Australia: The Land Where Time Began
Bat bones are the most common bones in the many Riversleigh deposits, numbering in the millions. They are from 35 different kinds of bats, 23 of which are found in the Oligocene-Miocene deposits and 12 in the Pliocene deposits at Rackham's Roost site.
As well as thousands of bones from many species, there are also perfectly preserved skulls, jaws, and in some cases, natural brain casts. On a world scale, the bat deposits are at Riversleigh are among the richest, in both species and numbers. The other known sites that can match the Riversleigh deposits for species numbers are found in cave and fissure fills from the Tertiary of Austria, France and Germany.
Bat bones don't fossilise well in deposits other than in caves and fissure fills, because the bones are relatively fragile and don't survive intact for long under most circumstances. The only sign of fossil bats in Australia before their discovery at Riversleigh were 2 isolated teeth, one form the Oligocene deposits at Lake Palamnkarinna in the Tirari Desert and one from the Late Oligocene deposits at Hamilton in Victoria. A bat was discovered in the Big Sink carbonate deposits, Wellington Caves, New South Wales.
One quarter of Australia's mammals, not counting marine mammals, is comprised of bats, numbering 65 known species. 11 of these eat fruit, flowers and nectar, including flying foxes and blossom bats - the megachiropterans. The microchiropterans are the other 54 species that are insectivorous, are smaller and less conspicuous. One exception to the insectivorous habit is the Ghost Bat from northern Australia that is carnivorous.
Only microchiropterans have been found at Riversleigh at the time of writing, most being leaf-nosed bats. These are named from the complicated skin flaps around their noses that are used for directing the ultra-high frequency sound emitted by the bats to navigate by sonar and to find prey in the dark. They can direct the pulses of sound into a wide arc, wide-angle sonar, then direct it into a narrow beam for zeroing in on prey. The larynx of the bats can produce sound pulses up to 100/second, that this type of bat emit through their noses. They have special grooves in their ears to channel the reflected sound. Using this echolocation they can gather a large amount of information about their surroundings, including the size, shape, density, direction, speed of its prey, as well as objects such as obstacles in their path.
Hipposiderids - Old World leaf-nosed bats
The Old-world leaf-nosed bats at Riversleigh are only distantly related to the phyllostomids, the New World leaf-nosed bats (American leaf-nosed bats). There are presently about 60 species of hipposiderid bats, between 4 and 12 cm in length, are found in caves or in mines in tropical and subtropical parts of Africa, Asia, Australia, and the Pacific Islands. They often roost in colonies of 40,000 or more in northern Australia, though they sometimes roost individually.
An unexpected finding regarding the Riversleigh bats is the some, 6 species of Brachipposideros are found in French deposits dated to the Late Oligocene to Middle Miocene. 9 species of the same genus have been found in the Riversleigh Oligocene-Miocene and Pliocene deposits. Presumably the intervening area between Australia and France was at one time populated by bats of this genus, but there remains have not yet been found. The rare, brightly coloured Orange Horseshoe Bat Rhinonicteris aurantius, is a living species of Brachipposideros that is 1 of 6 species of leaf-nosed bats from northern Australia. This endemic species survives only in the caves where the temperatures reach about 32o C and humidity is about 100 %. The bat faunas of the Riversleigh rainforests were dominated by the ancestors of this species during the Oligocene-Miocene, roosting in the limestone caves.
At Riversleigh, Microsite and Bitesantennary Site are very rich in bat bones. Most of the bat remains from Microsite are of one species, Brachipposideros nooraleebus. The Bitesantennary Site was an old cave occupied by up to 5 species of Brachipposideros that appear to have coexisted. The fossils found in the Rackham's Roost site indicate that by the Pliocene the number of species had dropped to 4 species. The Orange Horseshoe Bat was the only species that survived the increased aridity that occurred at Riversleigh during the later Pliocene and by the Pleistocene, was still hanging on, but is now rare.
The rare, insectivorous-carnivorous Ghost Bat, or False Vampire, (Macroderma gigas) is the largest microchiropteran in Australia, measuring 13 cm long, has a wingspan of 60 cm. It roosts in mines and caves across northern Australia. Unlike any other microchiropteran bats in Australia, as well as feeding on insects, it also eats fish, small mammals, other bats, other invertebrates, lizards and birds. Its prey can be as large as pigeons. There are presently 5 species of ghost bat or false vampire, members of the family Megadermatidae, are found in tropical and subtropical areas of the Old World, 2 in Africa, 2 in Asia and 1 in Australia.
The ancestors of the Australian ghost bats have been found at Riversleigh, at least 1 species in many of the deposits, but there are 2 species in some of the deposits, one of which is usually smaller than the other. It has been assumed that the size difference at a particular site is of significance, allowing the 2 species to co-exist by reducing the competition between them.
Where different species of the living false vampire bats coexist over part of their range in Africa and Asia they are either of significantly different size or are from distinctly different lineages. In Asia, the species Megaderma spasma and Lyroderma lyra are of significantly different sizes. In Africa, the species Lavia frons and Cardiodermia cor are from distinctly different lineages. The latter of the 2 species pairs eats much more vertebrate prey than the other.
2 false vampires from the Dwornamor Local Fauna at Riversleigh, M. godhelpi and the 'Dwornamor Variant' are of similar size, but are from different lineages. One false vampire from the Middle Miocene Gotham City site on the Gag Plateau is about the same size as the living Ghost Bat, but other Riversleigh false vampires are smaller than the living Ghost Bat, Macroderma gigas. At the time if its discovery it was the largest bat in the Riversleigh deposits. The Gotham City deposit appears to be the floor of an ancient cave. The remains of many prey species of the ghost bats that roosted in the cave have been found in the limestone of the cave floor. The remains on the cave floor are frogs, fish, skinks, birds, dasyurids, bandicoots, acrobatids and a very small koala. The prey of the Gotham City Site ghost bats included a high proportion of arboreal mammals, as does that of the extant ghost bats in the rainforests of eastern Australia. In the Rackham's Roost Site deposits the vertebrate prey species composition of the ghost bat diet was mostly terrestrial vertebrates such as rodents and small dasyurids. The Rackham's Roost Site formed when the rainforest had already retreated to the coastal fringe of the continent, so the rainforest animals were no longer included in the diet of the ghost bats of the area, replaced by the vast stretches of woodland that spread across northern Australia.
The most common fossil bats at Riversleigh are Leaf-nosed bats and false vampire bats. The same applied to the European sites of a similar age, especially between the Eocene and Middle Miocene. At that time the southern European climate was warmer than the present and large parts of the continent was covered by forest. Brachipposideros spp. have been found in deposits from the Late Oligocene to Middle Miocene in France. The group disappeared from southern Europe when the climate cooled. False vampire bats seem to have survived until the end of the Tertiary in southern Europe.
There hasn't been a dramatic change in ghost bat diversity in Australia, but there was apparently a decline in that of the Orange Horseshoe Bat's lineage. The range of the living Ghost Bat has declined significantly since the arrival of humans in Australia, and the decline accelerated with the arrival of Europeans.
Some types of bats are less common in the Riversleigh deposits, such as the molossids (free-tailed bats), vespertilionids (plain-faced bats) and emballonurids (sheath-tailed bats). Members of all 3 families are found both in the Old and New Worlds, whereas the leaf-nosed bats and the false vampire bats are restricted to the Old World.
In the Oligocene-Miocene deposits of Riversleigh there are at least 3 types of free-tailed bats. One of these species, Petramops creaseri, does not appear to be closely related to any of the 6 extant species of free-tailed bats in Australia. The 6 living species are species of Nyctinomus, a cosmopolitan genus, or Chaerephon and Mormopterus, both Old World genera. It appears to have been from a more 'archaic' extinct bat fauna that seems to have left no descendants in Australia. The false vampire and Brachipposideros species would also have been part of this old World bat fauna.
The primitive free-tailed bats (cuviermops spp.) and (Rhyzomops spp.) from the Late Eocene-Early Oligocene deposits of Europe seem to be the most closely related to P. creaseri. Some suspect that this lineage may still be extant, in the form of the living Mexican Free-tailed Bat (Nyctinomus brazillensis), which some think is the most primitive of the free-tailed bats. In warm, low-latitude caves of Mexico that are the site of colonies that can reach into the millions. Some free-tailed bats annually migrate 1300 km from Canada to Mexico, as these bats are exceptionally good at flying long distances.
Free-tailed bats don't manoeuvre as wall as leaf-nosed bats, being designed for fast, direct flight. P. creaseri probably hunted its prey in the open above the canopy or at the edges of the rainforest, not in the clutter of branches beneath the canopy.
Vespertilionids - Plain-faced bats
The presence of these bats in the Riversleigh Oligocene-Miocene deposits is known only from a few isolated, unidentified teeth. In the Pliocene, as is evident from the Rackham's Roost site, they were much more common, though still less common than the 'archaic' hipposiderids. Among the present Australian bat fauna (more than 30 species), the vespertilionids outnumber all other bat types. Their colonies in northern Australian caves make up about half of all the cave-roosting species. The fossil vespertilionids from Rackham's Roost seem most similar to species of Scotorepens and Chalinolobus, other species of these genera roosting with leaf-nosed bats and the Ghost Bat in caves in northwestern Queensland.
Emballonurids - sheath-tailed bats
Sheath-tailed bats are among the commonest of the animals that use the present-day limestone caves in the Riversleigh area. These large bats are characterised by smooth faces, large eyes, and have the habit of hanging spreadeagled against the walls of the cave, fissure or rock shelter, just past the edge of the daylight zone. They are not known in the fossil record in Australia until the time of the Rackham's Roost deposit, at about 4-5 million years ago. The earliest known European fossil sheath-tailed bats is about 42 million years ago. 2 species of Taphanous are found in the Rackham's Roost deposit, where there are large numbers of individuals. This concentration suggests they were at least as common in the Riversleigh rainforest days as the sole species, Taphanous georgiana, in the same area is today.
Rhinolophids - Horseshoe Bats
Rhinolophids are apparently absent from the Riversleigh deposits. These are closely related to the leaf-nosed and false vampire bats of the Old World, both being members of the superfamily Rhinolophoidea. They have very large ears and complex horseshoe-shaped leaves around the nose. They have not been found, at least at the time of writing, in the Riversleigh deposits from the Oligocene-Miocene or the Pliocene. The oldest known Australian isolated bat tooth comes from the fossil deposits in the Tirari Desert of South Australia. This isolated tooth could possibly be from a bat of this family. At present, there are 2 species of horseshoe bat that are only found in the forests of coastal eastern Australia. Southeast Asia has a much higher diversity of horseshoe bats than Australia, New Guinea has 4 species, and Borneo has 10 species. 13 species have been found in the Oligocene-Miocene deposits of Europe.
Pteropodids - flying-foxes, blossom bats, fruit bats
This is the only bat group that are not known in the Australian fossil record prior to the Pleistocene. This is unexpected as the bats are large and very conspicuous in the tropical and subtropical rainforests of Australia. They occur in the fossil deposits in Eurasia and Africa that date from the same time as the Riversleigh deposits. Also surprising because they regularly fly large distances from their roosts to find food, and in the islands to the north of Australia they have diversified greatly. They are the only bats on some of the islands. At the present, in Southeastern Asian rainforests they are important pollinators - small fruit-bats, blossom-bats and nectar-feeding bats. Many of these bats live near the entrance of caves. It was expected that any fruit bats living in the Riversleigh rainforest would be found in the limestone because many animals are found in deposits that were not formed in caves.
At the present there is 1 endemic pteropodid bat, and 10 non-endemic species. Many of the islands to the north of Australia have a much greater diversity of fruit-bats. In New Guinea, 18 of the 25 species are endemic. It has been suggested that the greater diversity in New Guinea may have occurred that at the time of arrival of the pteropodids in New Guinea from the north, some time in the Early to Middle Tertiary, possums from fruit-feeding, blossom-feeding and nectar-feeding types were in the process of colonising from the south. On the other hand, when they reached Australia there was a well-established herbivorous, arboreal marsupial fauna filling the niches of fruit-feeding, blossom-feeding and nectar-feeding. In present-day Australia there are many more possums than there are pteropodids, but in New Guinea the possums and pteropodids are approximately of equal numbers.
Michael Archer, Suzanne J. Hand & Henk Godthelp, Australia's Lost World: Riversleigh, world heritage Site, Reed New Holland
|Author: M.H.Monroe Email: email@example.com Sources & Further reading|