Biology of Australia
At the present, Australia has 2 species of crocodile, both found in the north, the saltwater croc, Crocodilyus porosus, and the freshwater croc, C. johnstoni, a specialised fish feeder found in freshwater. A study by Paul Willis of the University of New South Wales suggests that the low number of crocodile species has probably only been the case for the last 10,000 years. The present time is the first time since the Mesozoic that there was as few as 2 species of crocodiles in Australia. Fossil crocs have now been found in many parts of Australia. Between the Mesozoic and the present there have been no fewer than 3 species in Australia at any one time.
As well as from the Oligocene-Miocene deposits at Riversleigh, crocodiles have been found in the Oligo-Miocene deposits of South Australia, at Lake Palankarinna in the Lake Eyre Basin and Lake Pinpa in the Frome Embayment. In the Late Miocene Alcoota Local Fauna, and the Middle Miocene Bullock Creek Local Fauna, from the Northern Territory. In Queensland they are found on the Early Tertiary deposits from Murgon and Rundle. The total number of taxa in most of theses fossil deposits is about 3. The crocodiles from the Riversleigh deposits, at the time of writing, number 9 species from 5 genera. The genera at Riversleigh are Crocodylus, Pallimnarchus, Baru, and 2 new genera.
Among the Riversleigh crocodiles is the first freshwater crocodile, of which a left dentary has been found in gravels from the Pleistocene Terrace Site. A second specimen has since been found, also from the Pleistocene, on Floraville Station in north-western Queensland. Based on the morphology of these fossil freshwater crocodiles, it appears the freshwater crocodiles may have evolved in Australia from ancestral saltwater crocodiles.
Another crocodile species, possibly of the genus Pallimnarchus, has been found in the Terrace Site. Fossils believed to be of this genus have been found throughout the waterways of eastern Queensland, as they existed during the Pliocene and Pleistocene, as well as from sites in New south Wales, South Australia and Western Australia.
The largest Riversleigh crocodile, believed to be from the genus Baru, found at the time of writing was a 5-m giant, about the size of a famous Northern Territory saltwater crocodile called "Sweetheart", that had a habit of eating boats. It was accidentally killed in 1979. The species of Baru is known only from Site D, and from equivalent strata. It was a freshwater crocodile, but unlike the modern freshwater crocs, it did not specialise in fish. It appears to have also differed from saltwater crocodiles in that it probably grabbed prey at the edges of the water, but in stead of rolling or drowning its prey, as modern salties do, it had large, razor-edged, recurved teeth that would have caused severe lactations.
An almost complete skull of Baru darrowi was found in Site D. In the Bullock Creek Local Fauna from the Middle Miocene of the Northern Territory, a close relative of Baru darrowi was found, that may have evolved from the Site D croc. The crocodiles that may have been more of a threat to wildlife of the Riversleigh rainforests, were the very deep-headed, apparently terrestrial, about 1-2 m long, that could probably run down their prey on land.
In the Ringtail Site at Riversleigh, 2 small, well-preserved crocodile skulls were found overlapping in a single block of limestone, that proved to be from 2 distinct genera. They appear to be closely related, but have specialised in different directions. The 3 groups of Riversleigh crocodiles appear to be more closely related to each other than to crocodilians elsewhere in the world. Paul Willis' study led to the suggestion that Australia had an endemic suite of crocodile species for most of the Cainozoic, but all are now extinct. The present population, freshwater and saltwater crocodiles, have since invaded, probably from the north.
The crocodiles of Riversleigh seem to have occupied a much wider range of niches than modern crocodilians do anywhere. Some were specialised fish feeders, as are modern freshwater crocodiles, others semi-aquatic ambush predators, as are modern saltwater crocodiles, yet others were terrestrial carnivores, filling a similar niche to that of occupied by monitors. (varanids).
Michael Archer, Suzanne J. Hand & Henk Godthelp, Australia's Lost World: Riversleigh, world heritage Site, Reed New Holland
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