Australia: The Land Where Time Began
The Palaeozoic amphibians were of 2 types, the labyrinthodonts and the lepospondyls, together radiating along different lines from that of modern amphibians.
Labyrinthodonts probably evolved directly from rhipidistians, most likely from a common ancestor. Ichthyostegids that are restricted to the Late Devonian, are believed by some to be among the labyrinthodonts because of the closeness to the rhipidistians. Others have them in a separate category, but included in the fish-to-amphibian series.
Labyrinthodonts and Ichthyostegids are present in rocks from the Devonian to the Early Cretaceous. Some of the characteristics of the rhipidistians retained by the labyrinthodonts are labyrinthine dentine infolding of the teeth, large fangs associated with replacement pits in the palate, and the centra of the vertebrae composed of more than 1 bony ossification. Clearly differentiating the labyrinthodonts from the rhipidistians are limbs and the atlas-axis complex of the vertebral column.
Lepospondyls differed markedly from the labyrinthodonts, lacking fangs and replacement pits and the complex dentine infolding of the teeth. Each vertebra usually had a single ossification and the lacked the otic notch, so probably had no eardrum. The articulation of the skull to the vertebral column is different from the atlas-axis complex of the labyrinthodonts. They are generally much smaller than most labyrinthodonts. At the time of writing they have not been found in Australia.
In the late Palaeozoic these were a very successful group. They are usually split into 2 major types, based mainly on the build of their vertebrae. Temnospondyls, from the Devonian to the Cretaceous, and the anthracosaurs, the Devonian to the Permian. Some classify the earliest temnospondyls as a separate group, the Ichtyostegalia. The temnospondyls are the only group to have been found in Australia until the recent discovery of an anthracosaur. Most of the Australian temnospondyls are from the Early to Middle Triassic. It is the longest record and one of the most varied records in the world. An anthracosaur has now been found in Early Carboniferous deposits in Australia. See Nature.
A large crescent-shaped vertebral block (an intercentrum) associated with a pair of small blocks (pleurocentra) is characteristic of temnospondyls. They had a solid skull with interlocked cheek and skull roof. There are 4 digits on the hand, 2,3,3,3 phalanges. There were 5 toes on the foot, 2,3,3,3,3 phalanges. On the skull they had a pattern of sinuous ridges separating grooves and pits.
A single bony element, the pleurocentrum, dominated in each vertebra. If present, the intercentrum was greatly reduced. In most anthracosaurs, the skull roof was loosely attached to the cheek. The skull bone ornamentation of this group differentiates it from the temnospondyls, being characterised by fine, radiation grooves and a detailed arrangement of the skull bones. The hand had 5 digits, 2,3,4,5,3 phalanges. The foot had 1 or more extra phalanges in its 5th digit.
The Silverband Trackway-Early Devonian
The oldest evidence of vertebrates walking on land was found in a paving slab in the courtyard of an old Cobb & Co station, Glenisla Homestead. The paving stones came from a site 3 miles from the homestead, believed part of the Silverband Formation in the Grampians Group, of Siluro-Devonian age. Across one of the paving stones was a ladder-like pattern made by a small tetrapod walking across the ripple marks of the fossil beach, that was deposited in the Early Devonian, more than 400 Ma. There were no marks from a tail, so the animal was probably short-bodied, and has been estimated from the spacing of the footprints, and allowing for a tail, to be about 850 mm long. The impressions weren't detailed enough to show evidence of the number of digits, most likely because the sand was very wet at the time.
Most genera of Australian Triassic and Jurassic amphibians are from the family Brachyopidae.
Keratobrachyops australis, from the Arcadia Formation, Rewan Group, sandstone near the headwaters of Duckworth Creek, southwest of the township of Bluff in south-central Queensland.
Parotosuchus gunganj from The Crater, the Arcadia Formation, Rewan Group, 72 km SW of Rolleston, south central Queensland. (P. rewansis)
Parotosuchus aliciae from Duckworth Creek, the Arcadia Formation, Rewan Group, NW of Rolleston, south central Queensland.
Parotosuchus sp. from the Blina Shale, Erskine Ranges, 90 km east of Derby, Western Australia.
Chomatobatrachus halei from the Knocklofty Formation, at the Meadowbank Dam Site and Old Beach site near Hobart, Tasmania.
Acerastea wadeae from The Crater, the Arcadia Formation, Rewan Group, 72 km SW of Rolleston, south central Queensland.
Arcadia myriadens from the Arcadia Formation, Rewan Group, sandstone near the headwaters of Duckworth Creek, southwest of the township of Bluff in south-central Queensland.
Nanolania anatopretia, Early Triassic, Arcadia Formation (Rewan Group) of Queensland
Deltasaurus kimberleyensis from the Blina Shale, Erskine Ranges, 90 km east of Derby, Western Australia, and from sites in Tasmania - the Cluan Formation, Poatina Road near Launceston; The Knocklofty Formation, Midway Point & Cunningham localities near Hobart.
Deltasaurus pustulatus from the Kockatea Shale at the Bore Hole near Geraldton, Western Australia.
Derwentia warreni from The Knocklofty Formation, Midway Point & Cunningham localities near Hobart, Tasmania
Rewana quadricuneata from The Crater, the Arcadia Formation, Rewan Group, 72 km SW of Rolleston, south central Queensland.
Erythrobatrachus noonkanbahensis from Blina Shale, Noonkanbah Station about 180 km east-southeast of derby Western Australia.
Indeterminate sp. from The Crater, the Arcadia Formation, Rewan Group, 72 km SW of Rolleston, south central Queensland & from the Glenidal Formation, Clematis Group, near Moolayember Dip on the northeastern side of the Carnarvon Range.
Lapillopsis nana from The Crater, the Arcadia Formation, Rewan Group, 72 km SW of Rolleston, south central Queensland.
Plagiobatrachus australis from The Crater, the Arcadia Formation, Rewan Group, 72 km SW of Rolleston, south central Queensland.
Paracyclotosaurus davidi from the Winnamatta Group, Brick Pit, St. Peters, Sydney, New South Wales.
Parotosuchus brookvalensis from the Hawkesbury Sandstone, Brookvale, Sydney area, New South Wales.
Parotosuchus wadei from Narrabeen Group, Sydney area, New South Wales.
Notobrachyops picketti from the Ashfield Shale, Winamatta Group, Hurstville Brick Company quarry at Mortdale, Sydney area, New South Wales.
Early Cretaceous - Aptian
Siderops kehli, from the Uppermost Early Jurassic, about 2.5 m long, was found in rocks on Kolane Station, south central Queensland. This specimen was found with a small fossil millipede, Decorotergum, the only known Mesozoic record of these arthropods, in its mouth.
This labyrinthodont, possibly up to 3-4 m long, lived in the Early Cretaceous (Aptian) about 114 million years ago. By this time it was thought to have been extinct for about 100 million years. It must have been competing successfully with the all-conquering dinosaurs for over 100 million years. Its fossils have been found in outcrops between Cape Patterson and Inverloch, East Gippsland, Victoria. It apparently occupied a niche similar to crocodiles in warmer climates. A possible reason for its survival until so late is that it lived in a polar climate, which precluded competition from crocodiles, and that it may have adapted in a similar way to the giant Japanese salamander which survives freezing cold streams during Japanese winters. Another explanation could be peninsula Australia.
|Author: M.H.Monroe Email: firstname.lastname@example.org Sources & Further reading|