Open image in new window. This process is experimental and the keywords may be updated as the learning algorithm improves. This is a preview of subscription content, log in to check access. Collins, A. Amalgamating eastern Gondwana: the evolution of the circum-Indian orogens. Earth Science Reviews , 71 , — CrossRef Google Scholar. Dalziel, I. Pacific margins of Laurentia and East Antarctica as a conjugate rift pair: evidence and implications for an Eocambrian supercontinent.
Geology , 19 , — Fitzsimons, I. A review of tectonic events in the East Antarctic Shield, and their implications for three separate collisional orogens. Journal of African Earth Sciences , 31 , 3— Hoffman, P. Did the breakout of Laurentia turn Gondwanaland inside out? Science , , — A Neoproterozoic snowball Earth. Recent research on ancient sediments gave weight to the key role played by oxygen, by showing that oxygen levels in the sea rose from the Ediacaran to Cambrian period, to near-modern levels.
This would have allowed larger and more complex animals to evolve, and more complex food webs to develop. An increase in larger predators may have triggered rapid evolution of predators and prey, as natural selection adapted their bodies to better target or evade each other. There was also a global sea level rise at this time, which greatly increased the area of shallow seas on Earth, providing more habitable space that could be filled by new species.
Another possible environmental factor was an increase in calcium ions in sea water, due to volcanic activity and widespread weathering of the land. These ions would have provided the raw materials for animals to make shells, skeletons and other hard parts, making a greater variety of body plans possible. Some scientists speculate that internal, developmental features of animals themselves may also have played a role in the Cambrian Explosion, although it is very difficult to test this idea.
It is possible that animals acquired some key genes or that they crossed some threshold level of genetic complexity, making it possible to make a much bigger range of body shapes. However, the fact that some complex animals already existed in the Ediacaran period suggests that this was unlikely to have been a key trigger for the Cambrian Explosion.
Life in the ancient seas went through big changes during the Ordovician Period 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. 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. Gondwana shows a remarkably similar stratigraphy on all of the major southern continents, grading from Devonian tillite glacial gravels , through Carboniferous and Permian coal, Triassic red beds sandstone and conglomerate , and Jurassic through Cretaceous volcanic rocks.
The Devonian glacial til-lites are remarkable rocks, found on all five southern continents. These were deposited unconformably on older rocks, and in many places the unconformity surface preserves scratches made by glaciers hundreds of millions of years ago.
Boulders in the tillite also locally preserve glacial striations, plus isolated. Alex Du Toit's reconstruction of Gondwana showing areas affected by glaciation, striation directions, and other tectonic features. Most of these tillites are interbedded with nonmarine rocks that bear a distinctive fossil assemblage including a seed fern called Glossopteris that inhabited low swampy areas next to glaciated terrane.
The Glossopteris flora is found across much of Gondwana, and was used by Du Toit and Wegener to support their reconstructions of the late Paleozoic supercontinent. The Gondwanan stratigraphy grades up into Carboniferous to Permian coals, and then Triassic red beds, and finally Jurassic-Cretaceous volcanic rocks consisting mostly of basalt flows. These basalts are related to the breakup of Gondwana and Pangaea, and spreading of the modern oceans.
The animals of Gondwana included a great variety of reptiles, amphibians, fish, and invertebrates, some with key correlations across present-day oceans.
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