Australia: The Land Where Time Began
Huge numbers of snake fossils have been found in the Oligo-Miocene Riversleigh deposits. The snake fossils from Riversleigh has more than doubled the previously known extinct snakes of Australia. The rainforest must have been crawling with snakes. 4 families of snakes have been identified from the Oligo-Miocene faunas in the deposits at Riversleigh, and 3 of the 4 families are common in the living fauna. These 4 groups are the poisonous snakes, the pythons, the typhlopids, blind, burrowing snakes and the madtsoiids. The latter being extinct, their remains being known from the fragments of Gondwana, Australia, South America, Africa and Madagascar.
Some snake groups are not yet found in the Riversleigh deposits. These are the colubrids, that have no known fossil record in Australia prior to the Pleistocene, achrchordids, known from the Early Pliocene in Australia, and poids, not known from mainland Australia but occurring in New Guinea. It is thought that these 3 missing groups may yet be found in the Riversleigh deposits.
The snakes of Riversleigh are known mostly from their distinctive vertebrae, which display enough variation to allow for identification of the different groups to the family, genus and often the species level.
Pythons were the most common snakes in the Riversleigh deposits, some reaching lengths of up to 7 m. One giant species that was recovered from Site D was Montypythonoides riverleighensis, is closely related to living pythonids, but the genus has been extinct since the Tertiary. Another large python was discovered in the Bullock Creek Local Fauna from the Middle Miocene, Morelia antiquus. The living diamond Python, M. spilotes, belongs to the same genus.
This group, thought to be a non-poisonous constrictor, originated in the Cretaceous, occupying the southern continents until the Tertiary when they are found on the other continents, but in Australia they survived until the Late Pleistocene. The fossil madtsoiids of Australia are the most complete and best-preserved known in the world. The largest known from Riversleigh grew to more than 6 m, about the size of the South American anaconda. It is believed that the Madtsoiid from Riversleigh is related to the 6 m Wonambi naracoortensis from the Pleistocene of the Victoria Fossil Cave and Hensck's Quarry, near Naracoorte, South Australia.
These are the poisonous snakes, they don't kill by constriction, have a worldwide distribution, probably arriving in Australia from Southeast Asia. There are about 100 elapids in Australasia that appear to be closely related. Based on genetic studies, all these Australasian elapids seem to have arisen from a common ancestor about18-10 million years ago. The problem with the Riversleigh elapids in relation to the genetic results arises from the fact that most are about 20 million years old. This has caused some to wonder if the molecular work is giving an accurate estimate of the time when they arose in Australasia.
It has been suggested that the Riversleigh elapids are the ancestral elapids 5 million years before the later elapids arose by diversification from this base stock. This would allow the dates of separation indicated by the molecular clock to be accepted as it only started ticking about 5 million years after this elapid fauna existed.
These are small blind, burrowing snakes that are not venomous. Their eyes are merely dark spots beneath head scales. They eat mostly ants and termites and live under rocks, logs and underground. All of the 22 living Australian species belong to the genus Ramphotyphlops. Elsewhere in the world their fossils are rare, but in Riversleigh some deposits contain many vertebrae and ribs from this genus. This is the case in the Upper Site. Riversleigh is the only place in Australia where their fossils have been found so far.
Acrochordids (aquatic file snakes) & colubrids (water snakes and tree snakes)
In Australia, fossils of these groups have been found in sites dating from earlier than the Pliocene for the acrochordids, and earlier than the Pleistocene for the colubrids. Distinguishing characteristics of file snakes include very loose skin and rough, ridged scales, and their nostrils are on the top of their snouts. They are common in the Gregory River at Riversleigh and in all the northern Australian rivers that reach the sea.
It is believed the acrochordids most likely reached Australian freshwaters from the ocean along the northern margin of the continent. Based on their fossil vertebrae at Bluff Downs LF in north Queensland, it is believed they were present by the Early Pleistocene. They are distributed widely along the north west coast, colonising any bodies of fresh water where there are enough fish to support them.
The oldest known file snake have been found in Southeast Asian deposits from Siwalik, the deposits dating from the Middle Miocene. These Miocene fossils are of relatively derived snakes, suggesting that they were present in areas of Southeast Asia bordering the Indian Ocean well before the Miocene. As file snakes have not been found in the Riversleigh Oligocene-Miocene deposits, and they appear to have been present in the oceans north of Australia at this time, this leads to the conclusion that the depositional basins of Riversleigh were not open to the sea during the time span covered by the Oligocene-Miocene deposits. The ancestral Gregory River probably didn't become established until the Late Miocene, flowing to the north.
|Author: M.H.Monroe Email: email@example.com Sources & Further reading|