Australia: The Land Where Time Began
In the book Australia's Mammal Extinctions, Chris Johnson has a section on what is known of the origin of marsupials, and their spread from South America to Australia via Antarctica, and discusses the possible reasons for the dominance of marsupials in Australia.
The earliest mammals known from Australia date from about 120-105 Ma, there are 2 monotremes and several species that are not believed to be closely related to any living animals. Steropodon galmani from Lightning Ridge, and Teinolophos trusleri, a primitive platypus. Kollikodon ritchiei, from Lightning Ridge, was originally classified as a monotreme (Flannery et al., 1885). Later detailed studies indicate it may be a new type of mammal, and possibly a distant relative of monotremes (Musser, 2003). At about the same size as Steropodon, they were the largest known mammals from that time anywhere in the world.
The 2 animals known from the Cretaceous Flat Rock site in Victoria have been assigned to their own group, the ausktribosphenids. They were less than 10 cm long, and appear to be shrew-like animals. Their relationship to other mammal groups is unknown at the time of writing. The placental mammals are known only from the Northern Hemisphere until 65 Ma, but Rich et al. (1997) believe the ausktribosphenids are placentals. Many alternative classifications have been proposed for them, some believing they were actually monotremes. Woodburne (2003) has summarised the various interpretations, concluding that they were probably an ancient lineage, now extinct, of eutherian mammals that were related to the livings eutherians. These ancient mammals lived at a time when Australia was still part of Gondwana, the southern parts being cold and seasonal, and the more northerly parts warmer than now, and with a more uniform climate.
Forests of Araucariaceae and Podocarpaceae conifers covered the continent. It is believed these forests were mostly open-canopied, with a greatly diverse understory (Henderson et al., 2000). The climate is believed to have become warmer and wetter, beginning in the Late Cretaceous, Angiosperm-dominated dense rainforests expanded greatly, wet rainforests covering most of the continent by about 50 Ma in the Eocene. Typical of these forests were species of Nothofagus (Subgenus Brassosporus), that presently grow only in warm environments that are continuously wet, in New Guinea and New Caledonia. Gymnostoma, a rainforest-restricted lineage of Casuarinaceae, that occurs only in a small region of the wet tropics on the Atherton Tableland, was also common in the Eocene forests of Australia. Much of their Eocene range covered central Australia that is now in the arid zone. It was these forests that made possible widespread distribution of the earliest mammals to arrive in Australia.
The earliest known marsupial, Sinodelphys szalaya, has been found in deposits in northeastern China dating to about 125 Ma (Luo et al., 2003). This was a small insectivore/carnivore that had adapted for tree-climbing. It is the earliest known member of the Metatheria, the group that living marsupials belong, though it was not strictly a marsupial.
Metatherians are believed to have crossed from Siberia to Alaska, making their first appearance in the fossil record in western part of North America about 115 Ma in the mid Cretaceous. Kokopellia juddi, one of these early mammals was the earliest mammal to be classified as a marsupial (Cifelli & Muizon, 1997), though not all authorities agree with this classification.
Marsupials and placentals both flourished in North America between 115 and 65 Ma (Cifelli & Davis, 2003). Between 65 and 60 Ma they extended their range into South America, just after the close of the Cretaceous (Muizon et al.,1997). At the time of the crossing from North to South America, the 2 continents were connected only by a chain of volcanic islands, rising from what is believed to have been a shallow sea. It is assumed the animals island-hopped to South America on flotsam (Lillegraven, 1974). Once the marsupials travelled through the length of South America they could move all the way to Australia, as both South America and Australia were still part of Gondwana at this time. Marsupials, that are believed to be about 45 million years old, have been found on Seymour Island of the Antarctic Peninsula near the southern tip of South America (Goin et al., 1999). They resemble taxa from much earlier faunas of South America, though 2 species that are believed to have diverged in Antarctica differ from all known South American species. Based on these finds, Goin et al., (1999) suggest that 60 Ma could be the time when marsupials first colonised Antarctica from South America.
The only Cretaceous mammals known from Australia all come from 2 fossil sites, and the few fossils found indicate that there was a rich mammalian fauna in Australia at that time, and that they must have been diversifying for some time, usually believed to have been at least as far back as the Jurassic. The oldest mammalian fossils in Australia have been found in the Tingamurra fossil deposit near Murgon, Queensland, that has been dated to at least 55 Ma (Archer et al., 1993; Godthelp et al., 1992). The fossils from this site mostly differ from those found in both Antarctica and South America, indicating that they have undergone a long period of evolution after reaching Australia, apparently arriving in Australia soon after spreading to Antarctica from South America, arriving in Australia long before 55 Ma.
No fossil mammal is known in Australia between 105 and 55 Ma. I is assumed the ausktribosphenids went extinct at some point in that gap in the fossil record, as no later animals are believed to be descended from them. At 25 Ma the Ornithorhynchidae, the platypus family, first enter the Australian fossil record, the echidnas appearing a bit later. Prior to the Late Pleistocene extinctions, the echidnas became moderately diverse, at least 6 species distributed across and Australia and New Guinea (Musser,1999).
The only marsupial family that has been found that can be traced from South America to Antarctica, then to Australia is the family Microbiotheriidae. Dromiciops gliroides is the only living member of the family, found in the Nothofagus forests of southern South America. It is called monito del monte (little monkey of the mountains). In Antarctica, a fossil of a species of Microbiotheriidae has been found on Seymour island, adjacent to the southern tip of South America. The other 2 fossils of this family are found in the Tingamurra Fauna from the Murgon Fossil Site (Long et a.,. 2002). At both the Seymour and Tigamurra sites the fossils were from deposits where the vegetation was Nothofagus forest when the animals were living there. The living species, Dromiciops australis, is closer to the Australian marsupials than it is to the other South American marsupials (Cardillo et al., 2004). It is believed that modern Australian marsupials probably descended from a common ancestor with microbiotheriids.
The earliest known placentals found in South America occur shortly after the close of the Cretaceous, and are both less diverse and less divergent from the North American placentals they derived from than are the South American marsupials of the early Tertiary from the North American marsupials. This leads to the belief that the placentals arrived in South America later than the marsupials (Muizon et al., 1997). The same situation is found in Antarctica, the placentals are less diverse than marsupials, and are closer to their South American ancestors than are the marsupials to theirs. Again indicating that they arrived later than the marsupials (Goin et al., 1999). Godthelp et al. (1992) described Tingamurra porterorum from the Murgon site as a placental. This is the only placental, other than bats, that has been found in Australia from the Tertiary before the much later arrival of the rodents. Szalay (1994) and Woodburne & Case (1996) have challenged the description of T. porterorum as a placental.
Why marsupials spread from Antarctica to Australia but placentals didn't
A suggested reason for placentals failing to spread to Australia is that they reached the line of connection between Antarctica and Australia after the 2 continents had started to separate. By 70 Ma, the ocean gap between the landmasses may have already started forming (Smith et a., 2004). The marsupials appear to have reached South America before the placentals, and did the same between South America and Antarctica, so it seems likely they did the same on reaching Australia.
Another suggested reason is that both marsupials and placentals arrived in eastern Antarctica after the separation, the marsupials proving better suited to the sea voyage on flotsam. The known marsupials from Antarctica were all small, thought to weigh less than 350 g, (Goin et al., 1999) and are believed to have been arboreal, eating insects and possibly fruit. Being small, and with the possibility of insects, a highly nutritious, concentrated food source, also being carried on the flotsam, that allowed them to survive longer than a placental would, especially as they have lower metabolic rates than placentals, and placentals found in Antarctica tended to be larger herbivores that would require more food. Small arboreal species would also be more likely to be cast adrift on floating forest material than larger herbivores that were probably not arboreal.
Marsupials arrived in Australia at a time when rainforests were expanding across the continent, and with the only mammals already in Australia being the monotremes that were already highly specialised, they were able to diversify explosively to fill the many unoccupied niches, mostly in complex rainforests, reaching the peak of their diversity in the Early Miocene. At this time there were 50 % more families than there were in the last few million years (Archer et al., 1999).
After the Early Miocene the Australian climate began to become cooler, drier and more variable, the trend eventually leading to the arid conditions of central Australia (McGowran et al., 2000; Quilty, 1994). The climatic change was brought about by the formation of the Antarctic Circum Polar Current as the last of the Southern Hemisphere landmasses, Australia, broke from Antarctica.
Climate change in the Miocene
As the climate dried and cooled, the rainforests that had been so widespread in the Eocene contracted, being replaced by more open and drier forest types, and in places woodlands, shrublands and grasslands. Casuarinaceae commonly dominated these forests and woodlands, together with acacia, asteraceae (daisies) and Chenopodaceae (chenopods - bluebush and saltbush), and expansion of a shrub and herb layer beneath the canopy of open forest. In composition and distribution of structural types, the vegetation was similar to the present by the end of the Pliocene. There were some difference compared to the modern vegetation, rainforests were still widespread, and there were more extensive dry rainforest in which conifers, mainly Araucariaceae, were the dominant tree type. The dry forests differed from the present in that Casuarinaceae were often more common than eucalypts (Johnson, 2006).
The rainforest-dependant marsupial groups declined as the rainforests contracted, the family-level diversity declined and never recovered. The expansion of the new dry conditions gave rise to a burst of diversification in new directions, as the marsupials radiated to take advantage of the new niches opening up. As the dry vegetation expanded across central and northern Australia in the latter part of the Miocene insectivorous and carnivorous marsupials (dasyuromorph) experienced a diversification surge in the middle to Late Miocene. Small insectivorous marsupials, such as dunnarts and planigales, that occupied grasslands, shrublands and open woodlands, comprised the bulk of the new species resulting from this surge in diversification.
Pliocene and Pleistocene dry climate & large herbivores
During the Pliocene and Pleistocene climatic conditions were such that vegetation over much of Australia suited the evolution of large herbivores, of which kangaroos and wallabies (Macropodidae), were the most successful. There were 95 species of kangaroos and wallabies in Australia and New Guinea by 100,000 years ago (Cooke & Kear, 1999; Prideaux, 2004). In this rapid diversification the main evolutionary direction was towards adaptations for life in the increasingly dry country, such as increasing body size, changes in the teeth, digestive systems and skulls to allow processing of tough, abrasive plant material which was often low in nutrients. The sthenurine kangaroos had the most highly adapted structures for such a diet, many species occurring in shrubland and dry woodland.
Increasing body size and species numbers also took place in the wombats (Vombatidae) and the diprotodonts (Diprotodontidae). The large predators, thylacines (Thylacinidae) and marsupial lions (Thylacoleonidae) also increased in size.
At the beginning of the Pleistocene, about 2.6 Ma, the climate began to alternate between cool, dry and warm, wet periods, as the glaciers of the northern Hemisphere ice age expanded and contracted. As the glaciers expanded the global sea levels dropped and as the glaciers retreated the sealevels rose. The constantly changing climatic conditions would have added to an already variable climate in Australia, having profound effects of the Australian biota. Evidence from northern Australia for the last glacial cycle indicate that about 120,000 years ago the sea level was approximately about what it is now, or possibly slightly higher. Between 120,000 and 30,000 years ago there was a gradual drop of sea level to about 70 m below present level, then about 30,000 years ago it dropped rapidly to about 130 m below present level. Shortly after 20,000 years ago it began rising and by about 7,000 years ago it stabilised at about the present level (Lambeck et al., 2002).
Most of present-day New Guinea was still submerged during the early part of the Tertiary, large areas of southern and western parts being uplifted in the Oligocene, the exposed surface area being connected to Australia. This area would also have been occupied by marsupials (Flannery, 1995). Large areas of New Guinea were again submerged during the Miocene, only small areas of central and southern highlands remaining above sea level, isolated from Australia.
Between 15 and 10 Ma the edge of the Australian plate, which was moving north, slammed into the Pacific Plate in the area of New Guinea, leading to a big increase in mountain building. This orogenic episode gave New Guinea its present shape, with its high topographic diversity. The presence of high mountain ranges ensured New Guinea maintained its humid climate, even though northern Australia dried out during the Miocene and Pliocene.
Some marsupial lineages, such as tree kangaroos (Dendrolagus) and cuscuses (Plahanger, Strigocuscus and Spilocuscus), were all specialists of wet complex rainforests. During the Pleistocene glacial phases the Carpentarian Plain was exposed a number of times, but there was very little mixing of faunas between Australia and New Guinea, the main reason being the differences of habitat between the 2, wet rainforests in New Guinea and much drier vegetation in Australia at the colder, drier times when the land bridge existed.
An alternative origin, based on the evolution of marsupials in South America, has been proposed in a paper in the online journal PLoS Biology.Sources & Further reading
|Author: M.H.Monroe Email: email@example.com Sources & Further reading|