyuromorphians (from Early Eocene), bandicoots (from Early Eocene), marsupial moles (from Early Miocene), diprotodontians (from Late Oligocene), bats (from Early Eocene), whales (from Oligocene), rodents (from Early Pliocene), sirenians (from late Tertiary), carnivores (seals from Late Pleistocene, dingoes from mid-Holocene), Primates (humans) (from Late Pleistocene) (Archer, 1984; Rich, 1991). There are 2 known extinct orders, yalkaparidontians (Archer et al., 1988a) that are believed to be a relatively minor radiation of marsupials that were highly autapomorphic, that went extinct between the Middle Miocene and the Early Pliocene. Australian ?condylarthrans (Godthelp et al., 1992) appear to be nonvolent placental terrestrial mammals that went extinct some time between the Early Eocene and the Late Oligocene.
The mammals of the Australian Early Cretaceous are not well known, so not much information on habitats can be gleaned from them. Steropodon galmani is known only from a jaw fragment, indicating it was a generalised omnivore or carnivore, so nothing can be deduced about the habitat it lived in. The temperature tolerance of most groups present in the Lightning Ridge Fauna has not been agreed on, so no firm conclusions on the palaeoclimate at the time the site was being deposited can be drawn from the remainder of the fossils at the site (Molnar, 1991).
S. galmani was about the size of a rabbit, making it as large as or larger than any known mammal from the Mesozoic, even overlapping the size of the smaller dinosaurs (Archer et al., 1985). Because of the relationship between body size and surface to volume ratio, large size of mammals often indicates the latitude the animal normally lives in, the larger body size having a smaller surface to volume ratio, making it easier to maintain body temperature in cold environments. It has been suggested that in the Early Cretaceous dinosaurs were living within the Antarctic Circle in Victoria, where climates at the time could be expected to get to near 0o C (Rich & Rich, 1989). The Lightning Ridge Fauna are in New South Wales, not far from Dinosaur Cove, so the area could to be expected to be cold at the time of deposition. At the present echidnas in the Australian Alps hibernate to survive below freezing temperatures (Augee, 1991).
Late Palaeocene-Early Eocene
The only site known from Australia between Late Palaeocene and the Early Eocene is believed might be the the Tingamarra Local Fauna from the Murgon fossil site in southeastern Queensland. This site has been dated to about 55 Ma, based on an illite date that has been suggested to possibly be a minimum age (Godthelp et al., 1992). The mammals found at Murgon to the time of writing do not include any definite foliovorous species, so the vegetation surrounding the site at the time of deposition cannot be deduced, only insectivorous and omnivorous marsupials. The Casamayoran mammals from the Early Eocene in Patagonia also has only insectivores and omnivores, one of which is believed may have a representative in the Tingamarra Fauna (Godthelp et al., 1992). All that can be deduced from habitats where small carnivores dominate is that some of the animals were probably arboreal frugivorous species. Arboreal possums comprise most of the omnivorous marsupials at the present, leading to the suggestion that the forests surrounding Tingamurra may have included fruiting angiosperms. The non-arboreal vertebrate fauna of the area appears to be dominated by large reptiles, including a madtsoiid snake (J. Scanlon), and many large crocodylids (P. Willis), and trionychid turtles (Gaffney & Bartholomai, 1979). The domination of the vertebrate fauna by large reptiles has led to the suggestion the climate in southeastern Queensland at the time was probably mild to warm throughout the year during the Early Eocene. Other vertebrates in the deposits include small teleost fish, birds, frogs, and a variety of reptiles.
Early Eocene-Late Oligocene - the fossil 'Dark Age' of Australia
There is a gap in the mammal fossil record in Australia between 55 Ma and 25 Ma, a time during which Australia was breaking away from Antarctica between 45 Ma and 38 Ma (Veevers, 1984), precisely when interesting evolutionary events could be expected to be occurring. On the other continents the Late Eocene and Early Oligocene was a time of a great turnover and decline of mammalian faunas - in Europe the 'Grand Coupure', and in North America, the 'Terminal Eocene Event'.
Late Oligocene-Early Miocene
After the mammal fossil record again appears in the Late Oligocene there are many assemblages of mammal fossils continuing to the Early Miocene (Woodburne et al., 1985; Archer et al., 1991; Rich et al., 1991). Some knowledge of mammal communities of the Tertiary in Australia can gained from these deposits.
The biotas from this time from the Tirari Desert in the Lake Eyre Basin and from the Lake Frome Embayment to the east of the Flinders Ranges display a high degree of similarity, that includes such unique marsupial genera as Ilaria and Pilkipildra. These areas differ from each other mostly at the species level, with the exception of the freshwater river dolphins of the Namba Formation from Lake Pinpa on Frome Downs Station (Fordyce, 1991). The local faunas of Central Australia were mostly, if not all, deposited in fluvio-lacustrine sediments in which teleost fish, chelid turtles, crocodiles and occasionally waterbirds are the dominant vertebrates. Some suggestions have been made as to how the mammal remains came to be in these deposits, either being washed into depositional basins in streams, such as the Tarkaroola Local Fauna; (Rich et al., 1991), or accumulating at the edges of lakes by being taken by aquatic predators, such as the Ditjimanka Local Fauna; (Rich et al., 1991). Large animals could also possibly be trapped in mud around the edges of lakes that were drying, some skeletons having been found articulated, such as the Pinpa Local Fauna; (Tedford et al., 1977).
It has been suggested, based on the pollen present in the Etadunna Clays from the Tirari Desert, there were widespread grasslands broken up by rainforest containing such vegetation as Brassospora, a subgenus of Nothofagus (W. Harris to Woodburne et al., 1985) between the Late Oligocene and the Early Miocene. These claims have been disputed. Based on reanalysis of the material used by Harris, it has been suggested that the grass pollen may actually be from aquatic species, not evidence for the presence of grasslands in Central Australia in the mid-Tertiary as proposed by Harris (Martin, 1990). The presence of many browsing marsupials and arboreal possums, with no evidence of grazing mammals in the known biotas from Central Australia at the time the deposits were being formed would tend to support Martin's interpretation of the data (M Archer et al., in Hill, 1994).
Northern Australia appears to have been covered by lowland rainforest, that was species-rich, during the Oligo-Miocene, based on evidence from the Riversleigh assemblages. Archer et al., (1989, 1991) summarised the evidence for this conclusion that included sympatric arboreal foliovorous marsupials, as well as vertebrate groups of which all extant species are restricted to rainforest - log-runner birds, musky rat-kangaroos and ringtail possums of the Pseudochirops-type. The high taxonomic diversity, combined with a lack of markedly dominant species in the mammal communities from Riversleigh, is similar to what is found in rainforests of the present, but is not found in mesic communities.
Beyond the depositional basins some geological evidence has been found that outside the basins containing the oldest Riversleigh deposits of the Oligo-Miocene (System A deposits, D. Megirian) there may have been forest communities of a more open type. Biological evidence has not been found that grasslands surrounded the depositional basins in question. No adaptation for grazing has been found in the dentition of animals such as kangaroos from this time, many adaptations have been found that indicate browsing.
In the Dunsinane Site at Riversleigh, of ?early Late Miocene age, a leaf fragment of a type that is relatively small with a dentate margin, features not normally associated with rainforest species, being more common in temperate rainforest or vegetation of a seasonal nature (R. Hill), may be the first indication of the decline of the everwet closed forests of Riversleigh.
The rainforest faunas of northern Australia indicate a warm, wet climate that could possibly have been seasonal, during the Late Oligocene to Middle Miocene. The evidence is not as clear for the climates of Central Australia, the evidence trending to be conflicting, as far as indicating the dominant vegetation is concerned. Warm temperatures are suggested by the presence of many crocodiles and large turtles, and pollen of the Brassospora subgenus of Nothofagus suggest a wet climate. Opposing this evidence of a warm, wet climate is the assemblage that relatively depauperate in arboreal species, for example there were at most 3 foliovorous pseudocheirids in any local fauna, compared to up to 9 at Riversleigh. At Discovery Basin of Lake Pitikantia there is believed to have been a marked dominance of the terrestrial palorchestid marsupial Ngapakaldia tedfordi, about the size of a sheep, that may have been present in herds, like 'mobs' of kangaroos.
The local area apparently supported rainforest, but areas of woodland and open forest is suggested by the presence, in large numbers, of a generalised browser such as the palorchestids.
In Tasmania in the Early Miocene a subset of the same marsupials present contemporaneously in central and/or northern Australia is present in the Geilston Bay Local Fauna and the Wynyard Local Fauna, though there is no known evidence indicating the local climates or habitats.
Middle - Late Miocene
In central Australia some assemblages from the Middle Miocene that are known reasonably well have been interpreted as being diverse. The Kutjamarpu Local Fauna from the Tirari desert, South Australia (Woodburne et al., 1985; Rich et al., 1991). System C Local Faunas from Riversleigh, northwestern Queensland (Archer et al., 1989,1991). The Bullock Creek Local Fauna from the Northern Territory (Woodburne et al., 1985). The assemblages in these local faunas indicate wet forest environments with browsing herbivores, that may have been local, possibly riparian, surrounded by drier, more open environments further from the deposition sites.
The Bullock Creek Local Fauna contains the earliest known evidence of mammals with possible adaptations for grazing, such adaptations are also found in the highest stratigraphic levels in the System C Local Faunas at Riversleigh. A single relatively high-crowned macropodid kangaroo molar from the Bullock Creek LF is believed to indicate an animal that was adapted to feed on abrasive plants (B. Cook), that might have been a plant other than grasses. A number of high-crowned teeth of a proto-wombat have been recovered from the System C Local Faunas at Riversleigh, that suggests wombats appear to have been specialising as grass eaters, but the teeth could also indicate they had pre-adapted to grass eating by feeding on other, non-grass plants. The teeth of modern wombats, that are specialised grazers, are high-crowned and rootless, and are more specialised than those in the Riversleigh deposits.
In the Late Miocene there are 2 known fossil sites in Australia containing mammals, Beaumaris Local Fauna in southern Victoria and Alcoota in the southern part of the Northern Territory. The Beaumaris Local Fauna, a near-shore marine deposit, is of little use for deducing the palaeoclimates and palaeohabitats as it is too depauperate to provide much information. The Alcoota Local Fauna contains only a single dentary from a Pseudochirops-type possum that is thought to possibly be arboreal, but this is not certain. Petroseudes dahli, an extant foliovorous ringtail possum that is closely related to Pseudochirops-type dentary in the Alcoota deposit, inhabiting rock piles in parts of northern Australia, is only partially arboreal. It is believed that the rainforests of at least the southern parts of the Northern Territory had been replaced by the Miocene by either sclerophyllous forest, of more probably, woodland. Though the closed forests had most likely gone from the area the presence of herbivores in the deposit that were mostly of types of browsers, a few kangaroos and diverse diprotodontids, indicate that abrasive grasses did not yet form a significant part of the vegetation.
Between the Late Miocene and the Early Pliocene Australian mammal assemblages display a large change from few if any grazers to a number of grazers as well as browsers among the kangaroos, and the diprotodonts were larger, and there was a variety of wombats that had specialised for grazing. Evidence of this change came from Late Miocene assemblage at Alcoota and the Early Pliocene assemblages from northern and southeastern Australia. This trend towards grazing was continued throughout the Middle and Late Pliocene assemblages. It has been suggested that the generalisation may not apply in New Guinea, where some of the Awe mammals from the ?Middle Pliocene appear closer to Late Miocene forms from Australia, such as Kolopsis rotundus (Plane, 1967; Rich et al., 1991). This may in fact be a result of the Awe area of New Guinea being more stable during the Late Tertiary, becoming a refuge area for lineages that survived longer in New Guinea than in Australia (Archer et al., in Hill, 1994).
During the Early Pliocene the assemblages from some local faunas are among the first known in which there were high percentages of mammal forms that are still represented at the present. These are the Bluff Downs Local Fauna of northeastern Queensland, the Bow Local Fauna of eastern New South Wales, and the Sunlands Local Fauna of South Australia. There is a single taxon in the Alcoota Local Fauna, the Pseudochirops-like possum. In the Bluff Downs Local Fauna there are Macropus (Macropus), M. (Osphranter), Petrogale, Pseudochirops, Perameles and Planigales (Archer et al., 1994). The Hamilton Local Fauna from southwestern Victoria (Rich et al., 1991) is the only known local fauna from that time that is believed to have been deposited in a rainforest. At the genus level the taxa are typical of rainforests presently found in northeastern Queensland and New Guinea. The genera include Hypsiprymnodon, Strigocuscus, Thylogale, cf. Dendrolagus and Dorcopsis (Archer in Hill, 1994). There is evidence that there was forest of a more open type near the rainforest, that is suggested by the presence of gliding possums (Petaurus). At the present they are not found in rainforest, being found in more open types of forest.
The Kanunka Local Fauna from the Tiari Desert, central South Australia, is thought to possibly from Late Pliocene. If it is of Late Pliocene age it would be the only known diverse mammal assemblage from that time (Tedford et al.,1986). In this local fauna there are many Pleistocene-like taxa, with representatives of living genera such as Lagorchestes, Bettongia and Vombatus. There is no indication of arid conditions of the Tiari Desert of the present. It is believed the region was probably dominated by open sclerophyll forest and woodlands, and not grasslands and dunes, at least up to about 2.5 Ma (Archer et al., in Hill, 1994).
There are no known diverse mammals assemblages from Western Australia from the Pliocene. The Quanbun Local Fauna of the Kimberleys (Flannery, 1984) is the only known assemblage from the Pliocene, and it is faunistically impoverished.
There are many mammal assemblages known from all Australian states and New Guinea during the Pleistocene and Holocene (Archer & Hand, 1984; Murray, 1991). Most appear to be of Late Pleistocene age, though most lack radiometric dates. The Early Pleistocene Fisherman's Cliff Local Fauna and Portland Local Fauna represent assemblages that are distinctly different. The last surviving ektopodontids, ?frugivorous possums, otherwise known from the mid- to late Tertiary. There were also many taxa missing from the later part of the Quaternary. The least well known Quaternary deposits are in New Guinea, but enough is known to show that some megafaunal species survived until the latest part of the latest Pleistocene that went extinct in Australia about 30,000 BP. Such megafaunal species as zygomaturine diprotodontids have been found in deposits in Irian Jaya and Papua.
Changes in the balance of indicator species, such as the local extinction, at about 20,000 BP, of the western barred bandicoot (Perameles bouganville), an arid/semiarid adapted species, from the extreme southwast of western Australia (Merrilees, 1979). It appears that in Australia several periods of lower temperatures and higher aridity correspond to times of glacial maxima in the Northern Hemisphere. A major contraction of the rainforests of northwestern Queensland occurred as a result of the period of severe aridity that occurred about 17,000 BP (Kershaw, 1983). Because of an almost complete lack of deposits in eastern Australia that can be confidently dated to the Early-Middle Pleistocene it can only be assumed that the diversity of mammals of the wet forests of Australia declined, as well as a contraction of range, during these times of increased aridity.
After the Middle Miocene gigantism was a feature of Australian lineages, including marsupials, and especially among the herbivorous marsupials that survived the Late Miocene such as kangaroos, wombats and Diprotodontids. Diprotodontids were about the size of a calf during the Oligocene-Miocene. The mean size of diprotodontids had increased to abut that of a cow by the Middle Pliocene to Early Pleistocene. All the diprotodontids on mainland Australia were of the larger size by the Late Pleistocene. In Irian Jaya zygomaturines of calf size persisted (Flannery & Plane, 1986), and in Papua slightly larger zygomaturines that have been compared to pandas, persisted. Other vertebrates that were part of the megafauna of this time were Megalania prisca and Wonambi naracoortensis, a giant python (Murray, 1991).
It is not certain what effect this megafauna had on the vegetation of Australia, but it is believed to have enabled the large herbivores to browse to progressively greater heights and to transport larger seeds over larger distance. There would also the collateral damage of graviportal trampling of soils and surface plants.
The Late Pleistocene-Holocene trend to gigantism was reversed, the trend becoming dwarfism, at least compared to the sizes reached by the megafauna (Marshall & Corrucinni, 1978), the size of some species such as large Macropus grazing kangaroos reducing by about 30 % . It has been suggested that the decline in size may have resulted from the difficulty the larger mammals faced raising large young to independence when climates were so arid during glacial phases, at times covering 50 % of the continent during the Late Quaternary.
M. Archer et al., in Hill, Robert S., (ed.), 1994, History of the Australian Vegetation, Cambridge University Press.
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