The world of the Triassic Period (248-206 MYA)
was a very different place
than the later periods of the Mesozoic.
The climate was generally hot and dry, with plant
communities dominated by seed-ferns. Eastern
Australia seemed to be an exception to the generally
dry conditions, with swamps and waterways that would
eventually turn into the abundant coal deposits of
Queensland and New South Wales. Dinosaurs
were a minor component of the terrestrial
fauna, only diversifying at the very end of the
Triassic. Thecodonts were the top predators on
land; crocodile-like quadrupeds with enormous
heads filled with sharp teeth. They were probably direct
ancestors of the dinosaurs themselves.
Mammal-like reptiles were the most common types of
large land animal, making up to 85% of fossils in some
locations. It was
a time when the dinosaurs lived in the shadow
of other more successful animals - much as us mammals
were over-shadowed by dinosaurs in the Jurassic
and Cretaceous Periods.
In most of Gondwana during the Triassic, reptiles were the dominant form of life on land. In Australia however, Triassic fossils are dominated by amphibians, with reptiles a rare component of the fossil record. More than 20 species of labyrinthodont (large carnivorous amphibians) are known from the Triassic of Australia. There is a greater variety of labyrinthodont fossils known from The Crater site in south-central Queensland alone than any other single locality on earth. Given that some of the Temnospondyls, the most successful group of labyrinthodonts, may have originated in Gondwana (perhaps even Australia itself), it is not surprising that Australia seems to have been labyrinthodont heaven. Indeed, they persisted here long after becoming extinct in other parts of the world.
Covering all of the Australian labyrinthodont species would be a major undertaking, so here I shall concentrate on the reptiles and mammal-like reptiles of Australia's Triassic Period.
Almost all of Australia's Triassic reptiles, and indeed Triassic fossils in general, come from a site known as The Crater, close to Carnarvon National Park in south-central Queensland. These deposits, known as the Arcadia Formation, are a thick sequence of sandstones and mudstones that formed in freshwater pools, lakes and streams around 230 million years ago (MYA). In most places these soft deposits have weathered to form a thick layer of soil, however in a few areas the intact rocks form brilliantly coloured layers, ranging in hue from red, orange, dark brown and purple. Most fossil material found in these layers are badly worn scraps that are difficult to identify. In fact, a lot of the fossil scraps are discovered in coprolites; the fossilised dung of the larger carnivorous animals that lived in the area.
The name thecodont means 'socket tooth'. Whereas many reptiles have teeth growing from a continuous groove in the jaw, thecodonts (and later archosaurs like dinosaurs) had pointed carnivorous teeth that grew from individual sockets. They had various other improvements on the typical reptilian design, including more upright limbs and an improved ankle structure. Their decendants, the dinosaurs, would inherit and further improve these features.
The most primitive group of thecodonts were the proterosuchids. They are characterised by upper jaws with a kinked downturn at the end, with some semi-aquatic species reverting to semi-sprawling limbs, making them look superficially like crocodiles.
Thulborn 1979
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Fragments of the skull, vertebrae, limbs and pelvis are known for this species. It is named after the Hindu goddess of destruction, Kali, and is the oldest archosaur known from Australia. It had a broad snout, slender limbs, and a long neck and tail. It would have resembled a crocodile, with perhaps longer and more upright limbs.
Camp & Banks 1978
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Tasmaniasaurus is the most complete fossil reptile known from Australia, preserved in its entirety on a slab of sandstone. Found in 1960 in a sandstone quarry in Hobart, Tasmania, it was described by Camp & Banks in 1978, and redescribed by Thulborn in 1986. The bones of a small labyrinthodont are associated with the skeleton, and may represent its last meal. Tasmaniasaurus appears to have been semi-aquatic, with semi-sprawling limbs that made it look even more like a crocodile than most thecodonts did. Its skull was 20cm long, the jaws bristling with sharp conical teeth. It most closely resembles Chasmatosaurus, a well known species of thecodont from other parts of the world.
Mammal-like reptiles shared features in common with both mammals and reptiles. Dicynodonts were the most common type of mammal-like reptile. They had a vaguely hippo-like body, with a beak and large tusks decending from the upper jaw. The tusks were probably used to uproot water plants, and perhaps for defense or intraspecific combat. Dicynodonts were very common in Gondwana during the Triassic. Their fossils have been found in abundance in South Africa, India, Antarctica and China. It is not surprising that they also lived in Australia. However until the early 1980s their absence was somewhat of a mystery.
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This Australian dicynodont is known from a single quadrate bone (part of the upper jaw joint) found in 1982 that resembles those of Kannemeyeria and Lystrosaurus, both from Africa. The quadrate is one of the most distinctive bones in the body of dicynodonts, so it was fortunate that it was one of the bones preserved in the Queensland deposits. Of course, many of the undiagnostic scraps of bone from the same deposits may also be from similar animals, but being undiagnostic makes it difficult to identify them. In fact, one of those scraps is now thought to be a fragment of dicynodont tusk.
The quadrate is the bone in the back of the skull that articulates with the lower jaw. In dicynodonts it developed into a pulley-like grooved knuckle of bone, that allowed the lower jaw to slide backwards and forwards against the quadrate while the animal chewed plants. Only dicynodonts evolved this particular method of chewing. Blade-like structures in the lower jaws cut up plant material against hard plates in the upper jaw, like a carving knife on a cutting board.
In 1996 Early Triassic therapsid trackways from the Bellambi Colliery near Sydney were described as Dicynodontipus bellambiensis. The series of five-toed fore and hind prints suggest an animal similar to Lystrosaurus around 1m in length. The gait of the animal was more reptilian than mammalian, with semi-sprawling limbs.
| Dicynodonts were supposed to have become extinct around 220 million years ago,
at the end of the Triassic.
Recently skull fragments found near Hughenden and housed in the Queensland
Museum for the last 90 years were re-analysed by Thulborn & Turner.
The conclusion was that they most closely resemble dicynodonts such as
Lystrosaurus. The problem? The remains date to the Early Cretaceous, over 100 million
years after dicynodonts became extinct in other parts of the world.
Australia is already known for its relic populations, from Early Cretaceous labyrinthodonts (surviving another 120 million years), to modern marsupials and monotremes. A relic population of dicynodonts wouldn't be entirely unexpected. Heber Longman, curator of the museum early last century, thought it may have been a dicynodont even back then, but was never completely sure. |
Many people think of dinosaurs, and many other extinct reptiles, as just giant lizards. However lizards belong to a very specific type of reptile; the lepidosaurs. Given their usually small size and delicate nature, lepidosaur fossils are relatively rare when compared with the larger dinosaurs, thecodonts, and mammal-like reptiles.
Bartholomai 1979
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Kudnu may have been a lepidosaur, a group which includes modern lizards and snakes. Paliguanids are the earliest known types of lizard, and rarely exceeded 15cm in total length. Kudnu is known from a partial skull preserved in a red mudstone nodule from The Crater in south-central Queensland. The name comes from an aboriginal word for a mythical lizard-man. Other possible lepidosaur remains have also been found in the same area, but have not been described in detail.
Prolacertiforms were a group of lizard-like reptiles with long hindlimbs that probably ran on two legs when startled (like some modern lizards). They belong to an order known as the Eosuchians - the 'dawn crocodiles'. Their exact relationship to other reptiles is uncertain. Although superficially lizard-like, they also had some more advanced features, such as recurved teeth that grew from sockets. They may have been the ancestors of the thecodonts.
Bartholomai 1979
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The word 'Kadimakara' is an aboriginal term for animals that lived during the Dream Time. The legend says that these 'monsters' once lived in a huge canopy of trees that covered central Australia. When the trees disappeared, the Kadimakara wandered about in the desert until they died at Lake Eyre, where their bones can still be found. The legend hints at the once vast inland forests and lakes that filled Australia's now arid centre, where all kinds of animals once lived, from huge marsupials and flightless birds, to fresh-water dolphins and flamingoes.
Kadimakara was probably an insectivore, and closely resembles the South African species Prolacerta (also present in Antarctica). Kadimakara is known from two pieces of the skull, found on Rewan Station.
Procolophonids were lizard-like herbivores with short, squat bodies and sprawling limbs. Their remains are also known from the Arcadia Formation. Other procolophonids are known from South Africa, South America and Antarctica. They were close relatives of the mesosaurs, aquatic reptiles known only from the Permian of South Africa and Brazil, and parieasaurs, which although could reach the size of cattle still retained sprawled limbs. Procolophonids were a group of amniotes that split off extremely early in reptilian evolution, and developed alongside the more advanced reptile lineages that led to archosaurs (like dinosaurs and crocodiles) and modern lizards. They may however be a sister group to turtles.
During the Triassic, the ancestors of modern mammals were known as cynodonts. Cynodonts are known from South Africa, South America, and even Antarctica, so their presence in Australia during the Triassic would not be unexpected. So for however no cynodont remains have come to light. It is difficult to say whether their fossils have merely yet to be discovered, or whether for some reason they never made it to Australia at all during the Triassic.
Triassic dinosaur remains are so far completely unknown in Australia. Our only possible Triassic dinosaur, Agrosaurus, has since turned out not to be an Australian after all, but rather a British dinosaur in disguise. See Requiem for Agrosaurus for more details.
The only evidence for Triassic dinosaurs comes not from fossilised bone, but from footprints. Theropod tracks from the Blackstone Formation of the Ipswich Coal Measures, near Dinmore in Queensland, date to the Late Triassic (around 220-210 MYA). The smaller tracks have been assigned to the ichnogenus Grallator. These prints are no longer than about 7 cm (2.7 inches). Another much larger form of print comes from the same coal formation. These Eubrontes tracks measure up to 46 cm (18 inches) long, with a stride length of around 2 meters (six feet). However there is some doubt as to whether the larger prints are from a large Triassic theropod dinosaur, or are poorly preserved thecodont prints.
The smaller Grallator tracks are definitely those of dinosaurs,
and indicate that at least small theropods lived in Australia at the time, no doubt
trying to avoid being eaten by their more ferocious thecodont cousins, or
the giant carnivorous amphibians that haunted the lakes and streams. It would not
be until the Jurassic Period, after the great Late Triassic extinctions, that
the dinosaurs would inherit the earth.
