Australia: The Land Where Time Began
Birds from the Riversleigh deposits include dromornithids, casuariids and wading birds, song birds, as well as other orders of birds. The Riversleigh deposits are among the most important sites for fossil birds in Australia because of their diversity and the age of the deposits. Bird fossil sites from the Tertiary, such as those from central Australia, are mostly of waterbirds. Riversleigh has many terrestrial birds as well as waterbirds. It has the earliest deposits of bird faunas that did not require open water. This has allowed for a better understanding of the structure of Australia's bird faunas as they evolved through the middle and late Tertiary.
The large numbers of birds from different niches in the Riversleigh rainforests provide a different view of the ecology of Riversleigh than that obtained by studying the mammals of the area, making for a more complete picture of the time, adding to the accumulating knowledge of the palaeoecology, palaeoenvironments and palaeogeography of Riversleigh during the Oligocene-Miocene.
Dromornithids see Dromornithidae
Also known as Mihirungs or thunder birds, these were very large, at least 2 genera of them are present in the Riversleigh deposits, 1 species of Barawertornis and 1 of Bullockornis. They are known only from Australia, they survived until about 26,000 years ago, and were apparently known to the Aboriginal People who arrived in Australia about 60,000 years ago. Among the Dromornithidae is what is thought could be the heaviest bird ever, Dromornis stirtoni, at 3 m tall and an estimated weight of about 400 kg, from the Late Miocene Alcoota Local Fauna in the Northern Territory. They looked a lot like other flightless birds, the ratites, such as emus and cassowaries, but they are believed to be more closely related to the galliform birds. They are common in many of Australia's fossil sites.
They had lost the capacity to fly but their bones remain those of other birds that are designed to reduce the weight as much as possible while being hollow and strengthened by a system of struts. A number of the Riversleigh sites contain many dromornithid bones. Their bones are often associated with those of large aquatic animals such as turtles, crocodiles and lungfish. It seems possibly they spent some time in water, as do living cassowaries that are good swimmers.
The best known dromornithid fossil from Riversleigh has been called 'Big Bird', along with its bones were the rounded gizzard stones it had swallowed to grind its food.
At Riversleigh there is a species in the Oligocene-Miocene deposits, Dromaius gidju, that has characteristics found in both emus and cassowaries, that is smaller than the smallest of the living dwarf cassowaries of New Guinea. It appears it may have been able to run slower than an emu but faster than any of the 3 extant cassowaries.
The cockatoos, family Cacatuidae, are present in the Riversleigh deposits as a more than 20 million-year-old fossil that was about the size of a galah. It is indistinguishable from smaller species of the genus to which the galah belongs, Cacatua. In Australia only 1 species of cockatoo live permanently in rainforest, but outside Australia there are several smaller species that do so.
The Passeriformes comprises half of all living species of bird. Recent findings suggest that this group of birds originated in the Southern Hemisphere, instead of the Northern Hemisphere as has been believed for some time. Their first known appearance in the Tertiary of the Northern Hemisphere was in the Late Oligocene. They have a significant presence in the Riversleigh deposits, some from the Oligocene deposits at Riversleigh of equal age to the oldest known passerines from the Northern Hemisphere.
The genus Orthonyx, is a very old passerine group from Australia and Papua. They have a distinctive skeleton as a result of an unusual feeding method. Logrunners scratch in the soil with their feet, which isn't unusual, but they do it in an unusual way. They scrape the foot backwards while leaning on long spines of their tails, and kick the foot out sideways. They have well-developed thigh muscles connecting to the pelvis. They can be identified by the shape of the thigh bone. The Riversleigh species is smaller than the 2 living species, which also inhabit rainforest, in New South Wales, Queensland and New Guinea. It differs from species found in South Australia.
Among the bird fauna of Riversleigh is the first known fossil lyrebird, Menura. Living lyrebirds live in the eastern Australia rainforests. There are also swifts and possibly swiftlets, similar to the extant genus Collocalia. Swiftlets of this genus nest on the cave walls in north-east Australia and Southeast Asia that use echo-location.
A predatory bird found in the Riversleigh deposits had an unusual mode of feeding, evolving special adaptations to allow it to rotate its legs to reach into tree hollows to hook prey with its talons. The morphology of its legs is similar to that found in 3 extant species that are known to use their legs in such an unusual fashion, distinctive articular surfaces on the same leg bones, so it can be assumed that the Riversleigh bird shared a similar the behaviour with the 3 extant species. It is double the size of the living birds, and doesn't appear to be closely related to any of them, Polyboroides radiatus and P.typhus from southern Africa and Madagascar and Geranospiza caerulesens from South America.
Among the fossil birds from Riversleigh are also storks (some of which appear to be most closely related to birds outside Australia), rails, and many as yet unidentified bird bones.
Michael Archer, Suzanne J. Hand & Henk Godthelp, Australia's Lost World: Riversleigh, world heritage Site, Reed New Holland
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