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A walk through time, part 2

(See A walk through time, part 1)


The Cambrian Period (541–485 Ma) saw the advent of seafloor bioturbation by bottom feeders, the first major radiation of hard-shelled life (trilobites), and the appearance of vision in this group. 

During the Ordovician Period (485–419 Ma), one of the greatest evolutionary radiations took place, generally referred to as the Great Ordovician Biodiversification Event. Starting in the middle Cambrian, marine biodiversity increased to reach a level (about 1600 genera) that was not significantly exceeded during the remainder of the Palaeozoic, or the early Mesozoic, or possibly up to the Palaeocene. Brachiopods, trilobites, corals, echinoderms, bryozoans, gastropods, bivalves, nautiloids, graptolites, and conodonts show major genus-level increases through the Ordovician. Then, near the end of the Ordovician, over 25 percent of all marine animal families, over 50 percent of genera, and over 80 percent of species became extinct, marking one of the largest faunal turnovers on Earth. Dramatic changes in niches due to platform and shelf re-organization, major climate change, and oceanographic perturbations may have been contributing factors. 

During the Devonian (419–399 Ma), animals and early seed plants appeared on land. The animals include insects, amphibians (tetrapods), and lung fishes. The Devonian is also the time of greatest carbonate production with a peak of reef growth, and of the greatest diversity of marine fauna in the Palaeozoic. This ended in the late Devonian when major hypoxic or anoxic phases led to substantial mass extinctions. Over a period of perhaps 20 million years, a large number of families, genera, and species disappeared in the Frasnian and Famennian stages, including many trilobites. 

In the Carboniferous (359–299 Ma), reptiles evolved and major glaciations reigned in southern Gondwana, the enormous assembly of Antarctica, Australia, India, and other parts of southern Asia, and parts of Africa and South America. 

The end of Permian faunal and floral turnover about 252 Ma is the largest biological turnover event in history with the demise of over 55 percent of all faunal families, 80 percent of all genera, and over 95 percent of all species. The very last taxa of the prominent Palaeozoic trilobite group disappeared at the end of Permian. There is considerable evidence for a runaway greenhouse effect and significant increase in temperatures in the latest Permian and Early Triassic, possibly associated with the Siberian Trap volcanism. Associated charcoal-rich and soot-bearing layers indicate widespread wildfires on land.


The so-called K–T extinction event, properly called the Cretaceous–Palaeogene extinction event, was followed by a radiation of mammals and birds during the Palaeocene. The event itself saw over 17 percent of all families, 50 percent of all genera, and over 75 percent of all species disappear on land, and sessile life in the ocean decreased over 30 percent. Among oceanic plankton, the foraminifera became nearly all extinct before rebounding with many new taxa in the Palaeocene. Dinosaurs were already on the way out before their final demise. The period boundary event is linked to the Chicxulub (Yucatan Peninsula) meteorite impact crater, 180 km across, in addition to the Deccan Trap volcanism. 


The late Holocene, the beginning of the Industrial Revolution, or the middle of the 20th century with its massive urbanization and nuclear explosions, are candidates for a new chronostratigraphic unit that has been termed Anthropocene. 

Anthropogenic change to Earth’s biota, both in the plant and in the animal kingdoms is now pervasive. Such changes are clearly visible both on land and in the sea. Extinction rates, the most obvious indicator of change, are currently perhaps 100–1000 times background rates (Millennium Ecosystem Assessment 2005). These very high rates are recent, and so Earth has thus far only lost a small proportion of its biodiversity. However, many more species are classed as threatened, endangered, or critically endangered, and the continuation of present trends could produce a mass extinction event comparable to the ‘Big Five’ of Phanerozoic history in as little as a few centuries. 

Perhaps the most striking signal to date is that of cross-global species invasions, at a rate and on a scale unique in Earth's history. Terrestrial biotas worldwide now commonly include up to a quarter of total species (and up to a half of plant species) as invasive (McNeely 2001). Marine invasive species may be less in total, but are more rapidly growing. The numbers far exceed reported extinctions, with some 10 000 invasive species reported from Europe alone.

Thus ends our (too short) walk in deep time.

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