Lost worlds of Gondwana

Where is Gondwana located?

If we could look at the globe 400-550 million years ago, we would find a very different world. Almost all of the Northern Hemisphere was covered by an ocean called Panthalassa.

In the Southern Hemisphere, a huge landmass extended from the South Pole to the equator. There were no oceans separating South America, Africa, Australia, Antarctica and India. They were all merged in a single supercontinent, which scientists have named Gondwanaland or simply Gondwana.

It also included large parts of south and south-east Asia, and southern Europe, which were either attached to the mainland, or formed chains of islands and volcanic island arcs some distance off the Gondwanan coast. Some of these islands, such as South and North China, were the size of small continents.

First signs of complex animal life

This is the time when the first signs of complex animal life appear in the fossil record. The name of the Period comes from the Ediacara Hills in South Australia, where geologist Reg Sprigg discovered the (then) oldest known animal fossils in 1946. Ever since, Gondwana has been the most important source of our knowledge of early metazoan (complex animal) life. Important fossil localities outside Australia are known from Namibia, Newfoundland, Central Iran and Wales. All were part of Gondwana at that time.

The Cambrian Explosion

Evidence of life is scarce in rocks formed during the Ediacaran Period, but everything changed at the beginning of the Cambrian Period, just over 540 million years ago. This was the time when marine animals started to grow hard parts, or skeletons, for the first time. Remains of such animals were preserved in the rock as isolated parts, like vertebrate teeth and sponge spicules, as shells, including molluscs and brachiopods, and as complete carapaces of trilobites and similar animals. Such fossils are found in many fossil localities across Gondwana, from Newfoundland to Australia.

There are also a few locations where soft body parts of these animals are preserved. One of the richest and most important fossil localities of this kind is in Chengjiang, South China. These fossils provide convincing evidence that almost all major types of invertebrate animals that are alive today have existed since early Cambrian times. The Cambrian Explosion gets its name from the fact that most major groups of animals appeared over a (geologically) short period of time, as life diversified rapidly into a great variety of forms.

The Ordovician diversification of life?

Life in the ancient seas went through big changes during the Ordovician Period (440-490 million years ago). Animals living on the sea bottom started to grow upwards, higher above the sea floor, and the first coral reefs started to grow about this time. This resulted in an amazing increase in the diversity of life.

In the Ordovician world, more animals lived attached to the bottom and fed by filtering plankton from sea-water. Good examples of such animals are various corals, brachiopods, bryozoans or moss animals, and crinoids or sea lilies. Living alongside these were mobile animals, including molluscs and arthropods.

Tropical seas surrounding the Australasian sector of Gondwana were an important centre for the origin of new marine life, like maritime South-East Asia is today.

The end and new beginning

Towards the end of the Ordovician Period, life on Earth faced a difficult new challenge, resulting in the second biggest mass extinction event in its history. An enormous ice cap started to grow in the Southern Hemisphere, where most of the shallow seas supporting diverse marine life were located. During the final (Hirnantian) stage of the Ordovician, a significant part of Gondwana was covered by thick ice. Evidence for this is widespread across Africa, Brazil and the Arabian Peninsula, and has most recently been found by our research team in Iran. The growing ice sheet cooled the climate and caused sea level fall, drastically changing many shallow marine habitats.

The Hirnantian was named after Cwm Hirnant near Bala, in North Wales, where this stage of geological history was first recognised. By that time, Wales was actually far away from the rapidly cooling Gondwanan world and was approaching tropical Laurentia (the ancient North American continent), as part of a small 'break-away' continent called Avalonia. Wales was also the place where the so-called "disaster Hirnantia fauna" was discovered and described for the first time. This restricted group of animals evolved in temperate latitude Gondwana following the first major pulse of global extinction. At that time, with few competitors left, they were able to spread widely across the globe.

By the end of the Ordovician Period, two-thirds of marine species were extinct and the great variety of living things found in different parts of the world had all but disappeared. Lucky survivors took over expanding shallow seas after the ice cap melted in the early Silurian Period, from around 440 million years ago. Soon marine life flourished and diversified once more. But by that time, the four major continents, Gondwana, Laurentia (North America), Baltica (Europe) and Siberia, were slowly moving northwards and towards each other, eventually joining together to form a single land-mass, Pangaea (meaning "entire Earth"), by the end of the Palaeozoic. A different world with a different history was emerging.