Australia: The Land Where Time Began
At the present, 59 species of rodent make up nearly 25 % of the native Australian terrestrial mammal fauna. The fact that they were able to colonise Australia where there was a diverse marsupial fauna is not surprising, given that they are the most diverse of all mammals, having more species than all other mammals of the world combined.
Australia's rodents are all from the family Muridae, rats and mice. It appears the other rodent groups failed to reach Australia, even though they have a much longer fossil history. Murids originated in the eastern hemisphere, but some species such as the house mouse (Mus musculus), Black rats (Rattus rattus) and brown rats (R. norvegicus), have been distributed to all parts of the world by humans.
The Australian murids are usually divided into 2 groups, the 'old endemics', mostly confined to continental Australia, are believed to have evolved after the group arrived in Australia. Included among these are the false mice (species of Pseudomys), water rats (species of Hydromys and Xeromys) and rock rats (species of Zyzomys), and many others. The 'new endemics', including species of true rats (species of Rattus), seem to have entered the continent less than 1 million years ago.
Prior to the discoveries at Rackham's Roost there were only 3 known rat fossils from the Tertiary. The distribution of these fossils led to the conclusion that rodents arrived in Australia a bit more than 4.5 million years ago. The oldest known rodent fossil was an incisor from the 4.5 million years old deposit at Bluff Downs in northeastern Queensland.
2 rodent incisors had been found at Chinchilla, in southeast Queensland, that were believed to be about 4 million years old. The Hamilton fossil site in Victoria, that had a rich mammal fauna from about 4.5 million years, had not produced any rodent remains. Based on the spread of known fossils it was suggested that murids arrived from the north about 4.5 million years ago, reaching the southern part of the continent some time later.
Molecular evidence suggests that the Old Endemic group had evolved by 15 million years ago, species of present-day genera were present by 10 million years ago, The molecular evidence can't indicate whether they were living in Australia or to the north. The world's oldest known murid from Pakistan, Antemus chinjiensis, considered to be very primitive, and probably basal to the murid radiation, is about 15 million years old.
A small block of limestone from Rackham's Roost was found containing the teeth of 7 different kinds of rodents, and another from the same site contained teeth of Protemnodon snewini, an extinct kangaroo that was previously known from the 4.5 million year old deposit in Bluff downs Local Fauna. At the time of writing, 13 kinds of rodents had been found in the Rackham's Roost deposit. Such diversity leads to the conclusion that rodents had been present in Australia long before the deposition of the Rackham's Roost site.
Rodents don't appear in the Riversleigh Oligocene-Miocene deposits until after about 12-15 million years ago, and are not known from the Bullock Creek Local Fauna of Middle Miocene age, or from the Late Miocene Alcoota Local Fauna in the Northern Territory, or from the Late Miocene Beaumaris Local Fauna of Victoria. It has been concluded that it is unlikely that murids entered Australia about 6-7 million years ago, in the latest Miocene. The murid fauna of Rackham's Roost are extinct, but some of them have living descendants.
A new species of rock-rat (Zyzomys sp.) was the first to be recognised. At present rock-rats are found across northern Australia, the common rock-rat (Zyzomys argurus) is the most widely distributed of the 5 extant species. It is always found around dry rocky outcrops, as occurs on Riversleigh Station. Zyzomys species are the most abundant mammals found in the Rackham's Roost deposit, probably inhabiting the same rocky hillsides their descendants are still found on.
The ghost bats that live in caves along the Gregory River still eat rock-rats, so at least some things haven't changed much on Riversleigh for the last 4 million years. The Eurasian Antemus and related species were motley inhabitants of open woodlands and grasslands. It seems the Old endemics had established their habitat preferences before arriving in Australia. Near the end of the Miocene the Australian continent was drying and the rainforests receding towards the coast. The murids probably arrived at around this time when their preferred habitat was expanding and the rainforest marsupials were still moving into the newly available niches. It was probably the best time for the murids to arrive, having already evolved types that could take full advantage of the woodland habitats, giving them the advantage over the marsupials that were still in the process of moving out of the diminishing rainforests.
There were at least 6 species of false mice (Pseudomys spp.) in the Rackham's Roost deposits. Pseudomys was the most diverse group of murids found there, and it is still the most diverse group of murids in Australia. There was also a very small species of Leggadina.
It is thought the rodents found at Rackham's Roost are possibly the last of the original invaders, so may provide clues to the evolution of succeeding forms of Australian rodents. To date, no species closely related to the Old endemics of Australia have been found in surrounding regions. It has been suggested that this could be the result of the remnants of the original invasion being replaced by more advanced rodents, or the that the murids in Asia could have radiated so rapidly in Asia that the derived forms are no longer recognisable as relatives on the Old endemics.
There are caves in the limestone cliffs along the Gregory River where Pleistocene and Quaternary deposits in which mammal fossils have been found. These are mostly modern species, most still being present in the Riversleigh area. The 'new endemics' are often found in these deposits, including species of Rattus. One of these, the Long-haired Rat (Rattus villosissimus) can still be seen in the area.
|Author: M.H.Monroe Email: firstname.lastname@example.org Sources & Further reading|