Australia: The Land Where Time Began
The Triassic Labyrinthodonts of Australia See Triassic Australia
The Triassic is the time when the Australian labyrinthodonts are best known, most known fossils of them occurring in Early Triassic deposits. There are no known endemic families of labyrinthodonts in Australian assemblages. There are also no widespread families known from Australia. The situation is completely different from reptile faunas in Australia of this time. An intriguing difference exists between Australia and other parts of Gondwana in the Early Triassic in that in Australia amphibians dominate the fauna of the time, but in other parts of Gondwana it is the reptiles that dominate, being far more diverse than all other terrestrial vertebrates. Yet another feature where Australia is "different".
There are 4 main known deposits containing amphibians in Australia. The Arcadia Formation (part Rewan Group) in south-eastern Queensland, the Blina Shale and Kockatea Shale of Western Australia, the Knocklofty Formation, outcropping near Hobart, Tasmania, and the Sydney Basin, where material may be as late as the Middle Triassic.
The Early Triassic lake sediments of the Narrabeen Group are believed to be the source of 3 labyrinthodonts found many years ago. There is also a varied fish fauna in this rock unit, sharks, lungfish, palaeoniscoids and many other types of actinopterygians. The fossil evidence suggests the vegetation was of a type that would be found on a levee bank, scrub dominated by the pteridosperm Dicroidium Callipteroidium, cycads and conifers. A forest and heath assemblage is present towards the upper part of this unit. In both the heath and forest fringes the dominant plant was the seed-fern Dicroidium.
The Hawkesbury Sandstone from the Middle Triassic at Brookvale near Sydney, that forms Sydney's cliffs, contained a capitosaur labyrinthodont Parotosuchus, a lungfish and about 20 genera of actinopterygians, mostly palaeoniscoids and Holosteans. Another site from the same unit, Somersby, contains mainly the subholostean Promecosomina, as well as an eel-like subholostean Saurichthys. The subholosteans were a group approximately intermediate between the palaeoniscoids and holostean grade of organisation. There were also various types of fish, a freshwater shark - a xenacanth or pleuracanth, the lungfish Gosfordia. The xenacanth at this site is the most recent known occurrence of this shark in the world, so it was a relict form when it lived at Somersby.
Fish and amphibians have also been found in St Peter's Quarry, NSW, part of the Wianamatta Group of Middle Triassic age. A skeleton of the capitosaur Paracyclotosaurus was found here, together with a xenacanth shark and a number of types of actinopterygians, though holosteans dominate, not the more primitive ray-finned fish. The faunas of this site differ from those of the older Narrabeen and Hawkesbury assemblages were the more primitive actinopterygians predominate.
A temnospondyl trackway has been found in the same rock unit west of Sydney.
At a Queensland site, Early Triassic Arcadia Formation, deposited by a braided river complex, 90 % of the vertebrate fauna is composed of labyrinthodonts. Fish recovered from the site, lungfish and Saurichthys, and reptiles. Among the reptiles is a rare mammal-like reptile (a dicynodont), small lizard-like forms, Kodimakara and Kudnu, and a procolophonid and a thecodont. Another fluvial deposit above the Arcadia Formation, the Glenidal Formation, contained a long-snouted trematosaurid labyrinthodont.
The faunas of Australia are the most diversified known in the world - the oldest tetrapods, the most diversified labyrinthodont record, at the family level, the latest known occurrence of a labyrinthodont, surviving into the Cretaceous, they were extinct by the Jurassic elsewhere. Russia may possibly be a contender for the latest.
The Brachyopidae, a family of 5 known genera are unique to Australia. They have short, flattened skulls. They were 1 of 3 labyrinthodont groups to survive later than the Late Triassic. The other 2 late survivors are capitosaurs and chigutisaurids. An indication that a lot of labyrinthodont evolution took place in Australia is the presence of primitive forms such as Xenobrachyops and more advanced forms such as Blinasaurus.
In the rest of the world brachyopids are mostly rare or completely absent, as in South America where they are not known.
The chigutisaurids, found only in South America and Australia, are related to the brachyopids. One of the Australian species is Keratobrachyops australis from the Arcadia Formation, near Bluff, Queensland.
The brachyopids and chigutisaurids are often assigned to the Brachyopidae that appears to have had its centre of diversification in the Australian section of Gondwana. It had been present in Australia from the Late Permian to possibly the Early Cretaceous, making it the longest time range of labyrinthodonts in Australia. An almost complete skeleton of Siderops kehli from the Late Jurassic Evergreen Formation of Queensland is one of the the latest known surviving brachyopid remains in Australia. The latest known fossils of this group are found in the Strzelecki Formation of Victoria, of Early Cretaceous age.
Like the brachyopids, the capitosaurid labyrinthodonts had a worldwide distribution. Also like the brachyopids, they had flattened heads. They also had small limbs and reduced skeletal ossification. Capitosaurs were among the largest of the labyrinthodonts, some with skulls almost 1 m long.
The nature of the otic notch, marking the position of the tympanic membrane of the auditory system, on the back of the head is the basis for the division of this group in 3 parts. In the Early Triassic, the oldest capitosaurs had otic notches opening broadly to the rear of the skull. The Middle Triassic forms had closed otic notches. Between the Early and Middle Triassic, there were forms intermediate between these 2 forms.
Most capitosaurs are believed to have lived in rivers or lakes. There are 2 genera known from Australia, Protosuchus and Paracyclotosaurus.
Protosuchus is known from several sites in Australia. There are several known species, one of which is Protosuchus brookvalensis from the Middle Triassic Hawkesbury Sandstone near Brookvale, NSW. It was found in association with a rich fish fauna dominated by palaeoniscoids, as well as some lungfish, Ceratodus, and holosteans.
Paracyclotosaurus davidi was a capitosaur with a semi-closed otic notch. It was found in an ironstone nodule that was almost 3 m long in St Peter's Quarry from the Middle Triassic Wianamatta Group in the Sydney Basin. This was a large carnivore, 2.25 m long, living in the backwaters around Sydney during the Middle Triassic.
The long-snouted trematosaurids are present in the Early Triassic near shore marine Blina Shale in north-western Western Australia and from the Rewan Group fluvial deposits in south-eastern Queensland. As they possessed a well-developed lateral line, they probably spent most of their time in the water. This is the only amphibian group often found in marine deposits. They looked something like crocodiles, and are believed to have had similar habits to those of the living saltwater crocodiles, Crocodylus porosus.
Dissorophoids were a type of crossopterygian that had a rounded head and large eyes. They have been found in Permian and Carboniferous of Africa, Europe and America as well as Australia, but in Australia they were comparatively rare and are found in Early Triassic deposits. Lapillopsis nana, one of the Australian forms has been found in the Arcadia Formation in Queensland. It is similar to another dissorophoid from Africa, Micropholis stowi, also from the Early Triassic.
Another Australian form from the Early Triassic are the lydekkerinids. Chomatobatrachus is from the Early Triassic Knocklofty Sandstone in Tasmania. This group is known from a few genera in South Africa and Russia and a genus from Antarctica.
The rhytidosteids were another group of labyrinthodonts with skulls measuring about 20-25 cm long. Members of this group had either rounded skulls or triangular skulls. Some forms, such as Acerastea wadeae, from the Rewan Group of Queensland, had gastroliths, swallowed stomach stones that it used for grinding food, as is found in birds and some dinosaurs. An unusual feature found in some rhytidosteids, e.g., Rewana, is the construction of their vertebrae. In labyrinthodonts the vertebrae are composed of 3 singular elements, but in some rhytidosteids the elements are present in pairs, there were 2 neural arches, 2 pleurocentra, and 2 intercentra. During the late Palaeozoic and early Mesozoic the amphibians were experimenting with their vertebral column, most likely as they spent more time on land and needed new support for life out of water.
The plagiosaurs were another group from Australia, small forms with skulls that were short, broad and shallow. Their skulls often had enlarged orbits (eye sockets) and pustular ornamentation. Plagiobatrachus australis was found in the Early Triassic Arcadia Formation. Other specimens came from the Northern Hemisphere sites from the Late Permian to the Late Triassic. Some of these Northern Hemisphere forms had external gills, apparently as a result of paedomorphosis.
The lifestyles of Triassic labyrinthodonts from Australia has been arrived at by interpretation of the features of their fossil remains. This interpretation has been applied to temnospondyls based on a number of characteristics of their skeletons - overall size, skull shape, location of the eye sockets, size of the eyes relative to the length of the skull, degree of lateral line development, type of teeth, number of vertebrae in front of the pelvic area, degree of ossification of the skeleton, location and proportion of the limb muscle attachments, the type of sediment usually associated with the fossil - lake, river, floodplain or marine. Based on these features temnospondyls have been split into at least 5 different ecomorphic types, body forms associated with different kinds of lifestyle - terrestrial, semi-aquatic, freshwater, semi-aquatic euryhaline (some salt tolerance), fully aquatic freshwater, and fully aquatic euryhaline forms.
Australia had at least 2 of these ecomorphic types, possibly lacking a fully terrestrial type, fully aquatic salt tolerant forms, such as the mastodonsaurids, and the fully aquatic freshwater forms, though they may still be found.
Semi-aquatic Freshwater ecomorphs are found in some species of the capitosaurs, lydekkerinids, brachyopids and chigutisaurids. Some of the larger capitosaurids and brachyopids were probably obligatory aquatics, as suggested by the short length of the limbs relative to overall body length. These short limbs would probably not support the animal for long periods out of water.
When the lateral line is discontinuous a semi-aquatic lifestyle is indicated, and the skeletal ossification is sufficient to support an animal walking on land. Dermal armour was present in some lydekkerinids and capitosaurids. All the families in this ecomorphic type are associated with fluvial, paludal (swamp) and lacustrine deposits. The lydekkerinids are believed to have been small surface feeders, taking mostly insects and small fish. The larger members of this ecomorphic category were probably subsurface feeders, feeding on invertebrates, fish and possibly other smaller amphibians, possibly feeding by sucking the prey into the mouth as aquatic ambush predators often do. Capitosaurs are believed to have been something like crocodiles.
Some forms from the Rhytidosteidae (including Indobrachyopidae), Trematosauridae, and Plagiosauridae were apparently fully aquatic with wide salt tolerance. The characteristics this conclusion is based on are a lateral line system with continuous deep grooves, poor skeletal ossification, especially in the short limbs. Some had long bodies like crocodiles and have been found in river, lake, lagoon and nearshore marine deposits. Some of these labyrinthodonts have been found associated with ammonites and ichthyosaurs, species that are known to be aquatic forms. Some plagiosaurs had well ossified bronchial skeletons that support gills, further evidence for an obligate aquatic lifestyle
There was a great variety of feeding styles in this group. The long-snouted forms probably ate fish, while the short-snouted forms probably ate other vertebrates, such as tetrapods and invertebrates.
A marine transgression in the Early Triassic would have provided a range of niches for labyrinthodonts to move into, as such forms as rhytidosteids, and plagiosaurids seem to have taken advantage of the new opportunities. A major extinction event occurred the end of the Triassic that wiped out most of the labyrinthodonts, along with many other animals. Of the labyrinthodonts, only 3 groups of freshwater ecomorphs survived to the Jurassic - brachyopids, capitosaurids and chigutisaurids. One of these survived into the Early Cretaceous, but its family is uncertain.
|Author: M.H.Monroe Email: email@example.com Sources & Further reading|