Australia: The Land Where Time Began
Triassic Australia - about 251-199 million years ago See The Triassic Labyrinthodonts of Australia
All the continental blocks of the world were united in Pangaea during the Triassic. Globally, heat and aridity characterise the climate of the Triassic, with the arid belts indicated by evaporites extending up to 50o either side of the Equator, being more widespread and abundant than at any other time in the history of the earth. The evaporite deposits of the world were most widespread during the Middle Triassic, indicating it was the most intensely arid part of the most intensely arid Period. The Early and Late Triassic were apparently as hot, but more humid than the Middle Tertiary. Red-beds that are widespread in deposits dating from the Early and Late Triassic are believed to indicate alternating arid and humid conditions. It seems for once Australia "struck it lucky", being down at the South Pole, well away from the arid belt in the equatorial regions.
The distribution of reefs at this time is very similar to that in the Devonian, suggesting a similar temperature range, with maxima probably higher than the present and with warm seas. At the high latitudes were Australia was at the time, the climate being less extreme, probably warm-temperate and more humid, conditions which are thought to have possibly extended as far as the South Pole. For part of the Triassic the South Pole was situated near Bourke in New South Wales.
Most of the surface rocks in Australia that have been dated to the Triassic are of terrestrial origin. River and lake systems flowing across floodplains bordering the east coast near Sydney and the northwestern coast west of Broome in Western Australia, laid down the most extensive deposits that date to the Early-Middle Triassic. Areas of uplift in central New South Wales and the Kimberley region of Western Australia respectively provided the sediment.
In Victoria and southeast Tasmania downwarping occurred during the Early Triassic resulted in the formation of lakes and swamps, some of which formed coal deposits. In South Australia during the Late Triassic, coal deposits were laid down around Leigh Creek. Australian marine rocks from the Triassic are not common, some forming in central Queensland and northwestern Western Australia. These deposits were laid down in brackish environments where embayments occurred during the Early-Middle Triassic in areas that had subsided during the Permian.
The Triassic Period began after the end of the Late Carboniferous to Early Permian glaciation, and after the marine incursions retreated around the coast of the continent. Sediments deposited during this period consist almost exclusively of river and lake deposits, with only a minor component of marine rocks forming. At this time Australia was situated in fairly low latitudes and the climate is believed to have been humid and temperate, with marked seasonal changes in rainfall. It also followed the devastating mass extinction event at the close of the Permian.
The earliest-known mammals enter the fossil record of the North American and European part of Pangaea at this time. One of the earliest was Megazostrodon rudnerae, a small rat-like animal about 15 cm long, of which complete skeletons have been found, showing it had a pointed snout and a long tail.
During the Triassic, fresh-water sedimentation was a feature of the basins of the eastern part of the continent. Vast systems of lakes were gradually silting up in Queensland, river erosion of catchments being the main contributors. In the Bowen Basin, the sediments were coming from the basin margins. What is thought to have been one of the last remaining floodplains like those found in the Permian, in the Sydney Basin, was receiving vast amounts of sediment from rivers eroding the New England highlands. The sedimentary rocks that formed from these sediments are the Narrabeen Group. The massive Hawkesbury Sandstone and strata formed in the Late Triassic that was comprised of coarser sands, derived from areas to the south. This suggests that there was high country in the southeast, or even as far south as Antarctica. By the Triassic the only remaining coal swamps were only small areas at Leigh Creek in South Australia, Tasmania and northeastern New South Wales and nearby southeastern Queensland. At this time volcanoes were active along the eastern coast, on the continental shelf and to the east of the major basins.
There was a marine incursion down the Westralian Depression onto the North West Shelf in the Early Triassic, and there were 2 incursions further inland on the coast of Western Australia, on the northern margin of the Canning Basin and into the Bonaparte Basin. By the Middle Triassic these incursions had drained back to the sea, leaving the whole continent as dry land.
In the Early Triassic a river system with deltas formed near the Exmouth Plateau after the North West Shelf marine transgression retreated. Later in the Period there was another marine transgression in this area. The sediments formed sandstone during the Triassic were some of the main natural gas reservoirs, here as well other deposits from other parts of Australia. Unlike other parts of the world, where the deposits are mainly of marine origin, the Australian deposits of oil and gas that formed after the Devonian are mostly of terrestrial origin.
By late in the Triassic marine sediments were being deposited only in the northwest sector of the Westralian Depression because it had been filled-in elsewhere. In the rift that forms when landmasses are separating a typical series of events occurs as seen in the Westralian Depression, where there was an alternating cycle of marine transgression and retreat, with the deposition of sediment in the depression.
At about 249 Ma, during the Early Cretaceous (Induan-Olenekian, some low-lying coastal areas were covered for a brief period by marine embayments in the Maryborough Basin in Queensland and in Western Australia the Canning and Perth Basins. The Meso-Tethys Ocean bordered the northwest coast of Australia. Extending south from southeast Queensland to Sydney uplands were formed by folding of the crust and uplift in the east of the continent. During the Early and Middle Cretaceous huge volumes of sediment were carried by river systems, that were southeasterly flowing, across the lowland coastal floodplains of the Sydney Basin. The primary source of these sediments was the east coast uplands.
According to Kear et al. during the Triassic the climates of the poles were very different from the present regimes, with the coastal regions of Australia experiencing generally warm, humid conditions and the areas that were further inland and at lower latitudes were more arid, the climate was more seasonal with a monsoonal environment predominating. In rocks of Triassic age evidence has been found for the existence of coastal deltas in the Canning Basin, Western Australia, that were receiving sediments from upland areas in the northwestern Kimberleys. Other parts of Australia had mountainous regions at this time, in southeastern Queensland, that was volcanically active at this time, associated with ash deposits and mudflows, as well as in the continental interior. In northern Tasmania extensive lowland systems of lakes and peat swamps were bordered by uplands2.
In the Late Triassic, Carnian-Norian, 228.7-203.6 Ma, hot, dry climates were widespread across Australia. There is evidence of humid environments at a localised scale in the Flinders Ranges, South Australia, were lake complexes and coal-forming peat swamps were maintained by subsidence and faulting in upland areas. It is believed conditions were similar in the Ipswich Basin, southeast Queensland.
At this time the continent formed a southern peninsula of Pangaea that was in contact with other continental fragments that made up Gondwana such as India, New Zealand, and the western coastal terrain of New Caledonia, with New Guinea being a margin on the eastern coast and offshore shelf2.
In Australia most of the known fossil deposits of Triassic age are from the Induan (Early Triassic) to Lower Langdinian (Middle Triassic). Significant deposits included among these are the Arcadia Formation of southeast Queensland, of Induan-Olenekian age, a fluvial deposit. Outcrops of this formation occur in the Bowen Basin at the Crater Locality near Rolleston, the site of many amphibian and reptilian fossil finds. The Blina Shale and the Kockatea Shale, are both rich fossil sites. Exposures of these shales occur along the coast of Western Australia as well as on the continental shelf in this region. From these sites fossil invertebrates, amphibians and plants, all from a brackish delta environment, and near-shore marine embayments2.
In the Sydney Basin several deposits have produced a large number of fossils from the Lower to Middle Triassic (Olenekian-lower Landinian). These deposits in central eastern New South Wales, that were derived from extensive fluvial and lacustrine environments, are the Narrabeen Group, the Hawkesbury Sandstone and the Wianamatta Group. Included among the well-preserved fossils are plants, invertebrates and vertebrates, especially fish. These deposits are some of the most continuous fossil records from the Triassic that are known of on the entire continent. Fossils from the Upper Triassic, Carnian-Norian, have been found in coal-bearing strata of the Leigh Creek Coal Measures, that are most extensive in the Telford Basin, in the north of South Australia. 235 Ma lake systems and peat swamps dominated this upland region. Included in these deposits were many invertebrates, especially bivalves and rare vertebrate remains, that have been recovered from the tailings heaps resulting coal mining2.
The vegetation grew profusely with a high diversity of plant types - Dicroidium dominated vegetation were ferns, other seed ferns, ginkgophytes, cycads, conifers and horsetails. It was at this time that many plants developed the adaptations that would allow them to flourish in the dry times to come. Much of the sandstone seen between Sydney and Gosford was deposited at this time, and swamps were laying down the organic-rich sediments that were to become coal deposits around the Sydney area. The rivers and lakes had many fish types and large labyrinthodonts. Included among the fish were many types of bony fish (osteichthyans), one of which was Cleithrolepis, primitive sharks such as Xenacanths. Fish fossils from this time are common in the St Peters, Brookvale and Gosford areas of New South Wales, and the Knocklofty Ranges near Hobart. In the same deposits where the Dicroidium flora are found are tracks of Labyrinthodonts. Among the fossils of the amphibian-dominated fauna from many parts of Australia reptiles are found occasionally.
Amphibians - Triassic Australia - Temnospondyl Amphibians2
These 3+ m long predators had the appearance of crocodiles and according
to the authors3 they probably occupied niches that were mostly
aquatic in freshwater and estuarine habitats. During the
Permian-Triassic the diversity of temnospondyls peaked, then from the
Triassic-Jurassic boundary declined until they are not seen in the
fossil record by the Middle
The earliest known temnospondyls from
Among the taxa found in these deposits are at lest 2 species of Watsonisuchus, a mastodonsaurid with a wide distribution, being a large predator that was known from South Africa, India and Europe.
Acerastea wadeae, Arcadia myriadens and Rewana quadricuneata, as well as a tiny form with a skull length of less than 50 mm, Nanolania anatopretia, were rhytidosteids, a group that was well known across Gondwana.
Xenobrachyops allos, a primitive brachyopid that was distributed globally.
Lydekkerina huxleyi, a lydekkerinid, has been found in the Rewan Formation and deposits in South Africa from the same age.
Plagiobatrachus pustulatus, that the authors3 say is a possible plagiosaur that was an obligate aquatic temnospondyl mostly known from Europe and Africa.
Keratobrachyops australis, the basal chigutisaurid, a family found only in Australia and South America.
Lapillopsis nana, a tiny insectivore from Australia and India.
Representing some of the most primitive members of their families, the authors3 suggest they indicate temnospondyl linages from the Mesozoic originated in Gondwana or possibly Australia. According to the authors3 members of widespread lineages dominate amphibian assemblages from the Knocklofty Formation, Tasmania Basin, a fluvial-lacustrine habitat, and the Blina Shale, in the Canning Basin, Western Australia, both being of Induan-Olenekian age. Chigutosaurids are not present in these deposits, and Watsonisuchus, a mastodonsaurid, from the Blina Shale, flat-skulled suction-feeders that are believed to have fed on small fish, Blinasaurus henwoodi and Banksiops townrowi, are comparatively rare.
Chomobatrachus halei, a lydekkerinid, and Deltasaurus kimberleyensis, a rhytidosteid, both small temnospondyls about 1 metre long that were aquatic fish eaters, are present in the Blina Shale of Western Australia, and in Tasmania, in the Cluan Formation and the Knocklofty Formation. Species such as Derwentia warrenii, that were smaller-bodied rhytidosteids with a skull length of about 90 mm, and Deltasaurus pustulatus, that had a skull that was about 110 mm long, found in a drill core from the offshore Kockatea Shale near Geraldton, Western Australia, are suggested by the authors3 to have been predators feeding on small fish and invertebrates. Rotaurisaurus contundo, a very small lapillopsid from the Knocklofty Formation had a skull length of about 20 mm, has been suggested by the authors3 to have occupied predatory niches on land.
Erythrobatrachus noonkanbahensis, an endemic trematosaurid from the Blina Shale that was one of the large-bodied temnospondyls reaching about 2 m in length. The authors3 have described it as a long-snouted fish-eater they suggest probably inhabited estuaries along the coast and near-shore marine habitats.
In the Sydney Basin the mastodonsaurids and the brachyopids were the only temnospondyls present in the river systems along the coast following the significant diversity decrease that occurred in the Early to Middle Triassic. At Long Reef in the northern Sydney suburbs an incomplete mandible was found, the complete mandible being estimated by the authors3 to be more than 1 m long, and a femur was found in the Bulgo Sandstone of Olenekian age that proved to be the oldest described taxa of the temnospondyls, Bulgosuchus gargantua, a very large mastodonsaurid. They suggest this is one of the largest temnospondyls from the Early Triassic to be discovered so far. Bones and teeth that are believed to possibly be from other mastodonsaurids have also been found in the same deposits at Long Reef. Near Gosford to the north of Sydney a number of almost complete mastodonsaurid and brachyopid skeletons have been discovered in the Terrigal Formation, of Olenekian-Anisian age.
Skulls and other remains that are believed to be from Watsonisuchus have been found as well as an indeterminate large mandible believed to be a mastodonsaurid. The remains of brachyopids that have been found in the Long Reef deposit include a skull, pectoral limb girdle, an incomplete pectoral girdle that has been assigned to Platycepsion wilkinsoni. A well-preserved skeleton, that has a carbonised outline of the body, of another taxon that has still to be named, was also found.
Subcyclotosaurus brookvalensis, of Anisian age, another mastodonsaurid from the Middle Triassic is represented by a skull and fragmentary pectoral girdle has been found in the Hawkesbury Sandstone near Brookvale, Sydney. Also found in the deposit were amphibian footprints and indeterminate bone fragments. Trackways and an isolated skull of Notobrachyops picketii, a small brachyopid, have been found in a brick quarry in Mortdale, southern Sydney, in the Ashfield Shale of the Wianamatta Group, lake deposits of Anisian-Landinian age. At St. Peters in inner-western Sydney another large brachyopid has been found, as well as one of the most complete skeletons of a mastodonsaurid to be found so far. This animal, Paracyclotosaurus davidi, was 2.25 m long with short limbs similar to those of a crocodile and a massive skull that was flat. As it eyes were placed dorsally the authors3 suggest it may have been an ambush predator, possibly lying on the bottom of a lake waiting for prey to pass over its head close enough to be sucked into its mouth by the suction created when it suddenly opened its mouth.
The Leigh Creek Coal Measures, South Australia, deposits of Carnian-Norian age laid down in an intramontane swamp, has produced the only known Australian temnospondyl from the Late Triassic. The remains have some features in common with chigutisaurids, though it is not diagnostic.
The book by John Long has a section of labyrinthodonts2.
Short-skulled Labyrinthodonts are found in the Blina Shale, the Canning Basin, Western Australia, that are similar to those from the Permian in the Sydney Basin. There are also capitosaurs that are similar in appearance to crocodiles. The Blina Shale also contains fragments of Thecodont reptiles that were ancestral to Dinosaurs, that were either quadrupedal or bipedal. A similar amphibian fauna in the Tasmanian Knocklofty Formation also contains species of Thecodont that are closely related to forms found in South Africa and South-east Asia.
The Arcadia Formation, in Queensland's Bowen Basin, has a rich amphibian fauna among which there are some fossils of a thecodont and some lizard-like reptiles which show affinities to the Lystrosaurus zone in southern Africa. Lystrosaurus was a mammal-like reptile that occurred in Africa, South America, India and Antarctica, but hasn't been found in Australia. Triassic rocks in Queensland have been found that have footprints of amphibians and reptiles. Footprints and bones have been found in Triassic rocks of the Sydney Basin. In the St Peters brick pit in Sydney the skeleton of a Capitosaur, Paracyclotosaurus has been found.
In the Early Triassic the lakes and rivers teemed with freshwater fish. Deposits containing fish are found in the Sydney Basin. At Somersby, near Gosford, a fish bed contained lungfish, sharks, Saurichthys, a long-nosed predator, and many of a common type at the time, Cleithrolepis. A horseshoe crab, Austrolimulus has been found among the remains of insects, plants, and large amphibians, at the Beacon Hill Quarry, Brookvale, Sydney. Another horseshoe crab, Dubbolimulus has been found near Dubbo, western New South Wales. There are not many sedimentary rocks known from Australia resulting in a poor understanding of the marine invertebrates from this age in Australia. Included among those that are known are cosmopolitan types of ammonites that have used to correlate different horizons. At this time there was only a single landmass, Pangaea, and a single ocean that was warm over its entire expanse. Dinoflagellates arose at this time.
Some of the best known amphibian and reptile sites from this time in Australia are outcrops of the Arcadia and Rewan Formations near Rolleston and Bluff in south central Queensland, in New South Wales, the sandstones around Sydney, the Narrabeen and Wianamatta Groups and Hawkesbury Sandstone. Outcrops of the Blina Shale near Blina Station in the Erskine Ranges in the north of Western Australia. The Knocklofty Formation sites around Hobart.
Rare early reptiles have been found only on the Knocklofty and Queensland sites. All other Australian sites contain only amphibians and fish. In Queensland the fragmentary remains of a therapsid, mammal-like reptile, have been found. The rarity of these animals in Australia is anomalous, as they were abundant across the rest of Gondwana.
At this time Australia had many amphibians, all except a recent find, are Temnospondyls. Dinosaurs from this time have not been positively found, but some dinosaur remains, Agrosaurus, have been found in an unknown site in Queensland. They are believed to be from the Late Triassic or Early Jurassic.
Throughout the Triassic faunal and floral changes were taking place globally. The gorgonopsians had been very successful across Gondwana, though are as yet unknown in Australia, in the Latest Permian and Early Triassic in the niches of medium to large sized carnivores. During the Triassic they were replaced by the thecodonts, the group that later gave rise to the dinosaurs.
It seems the thecodonts appear to have arisen at the right time to take the place of Gorgonopsians as they went extinct. The thecodonts were then ready and able to radiate to fill the niches vacated by the gorgonopsians.
Until the Late Triassic thecodonts and dicynodonts shared Gondwana for millions of years, then in the Late Triassic thecodonts replaced the carnivorous dicynodonts. Rhynchosaurs, Diademodontoids and aetosaurs replaced the dicynodonts in the Middle to Late Triassic. The Rhynchosaur-Diademodontoid Empire was short lived as they were replaced by the dinosaurs in the Late Triassic.
The success of the dinosaurs was rapid and seems to coincide with rapid climatic and floral changes. The Dicroidium flora was replaced by the conifer-dominated plant assemblage that flourished throughout most of the ascendency of the dinosaurs. The dinosaurs were waning towards the end of the Cretaceous, even before the mass extinction event at the close of the Cretaceous. Their demise coincided with the rise of the angiosperms (flowering plants), which gained dominance by the start of the Cainozoic. It is often assumed that the dinosaurs succeeded by outcompeting the earlier reptile groups. It is believed possible that they actually succeeded by filling niches as they became available by the extinction of the previous niche occupants. Maybe they owe their success to their ability to radiate rapidly when niches became available, the ultimate opportunists.
See Australian Dinosaurs for more
See Australian Dinosaurs for more on dinosaurs
There is very little evidence known of dinosaurs in Australian from the Triassic. It had previously been reported that the crew of HMS Fly collected remains on Cape York Peninsula in 1844 that were identified as Agrosaurus macgillivrayi. This is now known to be incorrect, they are actually of a prosauropod, Thecodontosaurus from deposits in Somerset, England. The only indication of dinosaurs in Australia from the Triassic are footprints of a carnivorous theropod that have been found in 2 locations in southeast Queensland, in the Callide Basin, of Rhaetian age (uppermost Triassic) and the Upper Triassic deposits in the Blackstone Formation in the Rhondda Colliery. Included in these is a series of large tracks, 460 mm long, that resemble the ichnotaxon from the Mesozoic Eubrontes, corresponding to a carnivorous dinosaur that was about 6 m long.
Therapsids - mammal-like reptiles
During the Triassic the therapsids were one of the most ubiquitous elements of the terrestrial faunas. They are known globally during the Triassic in many places being the dominant component of most faunas. As in many other features Australia is different from the rest of the world, having few known therapsids, and the known specimens are found in Queensland's Arcadia Formation as fragmentary material. Dicynodonts comprise the bulk of the known taxa.
The dicynodont fossils from Australia comprise fragments of skull that have a structure that has been compared to Lystrosaurus, a kannemeyeriid, a cosmopolitan group that represents all dicynodonts from the Triassic. Lystrosaurus is regarded as an index fossil for deposits from the Early Triassic across Gondwana, South Africa, Antarctica and India. A more definitive assignment is not yet possible as little information has been gathered from study of the specimens.
In the Coal Cliff Sandstone, Narrabeen Group, in the Bellambi Colliery in the southern Sydney basin, there is a series of pentadactyl footprints that the authors3 say could possibly have been made by a dicynodont. Representing an animal that is believed to be about 850 mm long, suggested to be Lystrosaurus-like, with a sprawling gait similar to that of a lizard. Recovered from the Arcadia Formation are 2 small vertebral centra that the authors3 suggest could possibly be a cynodont. Cynodonts are closely related to mammals. In appearance they were superficially dog-like
Before Australia was island Australia it was peninsula Australia, it was a peninsula on the vast Gondwanan supercontinent. During the Triassic the tetrapod faunas of Gondwana were dominated by reptiles. On the mainland of Gondwana the reptile dominated plains fauna, called the Lystrosaurid Empire, that covered most of Gondwana, and in the far east at that time was the amphibian-dominated Trematosaurid-Rhytidosteid Empire in peninsula Australia.
The labyrinthodonts were very successful in Australia. It might have been the case that relict populations radiated on the Australian peninsula partly because the more recently evolving species on the mainland, such as dinosaurs, took longer to spread to the peninsula and fewer did so, than spread across the rest of Gondwana.
the authors3 suggest that in Australia the terrestrial record of the Early to Middle Triassic is best typified by the Narrabeen Group, the Hawkesbury Sandstone and the Wianamatta Group in central-eastern New South Wales. The 3 units comprise a continuous series of shales, sandstones and claystones, representing a prograding shoreface/coastal plain sedimentation, that are found widely across the Sydney Basin.
|Author: M.H.Monroe Email: firstname.lastname@example.org Sources & Further reading|