The Alcoota fauna, Central Australia : an integrated palaeontological and geological study

A rich concentration of fossil vertebrates has been found in middle Tertiary sediments near Alcoota station in the Northern Territory, Australia. The sediments, which are about 70 feet thick, comprise a basal drab lacustrine siltstone overlain by red fluviatile siltstone, sandstone, and conglomerate, capped by chalcedonic limestone. They are generally flatlying and are designated the Waite Formation. The Waite Formation rests unconformably on lateritized Archaean gneiss and quartzite. The geological evidence clearly' indicates that the laterite antedates the Waite Formation and is, therefore, no later than Miocene in age. The vertebrate assemblage recovered from the drab lacustrine siltstones at the base of the Waite Formation is designated the Alcoota fauna. It includes a crocodile seemingly related to C. porosus of New Guinea and a number of large emu-like birds. A new wallabysized macropodid is apparently related to Dorcopsis and Dorcopsulus, which are also currently found in New Guinea. A new genus of large macropodid may represent an early sthenurine, but it also shows some affinity to the Pliocene and Pleistocene genus Protemnodon. The presence of the protemnodont lineage during Alcoota time is firmly established by four isolated upper first incisors, but these teeth probably do not belong to the new genus. The bulk of the animal remains belong to the extinct family Diprotodontidae. One of these is a new species of Palorchestes. The others have been described previously as three new genera and species. Cranial material of these three genera, not available earlier, is described. The geological and palaeontological evidence indicates that the middle Tertiary' climate around Alcoota was at least subtropical. The seasonal precipitation was undoubtedly much more effective than at present. In the dry season, animals were apparently drawn in large numbers to the shores of a lake which had developed in a depression in the old lateritized terrain. Each year some of the animals perished around the shore, and their remains were entombed nearby. In stage of evolution, the members of the Alcoota fauna are younger than those from the Kutjamarpu fauna (middle Miocene) of South Australia and older than those of the Beaumaris (early Pliocene) fauna from the Sandringham Sands of Victoria. The age of the Alcoota fauna, in these terms, is late Miocene or possibly early Pliocene.