Australia: The Land Where Time Began
Murgon Fossil Site
Murgon is situated in south-east Queensland, along the Bunya Highway, 270 km northwest of Brisbane. The location of this fossil site is a place of rolling hills, some of the higher of which are capped with basalt from lava that flowed down valley floors about 25 million years ago. The hardness of the basalt has resulted in them forming a fossil landscape as the softer surrounding rocks have been eroded down leaving them elevated above the areas that were originally above them, an inverted landscape. This has prevented the valleys that contained the swamps where the fossil deposits were forming from being eroded along with the former higher ground around them. One of these basalt capped hills is Boat Mountain, named from its resemblance to an upturned boat. The exposed sides of Boat Mountain allow access to some of the fossils that are protected beneath the cap.
During the Early Eocene, about 45.5 million years ago, this site was a shallow swamp or lake. The Tingamurra Local Fauna deposits formed from freshwater clays laid down at this time. It is the only known site in the Australian part of Gondwana older than 24 million years that contains marsupial fossils. At the time the Tingamurra Local Fauna deposit was forming Australia was still part of Gondwana. It is also the only known fossil site in Australia from the beginning of the great diversification of mammals after the demise of the dinosaurs.
Other firsts and only include the oldest known songbird, Australia's oldest known marsupial, frog and crocodile, one of the world's oldest known bats, the only known eiopelmatid frogs, and only salamander fossils found in Australia. There were also nonviolent placentals, madtsoiid snakes, trionychid turtles and birds.
The Murgon songbird is 20 million years older than any songbirds known anywhere else in the world. It has yet to be named, as it is known only from a leg bone, distinct enough to show it is a songbird, but more is required to allocate it to a species, or anything else to be known about it. This suggests that the songbirds may have arisen in Australia, spreading to the rest of the world, possibly along the reverse of the route taken by bats that arrived in Australia. Birds don't usually leave fossils, partly because their bones are so fragile, and partly because they tend to live in places where fossils are not easily formed. At Murgon they inhabited a swamp, which may be how this single bone came to be fossilised. The dearth of fossil birds makes the unique status of the Murgon songbird uncertain. The only certainty is that they were in Australia at the time the Murgon fossil deposits were being laid down. All that can be said about this bird is that it had a wing span of about 35 cm.
Kambara implexidens, from the Queensland Early Eocene, was a mekosuchine, a group of primitive Gondwanan crocodiles. This genus is the best known Palaeocene to Oligocene crocodiles in Australia. 2 closely related species of this genus has been found at the Murgon site. At about 2 m long, it was a similar size to K.murgonensis, but more slender.
Kambara murgonensis, from Murgon, about 2 m long. The tooth arrangement indicated that it was a crocodile, but unlike crocodiles and like alligators, the teeth of the upper and lower jaw didn't interlock when the mouth was closed, the teeth of the bottom jaw resting inside those of the upper jaw, as in alligators.
Both species of Kambara at the Murgon site were found in what appears to be the same strata. If they actually did coexist it would be unusual for crocodiles, as they are usually so territorial they would not tolerate each other. It may be that the waterhole/billabong they lived in was ephemeral, drying up periodically. If this was the case they may have forced together as the water body shrank, or they may have lived there sequentially, as when the water body refilled after a dry period, being populated by a different species from the species that occupied it during the previous wet phase.
Among the marsupials at Murgon, the oldest record of marsupials known from Australia, were types that are unrecognisable when compared to the modem marsupials of Australia. Some of the new marsupials at Murgon are similar to South American species. Chulpasia spp have also been found in deposits of a similar age in Peru. As with all mammals at Murgon, these animals are known only fro their teeth, so all that can be stated about their appearance is that they were probably about 20 cm long, and even that is an estimate. It does seem likely that they may have been mainly herbivorous, possibly eating a mixed diet of seeds, fruit and insects.
A bandicoot is known only from its teeth that would probably have been about 25 cm long, and possibly of similar appearance to a modern bandicoot, probably eating fruit and small animals on the forest floor. As it is double the age of any other known bandicoot, suggesting that this Australian group appeared very early in the evolution of marsupials.
Djarthia and Thylocotinga are among several insectivorous marsupial found at Murgon. Djarthia, about the size of a mouse, is known from some fragmentary jaws with teeth and cheekbones. Thylocotinga was possibly the size of a small cat, is known from some isolated teeth. It has been suggested that there is a similarity to South American forms, often coming from deposits of similar age on both Continents, in spite of the Australian species being definitely of local origin. Thylacotinga is believed to have characteristics of both South American groups of marsupial, but could be assigned to neither. One marsupial from Murgon could possibly represent a South American group, and is believed to probably be ancestral to all Australian groups. Australia had been part of Gondwana, connected to Antarctica, which was also connected to South America, for millions of years, making it likely that the inhabitants of South America, Australia and Antarctica would have comprised similar animals and plants, radiating from these related ancestral forms in each continent, especially after separation began.
There is a gap of about 30 million years between the mammals of the Tingamurra Local Fauna and the next oldest mammal fauna of Australia is found at Geilston Bay, Hobart, Tasmania.
A tooth less than 3 mm long, of Tingamarra, was found at Murgon that is believed to have belonged to a group of extinct placental mammals, condylarths, the group that is believed to be ancestral to later placentals in the rest of the world, but in Australia it went extinct, apparently out competed by the ancestral marsupials. As well as the original tooth some other bones have now been found, an ankle bone and ear bones, and another tooth. All bones are believed to have come from Tingamarra that appears to have been about the size of a large rat. All the bones show characteristics of placentals. There are some who deny Tingamarra is a placental, suggesting that it was a marsupial that had evolved placental-like characteristics, but the evidence from the teeth, the microstructure of the enamel display patterns seen only in placentals, suggests it is most likely a placental.
Before their discovery at Murgon bats were thought to have arrived in Australia at a much later date after evolving in the Northern Hemisphere. Then one of the world's oldest known bats, Australonycteris, was been found in Australia, at a time not much younger than the oldest known bats from France. Assuming they had indeed evolved in the Northern Hemisphere they apparently arrived in Australia possibly less than 1 million years after they evolved. Having accepted that they arrived from somewhere, exactly where became a problem. To the north was a vast unbroken expanse of ocean with no known continental landmasses. Australia was situated about 2500 km further south than at the present. The problem with their arrival from South America, is seemingly obvious given the connection via Antarctica, but there are no known fossil bats from this time in South America.
A surprise with the fossils of Australonycteris at Murgon is that they were about 20 cm across and seemed to have looked very similar to modern bats and the periotic bone, of the region of the ear that has been found demonstrates that at this early stage of their evolution they already had the ability to echolocate.
Microfossil assemblages from the Late Carboniferous at Murgon include scales and teeth resembling those of neoselachians (modern sharks), these advanced forms being associated with primitive ray-fins (palaeoniscoids).
The fossils of turtles are even more common than those of crocodiles in the Murgon deposit. The Tinganarra soft-shelled turtle is among those discovered here, trionychid turtles, with shells comprised of various bones that are not knitted together, so being not as strong as in forms where the bones are knitted together. As a result the shells of these turtles are often found as isolated bones. Among the shells that have been fossilised in place the longest known is about cm long. The shells remain in the more "soft" condition than in most other turtles to allow the shells to deform as they squeeze into small spaces, probably after prey, which is believed to have been animals such as crayfish, frogs and small fish. The Tingamarra soft-shelled turtle died out about 40,000 years ago, though similar soft-shelled turtles still survive in New Guinea, Asia and Africa.
Madtsoiid snakes survived past the end of the Eocene, 35 million years ago, only in Australia, finally becoming extinct about 60,000 years ago.
Alamitophis tingamarra A small madtsoiid, about 80 cm long, from the Eocene. It was the smallest of its genus. Other members of the genus are found in Patagonia. As it was small, it probably ate small prey such as frogs, lizards and small mammals. It is known from vertebrae and dentaries.
Their closest relatives are other members of the genus from the Late Cretaceous deposits of Patagonia, though they are also related to other extinct Australian snakes, Yurlunggur and Wanabi naracoortensis, large snakes from the Miocene and Pleistocene.
Patagoniophis sp. have also been found in Murgon and Patagonia.
Letters to Nature
Nature 356, 514 - 516 (09 April 1992); doi:10.1038/356514a0
Australia's oldest known non-volant placental mammal, Tingamarra porterorum, was represented at this site by a tooth, was found at Murgon. It is believed it is probably a condylarth-like animal. Prior to its discovery it had been thought that placental mammals didn't arrive in Australia until about 5 million years ago. This find showed that both placentals and marsupials had been in Australia since at least 55 million years ago, as determined by radiometric dating. This challenges the presumption that marsupials dominated the therian assemblages prior to the arrival of placentals in the late Tertiary, inferring that they replaced marsupials soon after they arrived. It seems the opposite was true, the marsupials out-competed the placentals in the increasingly unusual Australian environment. All known marsupials found in the Tingamurra Local Fauna are more derived (dilambdodont) than peradectids. None are definitely a member of known families in Australia, though some might be uniquely plesiomorphic dasyuroids or perameloids. One is autapomorphically specialised (a derived trait unique to a terminal group - found in only 1 member of a clade, not even in closely related species, family, etc), indicative that the Australian Peninsula of Gondwana was at least partially isolated.
Journal of Mammalian Evolution, Volume 06, Number 3, September 1999 , pp. 289-313(25)
A new genus and species of marsupial has been found at the Murgon fossil site. Djarthia murgonensis, a member of the Tingamarra Local Fauna, is described on the basis of teeth. In D. murgonensis, the combination of marsupial synapomorphies and symplesiomorphies suggests it should be placed in Didelphidae or Australidelphia. The tarsal morphology of this genus is presently unknown, so it cannot be assigned to Ameridelphia and Australidelphia resepctively. Therefore it cannot be confidently assigned to either clade. If it is australidephian it supports the hypothesis that the Australian marsupial radiation occurred from a common ancestor with didelphoid-like dental features. Previously it has been contended that South American and Australian marsupial faunas are manifestly distinct, apart from the australidelphian affinity of South American microbiotheres.
Because the tarsal anatomy of D. murgonensis, and some other generalised Australian fossil taxa are unknown, and no synapomorphies of the dentition has been revealed by character analysis, it is not possible to define either Ameridelphia or Australidelphia to the exclusion of the other, the authors consider present evidence neither supports nor refutes the argument that Australasian and American marsupial faunas are distinct. And they consider the phylogenetic position of Ankotarinja tirarensis and Keeuna woodburnei. They recommend that these central Australian fossil taxa should be assigned to Marsupialia incertae sedis until sufficient material is available to assign them to either Australidelphia or Ameridelphia.
Cranial and post-cranial fossils of D. murgonensis indicates that it is a member of the Australidelphia. This pan-gondwanan clade includes all extant Australian marsupials and South American microbiotheres. This makes it the oldest crown-group marsupial in the world to be represented by dental, cranial and post-cranial remains. It is 30 million years older than any other known Australian marsupial. It is the most pleisomorphic Australiadelphian. Phylogenetic analysis shows that it is outside all other Australian marsupials. Because it is the oldest unequivocal Australidephian and the most plesiomorphic, it is believed it could be close to the ancestral morphotype of the Australian marsupial radiation, and back-dispersal from Eastern Gondwana of the South American microbiotheres.
|Author: M.H.Monroe Email: firstname.lastname@example.org Sources & Further reading|