Gondwana

Gondwana, also called Gondwanaland, the second largest continent of the Palaeozoic Era (545 to 248 million years ago), making up more than 70 per cent of the Earth’s continental area at its maximum extent. Confined to the southern hemisphere, this enormous land mass was involved in a series of collisions with other continental blocks, culminating in the formation of the supercontinent known as Pangaea during the Permian Period. The subsequent rifting of Pangaea during the Mesozoic Era resulted in the origins of the modern continents, and the massive Gondwanan element fragmented into South America, Africa, Madagascar, India, Australia, New Zealand, Antarctica, and a jigsaw of other smaller pieces found through Central America, the Mediterranean, the Middle East, the Himalaya, and parts of the Pacific and Arctic Oceans. The name Gondwana is derived from Sanskrit and means literally the forest [land] of the Gonds—a people of northern India.

The existence in the remote geological past of a continuous continent extending from South America to India was conceived in the late 19th century based on similarities in the living and extinct plants and animals of these regions. At this time, the prevailing theory of the Earth saw continents as fixed in their relative positions, and the problem of linking up the various elements of Gondwana was solved by hypothesizing the existence of ancient land bridges. In 1912, this concept of Gondwana changed radically with the development of the theory of continental drift by the German meteorologist Alfred Wegener. According to Wegener, continents are not fixed but drift around the surface of the Earth coalescing and subsequently fragmenting. The need for hypothetical land bridges linking up the elements of Gondwana therefore disappeared because the component continents had once been joined in a single supercontinent.

Wegener’s ideas were further developed by the South African geologist Alex Logan du Toit who, in 1937, produced the first plausible reconstruction of the shape and composition of Gondwana. In support of their ideas, Wegener and Du Toit pointed to an astonishing number of close affinities of fossils, rocks, and geophysical structures on opposite sides of ocean basins. These included the remarkable geometric fit of South America and Africa, similarities in rock age groups and types on the southern hemisphere continents, the similarity in Palaeozoic fossils between adjoining areas that are now thousands of miles apart, and the widespread occurrence of ancient glacial deposits in Gondwana. In particular, certain aspects of the Palaeozoic fossil record of Gondwana were quite distinctive, including endemic reptiles (for example, Lystrosaurus, Mesosaurus) and plants (for instance, Glossopteris, Dicroidium).

Despite abundant circumstantial evidence, the theory of continental drift lacked a convincing mechanism to explain the movement of continents, and for this reason it excited great controversy. The notion of drifting continents and the new concept of the structure of Gondwana only became widely accepted in the 1960s, following the discovery of palaeomagnetism and the development of the theory of plate tectonics. Palaeomagnetism—the study of changes in polarity of the Earth’s magnetic field through time—provided the first convincing evidence that continents had moved. Plate tectonics is a revised theory of the Earth’s surface and lithosphere explaining the growth and movement of continents, and other geological phenomena such as the origins of mountains and earthquakes. Subsequent study has vindicated the main elements of Du Toit’s original reconstruction of Gondwana. Current models show a slightly tighter fit of continents and make other small adjustments.

Gondwana and its fragmentation have been reconstructed with some precision, but the birth of this giant continent is much less certain. The formation of Gondwana as a distinct entity has been linked to the breakup of Rodinia, a pre-existing supercontinent. Assembly of Gondwana is thought to have been more or less complete by the beginning of the Phanerozoic Eon. Although many small fragments were chipped off throughout its history, the continent remained essentially intact until its breakup in the Jurassic and Cretaceous Periods. Gondwana therefore existed for a period of close to 500 million years.

Palaeomagnetic data indicate that Gondwana was located predominantly in the southern hemisphere, extending from the equator to the South Pole. During the early Palaeozoic the continent had a clockwise rotation. The western part moved from polar to subtropical latitudes while the eastern part remained at relatively low subtropical latitudes, moving rapidly to a polar position in the late Palaeozoic and early Mesozoic. During the Jurassic and Cretaceous the whole of Gondwana moved north. Only Antarctica has remained within 1,000 km (620 mi) of the South Pole since the Carboniferous, but South America has not changed its latitude by more than 20° in the past 300 million years.

During the Late Triassic and Early Jurassic, Pangaea began to break up, beginning with the separation of Gondwana from Laurussia. The rifting of Gondwana itself began in the Early Jurassic, leading to the establishment of a seaway between the South America/Africa region and the Antarctica/Australia/India region. This was followed in the Early Cretaceous by the separation of South America from Africa, and the Africa-India plate from Antarctica. Finally, in the Late Cretaceous and Palaeocene, the breakup of Gondwana was completed when Australia and New Zealand separated from Antarctica, and other small continental blocks (for example, Madagascar, Seychelles) separated from India as it moved northwards. In most cases, these riftings were accompanied by the outpouring of vast amounts of basalt lava (for example, Karoo-Ferrar-Tasman basalts during the east/west Gondwana rifting; Parana-Etendeka basalts during the South America/Africa rifting; Kerguelen-Rajmahal basalts during the Antarctica/India/Australia rifting; Madagascar basalts during the Madagascar/India rifting; and Deccan basalts during the Seychelles/India rifting). The causes of the breakup of Gondwana are at present not well established. Models range from lithospheric extension in response to subduction, to the action of deep-seated mantle plumes. The huge size of the continent itself may have been a contributing factor.

The climatic history of Gondwana shows that the far reaches of this enormous continent experienced everything from warm, humid equatorial rainforests, arid subtropical deserts, temperate forests, to frigid polar ice caps. The approximate locations of major climatic zones are based on lithological indicators of climate, such as coals, evaporites, bauxites, and tillites, as well as palaeomagnetic data. Climate was influenced by the size of the continent, its movement across climatic zones, and shifts in climate on a worldwide scale from so-called “Hot House” or “Greenhouse” to “Ice House” conditions. There is evidence of widespread glaciation in Gondwana during the Palaeozoic. Extensive Gondwanan ice sheets were a feature of Ice House phases during the Late Ordovician, and from the Late Carboniferous through to the Early Permian. During intervening Hot House phases no permanent ice existed, and a cool temperate climate prevailed at the South Pole. During the Permo-Carboniferous glaciation, ice sheets may have expanded to within 30° of the equator, resulting in significant compression of the Earth’s climatic zones. Cool temperate, warm temperate and dry subtropical conditions were present over parts of Gondwana throughout its history, but humid tropical climates were an intermittent feature.

The geological history of Gondwana encompasses the fossil records of the Palaeozoic and most of the Mesozoic. The continent hosted many endemic species of plants and animals, particularly during the Palaeozoic. The Permo-Carboniferous glaciation had an enormous impact on biological diversity in Gondwana. Regional endemism seen today in southern hemisphere floras and faunas results partly from the isolation of continents following the rifting of Gondwana, and partly from extinctions on a regional scale.

Fossil evidence from Australia demonstrates the presence of a land flora by the Late Silurian. During the great Permo-Carboniferous glaciation plant life in much of Gondwana was impoverished in comparison to that which flourished at equatorial latitudes. The flora at mid and high latitudes diversified following the melting of ice sheets in the Late Permian. During the late Palaeozoic and early Mesozoic gymnosperms (conifers and related plants), horsetails, lycopods (clubmosses), and ferns were abundant. One of the most distinctive plants of the Permian and Triassic Periods is Glossopteris—an extinct endemic gymnosperm tree with large, tongue-shaped leaves. The Glossopteris flora was gradually replaced during the Triassic by a flora dominated by other distinctive gymnosperms such as Dicroidium. The degree of endemism within Gondwana decreased during the Triassic. Jurassic floras are dominated by Bennettitales (extinct seed plants with a superficial resemblance to modern cycads), ferns, and conifers. Flowering plants (angiosperms) first appeared in the Early Cretaceous and were soon widespread throughout the southern hemisphere.

Gondwana seems to have been the birthplace of tetrapods, which first appeared in eastern equatorial latitudes during the Upper Devonian but soon spread to the northern continents of Laurentia and Baltica. During the Permo-Carboniferous glaciation, most tetrapods are found in the so-called Edaphosaur-Nectridean Province—a region that encompassed equatorial Laurussia and Gondwana. The Mesosauridae were an exception. This endemic group of specialized long-snouted reptiles is found in interglacial southern Gondwana. The retreat of ice sheets in the Late Permian heralded an increase in the diversity of Gondwanan tetrapods. This was made up mainly through migration of Laurussian species into Gondwana. Almost every major dinosaur group was represented in Gondwana except the dome-headed and horned dinosaurs (Pachycephalosauria and Ceratopsida).