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Lost worlds of Gondwana

Earth about 530 million years ago, shortly after the Gondwanan supercontinent formed
Image 1: Earth about 530 million years ago, shortly after the Gondwanan supercontinent formed.
The oldest mineralised fossils of marine animals date from this time: (a) mineralised plate of worm from Northern Iran; (b) brachiopod from Indian Kashmir; (c) trilobite from Alai Range, Kyrgyzstan; (d) enigmatic fossil from Jordan; and (e) conodont tooth. Scale bars = 0.2 mm. Trilobite fragments about 4 mm wide.
About 480 million years ago, Avalonia separated from Gondwana
Image 2: About 480 million years ago, Avalonia separated from Gondwana, and Wales started a long journey towards the tropics, meeting the east coast of Laurentia (North America) 70-80 million years later. Marine life rapidly diversified on tropical shelves, shown by these fossils from Australasian and Middle Eastern sectors of Gondwana: (a) an early coral; (b) ostracod, a minute crustacean; and (c) encrusting bryozoans on brachiopod shell.
 440 million years ago, the world was recovering after ice age and mass extinction.
Image 3: 440 million years ago, the world was recovering after ice age and mass extinction. Most post-extinction survivors settled in the tropics, only later migrating to higher latitudes. Early Silurian rocks from Iran preserve a rare record of that migration: (a) common brachiopods and (b) bryozoans; (c) microscopic plates from sea cucumbers; (d) rare sponges; (e) trilobite Calymene; (f) conodont teeth and (g) cone-shaped brachiopods.

The Museum has large fossil collections of Early Palaeozoic age (540-400 million years old) from various parts of the world. Many come from regions that in the geological past formed a huge single supercontinent, called Gondwana. Wales was also part of Gondwana, until it broke away some 480 million years ago.

Alien, ancient world

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

Gondwana was formed some time during the Ediacaran Period, by about 550-530 million years ago, as a result of the collision of several ancient continents. 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.

From 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.

... to 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. Trilobites were still abundant, but gradually became less important.

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.

Article by: Leonid Popov, Honorary Research Fellow

Article Date: 23 December 2013

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Glossary

Invertebrates
Animals without a backbone.

Molluscs
A group of soft-bodied invertebrate animals usually with a hard external shell. Familiar molluscs include the cockle, oyster, snail, slug, octopus and squid.

Arthropod
An invertebrate animal with a hard outer carapace and lots of jointed legs, e.g. insects, crabs.

Metazoan
a multicellular animal

Palaeozoic
Geological era lasting from 542-251 million years ago.

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