Australia: The Land Where Time Began

A biography of the Australian continent 

The Tetrapods - From the Early Devonian to the Triassic, 409-208 Ma

The earliest known record of terrestrial vertebrates comes from the Early Devonian of Australia in the form of tracks left in the sand of a backwater from more than 400 Ma. It is believed to have been made by a labyrinthodont. This group of early amphibians were the dominant members of terrestrial faunas in Australia in the late Palaeozoic and early Mesozoic. The only record of reptiles could possibly be an artefact of fossilisation..

Australia was a peninsula on the edge of Gondwana, and is thought to have possibly been biogeographically separate from the rest of the world. In the Devonian Australia was situated on the equator but by the Late Carboniferous and Early Permian it had been shifted to the present position of Antarctica. Pangaea had formed from the coming together of Gondwana and Laurasia, forming a supercontinent that formed a continuous landmass stretching from pole to pole. The changed patterns of circulation in air and water brought large climatic changes, glacial conditions prevailed over much of the supercontinent, leading to lowered sea levels as much of the water was locked up in glaciers, and cold water masses were moved towards the tropics. This cold water moving to the equatorial regions is believed to have at least partially, led  to the dramatic mass extinction that occurred at the end of the Permian.

Many of the large terrestrial vertebrates of the time were eliminated, some smaller labyrinthodont amphibians and mammal-like reptiles survived. It was these smaller survivors that later radiated explosively to fill the niches vacated by the victims of the mass extinction. At the end of the Triassic another mass extinction wiped out most of the survivors from the Palaeozoic. When the latest survivors radiated they gave rise to mammals and dinosaurs that dominated the faunas for the rest of the Mesozoic.

Problems to be overcome

One of the problems to be overcome before life on land could move away from the water was to replace the support for the body previously supplied by the water. Once out of the water for prolonged periods the weight of the body would become a problem. The notochord of the ancestral fish was all that was necessary while the body was supported by water, but more was required out of the water, The solution to the problem of locomotion had begun before the move to land was made, pre-adaptations of the bony support of the pectoral fins, in the form of strengthening that probably occurred to assist in making rapid lunges by the ancestral  fish, as it was believed to have been an ambush predator.

One of the earliest known amphibians, Ichthyostega, had developed massive ribs to aid in support and protection of the internal organs, but retained the notochord. This amphibian retained a fin on its long tail supported by the neural (dorsal) and haemal (ventral) processes on the vertebrae. This structural support for the tail suggests it used the tail for swimming, hence it probably spent a lot of time in the water.

Head support was necessary to allow efficient walking, breathing and feeding. To achieve this the dermal bones of the skull and the endochondral bones of the braincase and jaws underwent a lot of fusion of the bones making up the skull of the fish and skull roof and cheek area were now firmly attached. Loss of bones also occurred in the early tetrapods. The bones such as the opercular series that had covered the gill area of series that had covered the gill area of crossopterygians and had been attached to the shoulder girdle in stages leading to amphibians. Ichthyostega retained 2 of the opercular series.

Ichthyostega developed the atlas-axis complex. This structure involved the first 2 vertebrae that now attached to the occipital condyle at the back of the skull in a ball and socket structure. In the transition from fish to amphibian the proportions of the anterior and posterior portions of the skull changed. The anterior portion elongated in the tetrapods, while the posterior portion shortened. But in spite of these changes, there were still many homologous bones in the crossopterygian predecessors and the back of the skull in a ball and socket structure. In the transition from fish to amphibian the proportions of the anterior and posterior portions of the skull changed. The anterior portion elongated in the tetrapods, while the posterior portion shortened. But in spite of these changes, there were still many homologous bones in the crossopterygian predecessors and amphibians.

One feature that carried over to the early amphibians is the labyrinthodont teeth possessed by the osteolepiform crossopterygian These teeth differed from those of humans and most other vertebrates. They had a characteristic array of dentine infolding. In the early tetrapods these teeth were often large and fang-like.

The unique arrangement of the bones supporting the paired fins of rhipidistian crossopterygians is more similar to the limbs of tetrapods than that of any other fish, but in the fish they were only used for swimming, the closest they came to a similar function was in allowing the fish to make a powerful lunge at prey. In the tetrapods the shoulder girdle - scapulocoracoids, cleithra, interclavicles, clavicles - became became a separate, distinct structure from the bones of the skull, unlike the condition in the fish, where all these bones remained interlinked.

A firm connection between the vertebral column and the hind limbs developed, a single vertebra or several vertebrae fused via a sacral rib with the pelvis. The bones and joints of the rhipidistians were remodelled, the main change being increased flexibility of the distal parts of the limbs to allow for the wrist and ankle joint to twist and hinge the feet. They moved by extending the feet forward or outward and the muscles pulled the body towards the extended arm or leg. The hands and feet developed by consolidating the phalanges into distinctly defined digits. The hands of early amphibians had 1 of 2 patterns, one with 5 digits with phalanges, the other with 4 digits with or phalanges. Ichthyostega had the latter, with an extra digit on the hand and an extra 2 digits on the foot.

An adaptation of early amphibians was the development of heavy scales that reduced water loss through the skin.

Sensing the environment was another problem when an animal left the water. The lateral line, so useful in fish, is useless on land, so the fact that rhipidistians, Ichthyostega as well as most Palaeozoic and Mesozoic amphibians retained it means they were still very strongly tied to water, spending a lot of time in it.

The presence of a nasolacrimal duct, which is associated with producing liquid to keep the eyes and nasal epithelium moist, in Palaeozoic amphibians indicates that they were spending extended lengths of time out of water. This duct was not a new feature of the amphibians, having been already present in some rhipidistians, apparently they were already spending time out of the water before the transition.

The otic notch is a concave depression at the back of the skull of many labyrinthodonts. It is believed the ear drum may have fitted into this notch, the associated bone transmitting sound to the stapes, the inner ear, which developed in the early tetrapods and its refinement continued through to mammals. In the early tetrapods the stapes may have functioned as a support for the brain case, its sound reception being enhanced in the more advanced amphibians.

The last of the problems faced by the animals that moved to the land was reproduction. Amphibians never managed to completely cut their ties to the water, having to return to the water to reproduce. The final tie to the water wasn't made until the arrival of the reptiles with their development of the first amniotic egg that didn't require water for its development.

See Earliest Tetrapods, Origin from Marine Environment

See An Early Tetrapod from Romerís Gap

Sources & Further reading

  1. Long, John A, 1998, Dinosaurs of Australia and New Zealand, University of New South Wales Press.
  2. John A Long The Rise of Fishes - 500 Million years of Evolution, University of New South Wales Press, 1995
Author: M. H. Monroe
Last Updated 07/09/2014 
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