Australia: The Land Where Time Began
The earliest known Australian mammals have been found in deposits in southeastern Queensland that have been dated to about 54 million years ago in the Early Eocene. In this deposit is found, among the remains of crocodilians, birds, and turtles, teeth from the Tingamarra Fauna, the earliest known mammal fauna in Australia. The teeth representing the animals show the closest relationship to South American marsupials that have so far been found in Australia.
After a gap of 30 million years, the next oldest Australian mammal fauna is found at Geilston Bay, near Hobart, Tasmania. Again the fossils are mostly in the form of isolated teeth and fragments of jaws. The marsupials in this assemblage are recognisable as belonging to groups known from the latter part of the Cainozoic - a phalangerid, a burramyid, a diprotodontoid, and a possible dasyurid.
Lake Eyre and Tarkarooloo Basin
Fossil vertebrate deposits have been found to the east of Lake Eyre and Lake Frome in South Australia. They are situated on the margins of playas (dry lake beds), and the fossil-bearing deposits are on top of each other so that a sequence through time can be easily worked out for the fossils. This situation occurs most prominently at Lake Palankararinna.
In the Lake Eyre and Lake Frome regions the fossil-bearing rocks of Tertiary age were deposited typically as white sands and pale green to black clays on the lake floors and in the channels of small streams at a time when the area was much wetter than at the present. Around these streams when they flowed continuously were rainforests, and in the areas between the streams were woodlands and possibly grasslands. Salinity of the lakes increased throughout the Tertiary, but at this time there were flamingos and palaeodids (flamingo-like birds). There was also a variety of mammals, many of similar appearance to their modern descendants. In the Lake Frome region, that was apparently connected to the Southern Ocean, there were freshwater dolphins in the streams and lakes.
Fragmentary fossils of small dolphins from the Pinpa and Ericmas faunas have been found fluvial-lacustrine Middle Miocene deposits in Namba Formation, Tarkarooloo Basin, in the Lake Frome area. They are of an indeterminate genus and species from the extinct family Rhabdosteidae (= Eurhinodelphidae). They are the first known occurrence of rhabdosteids from Australia. Prior to this discovery they had been known from Patagonia and possibly New Zealand. The presence of these fossils is evidence of a marine connection of the Tarkarooloo Basin before or during the Middle Miocene. The had not previously been known from fluvial-lacustrine sediments.
The fully toothed platypus Obdurodon were also present. In the forests along these watercourses was an arboreal assemblage that included well known as well as lesser known forms. There were also diprotodontiods, smaller than their descendants later in the Quaternary, that were a dominant part of the fauna. The diprotodontpoids were one of a number of wombat-related marsupials (Vombatiformes) that underwent extensive radiation, reaching a peak at this time, the first diverse assemblage of mammals in Australia. The only Vombatiforms still living were represented only by a few teeth. The wynyardiids and iliariids are known only in these associations. Iliariids were the largest marsupials of the the time. Essentially all that is known of the iliariids is their distinctive molar teeth, the crowns of which have a complicated pattern not seen in any other mammal.
The first specimen of a wynyardiid was found at Wynyard in Tasmania, in a loose block of rock broken from the nearby sea cliff. Dating of the fossil to the Early Miocene was made possible by the presence in the rocks of the cliffs of numerous marine shells. Wynyardia is believed to be close to the ancestral form of the larger proterodontian marsupials that dominated the later Australian megafauna. They are characterised by having a single pair of prominent lower incisor teeth. They include the modern animals such as koalas and wombats, kangaroos and many possums (phalangeriforms).
A bizarre form among the diprotodontians were the thylocoleonids. Among mammals the usual evolutionary trend is from carnivore to omnivore to herbivore, but the thylocoleonids went the other way. They are obviously diproterodontians. Study of the basicranium (the base of the skull) suggests that they are closely related to the vombatiforms. Their ancestral forms had lost the canine teeth and developed a gap in the tooth row between the incisors in front and the last premolars behind, useful for manipulating food in herbivores. They enlarged their most interior incisors for use in grasping and piercing prey, as do canine teeth.
The possum-sized Priscaleo and the dingo-sized Wakaleo from the middle Tertiary of South Australia are the earliest known marsupial lions. Both had the highly specialised tooth pattern characteristic of the group, implying that this evolutionary path had been followed prior to their appearance near the Oligo-Miocene boundary. The thylacoleonids were probably a separate distinct group of diprotodontians originating much earlier in the Tertiary.
East of Lake Frome and Lake Eyre early browsing kangaroos have been found in Middle Cainozoic sediments. The grazing kangaroos that were the dominant kangaroos in the Pliocene and Quaternary hadn't yet evolved. The potoroos in these faunas were clearly more primitive than then living species.
Middle Cainozoic possums from central Australia included a mixture of extant families such as pseudocheirids (ringtailed possums) and petaurids (gliders and Leadbeater;s and striped possums), and some that appear only at this time, the phalangerid-like Miralindae and the peteurid-like Pilkipildrae. All are known only from their teeth, mostly isolated. Based on this small amount of evidence it seems no major changes have occurred among their descendants since that time. They seem to have been arboreal since that time.
The phalarangerids, including the brush-tailed possums (Trichosurus), appear late in the fossil record, unlike other possums that have been present since the first appearance.
|Author: M.H.Monroe Email: firstname.lastname@example.org Sources & Further reading|