Invasion of the land

As with any human innovation, the adaptations of early plants which enabled them to cope with these environmental problems were crude by what came later, but they were sufficient to enable the plants to survive in the world they found themselves in. With further evolution, the maximum heights of plants, for example, became greater. Within 50 million years of the first land plants, there were club mosses and ferns that were the size of modern pine trees.

As plant evolution continued, one of the major areas of innovation was in the mode of reproduction. As time passed, reproductive strategies evolved so that land plants relied less and less on having at least one part of the their life cycle take place in water or a moist environment. With such changes, plants were able to spread into harsher habitats such as deserts.

One of the earliest vascular plants and largest for its time was Baragwanathia, found at Siluro-Devonian sites near Melbourne. These earliest land plants were rod-like, with no development of leaves or true roots. Standing more than a metre high, they dwarfed their contemporaneous northern hemisphere cousins.

As soon as land plants evolved, insects appeared to prey upon them. Other plants predators also quickly evolved. Even among the earliest of plant fossils, damage owing to grazing by animals can be seen and adaptations by the plants to discourage such predation are also evident. The ongoing 'arms race' between plants and their animal predators thus has a long history.

By the Late Devonian, true roots and leaves had appeared and plants were becoming somewhat larger and more diverse. Because their mode of reproduction still required a moist environment, land plants of this age were confined to the water's edge.

In the succeeding Carboniferous Period, much of the coal deposits in the northern hemisphere were formed as the land flora diversified and flourished there. Prominent among these plants were ferns, seed-ferns, clubmosses and towards the end of that period, the first gymnosperms or 'naked-seed' plants which today include pine trees. The development of the gymnospermous condition meant that plants with that mode of reproduction did not have to spend any part of their life in water or a moist habitat. The way was open for such plants to spread away from the edges of water courses and swamps.

The widespread deposition of plants in vast swamps during this time resulted in the large coal fields of Europe which gave the Carboniferous Period its name. The Carboniferous was the only time when the principal coal deposits of the world were formed at tropical latitudes. Later, almost all coal was formed at temperate and even high latitudes.

While vast coal fields were being laid down in the tropical swamps of the northern hemisphere, the southern Gondwana continents were clustered around the South Pole. The centre of Late Paleozoic Gondwana glaciation began in Africa and gradually shifted across India and ultimately into Australia. As a consequence, the floras of that time on the Gondwana landmasses are quite distinct from those in the northern hemisphere.

The first invertebrates on the land

The first truly terrestrial invertebrates were other arthropods closely related to spiders, whose remains are known from cherts (flint-like rocks) of Early Devonian age. Other terrestrial groups known from the Devonian are mites, pseudoscorpions, insects, millipedes and possibly spiders. Flying insects first appear in the fossil record in the Carboniferous, but they were diverse enough by this time to suggest that flight may have developed during the preceding Devonian Period.

The appearance of amphibians

While we think of 5 or less as the number of toes on land vertebrates, in the Late Devonian, there was much experimentation going on in foot structure. While some with five toes are known, others had 6, 7, or even 8. Why the number 5 prevailed and the others did not is probably more a matter of chance than superior functional qualities of the pentadactyl condition.

The earliest amphibians were an intriguing combination of fish and land dweller, a sort of fish with feet. Some retained a dorsal fin on their back and had lateral line systems on their skulls, organs quite well adapted to life in water but useless on land.

The Carboniferous record of amphibians is extremely sparse, being concentrated primarily in western Europe and the eastern United States. In the northern hemisphere, what we know of them are rare fossils preserved in the coal deposits of that age which were laid down in steamy swamps. Just recently, the first Australian assemblage of this age was found. When initial studies of this material are completed, it may give us a much better idea of how amphibians evolved. With our scanty knowledge of Carboniferous land vertebrate history, there is little evidence to back up conjectures about just how the evolution of these animals proceeded. It is clear, however, that this period was very important in the history of all terrestrial vertebrates, because it was during the Carboniferous that the reptiles arose from the amphibians.

With the appearance of the reptiles in the Carboniferous, the history of amphibians does not end. Far from it, the fossil record in fact markedly improves in the following Permian and Triassic periods. It is then that we see the greatest diversity of forms that amphibians ever assumed. They ranged in size from frogs to crocodiles. Many took on the appearance of deep bodied, blunt-faced crocodilians but others had quite bizarre shapes. One of the strangest of these, Diplocaulus, had a triangular-shaped head much broader than its body, so much so that the lateral margins stuck out to the side like a pair of horns. All of these Palaeozoic and early Mesozoic amphibians are called labyrinthodonts and lepospondyls, based on features of their teeth and vertebrae. They are all extinct.

One of the truly remarkable discoveries in Victoria in the past twenty years are the remains of Early Cretaceous labyrinthodont amphibians. Until this material was found, it was thought that the last of these had died out in the Early Jurassic. To find them persisting in Victoria in the Early Cretaceous was more unexpected than if a living dinosaur were found today because the Victorian specimens extended the range of labyrinthodonts by about 85 million years. Living amphibians have a greater tolerance to cold than do living reptiles. Therefore the key to why labyrinthodonts survived in Victoria long after becoming extinct elsewhere may be related to the fact that Victoria was then a cold, polar region.

This great radiation of amphibians in the late Palaeozoic and early Mesozoic does not include the frogs, salamanders, newts and caecelians which are the only amphibians alive today. They were a later development, appearing in the Late Triassic when the labyrinthodonts and lepospondyls were declining in diversity. It was at this time that many other vertebrate groups were also making their first appearance such as dinosaurs, mammals, and flying reptiles or pterosaurs.

The skeletons of the earliest frogs were only slightly more generalised than their living descendants. Like them, the skeleton was highly modified for jumping. It was these specialisations, such as the fusion of the tail into a single bone and the reduction of the bones in the vertebral column and limbs that marks the frogs as not primitive at all. Despite the popular perception that they are primitive, in fact they are among the most specialised of vertebrates.

The pattern of frog evolution is typical of many groups. When they appeared, they were only slightly different from their later descendants. Clearly what happened was that early frogs had a quite successful body form and their descendants thrived by evolving only the slightest modifications to the earlier pattern. And like so many groups that have such a history, their precursors among more generalised amphibians are quite uncertain. It is likely that these unknown ancestors among the labyrinthodonts or lepospondyls rapidly evolved in terms of geological time, to make the change into frogs. Once that body form had evolved, it did not alter it much. Thus the frogs we see around us would not have looked remarkably out of place alongside dinosaurs 100 million years ago.

References:

Coates, M. I. & Clack, J. A. 1990. Polydactyly in the earliest known tetrapod limbs. Nature 347: 66-69.

Edwards, D. 1980. Early land floras. In Panchen, A. L. (ed.), The terrestrial environment and the origins of land vertebrates. Academic Press, London.

Gordon, M. S. & Olson, E. C. 1995. Invasions of the land. Columbia University Press, New York.

Little, C. 1990. The terrestrial invasion. An ecophysiological approach to the origins of land animals. Cambridge Studies in Ecology.

McAlester, A. L. 197. The history of life. Prentice-Hall, New Jersey.

Selden, P. A. 1990. Invertebrates (Terrestrialization). In Briggs, D. E. G. & Crowther, P. R. (eds), Palaeobiology. A synthesis, Blackwell, Oxford, 64-68.

Selden, P. A. & Edwards, D. 1989. Colonisation of the land. In Allen, K. & Briggs, D. (eds), Evolution and the fossil record, Belhaven Press, 122-152.

Taylor, T. N.1982. The origin of land plants: a paleobotanical perspective. Taxon 31: 155-177.

Taylor, T. N. & Taylor, E. L. 1993. The biology and evolution of fossil plants. Prentice-Hall, New Jersey.

Thomas, B. A. & Spicer, R. A. 1987. The evolution and palaeobiology of land plants. Coom Helm, London.

Warren, A. A., Jupp, R. & Bolton, B. 1986. Early Devonian tetrapod trackway. Alcheringa 10: 183-186.

Warren, A. A. & Wakefild, N. A. 1972. Trackways of tetrapod vertebrates from the Upper Devonian of Victoria, Australia. Nature 238: 469-470.