The Phanerozoic Eon:
542 Million years ago until last weekend or so ...
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The Phanerozoic represents a relatively brief period of half a billion years (brief that is relative to the age of the Earth and the universe) that constitutes the age of multicellular animal life on Earth. During this time micro- and multicellular organisms left a detailed fossil record, and built up complex and diverse ecosystems, and life has evolved through countless transformations and millions upon millions of species.
The term Phanerozoic - "revealed life" - is generally applied to the Paleozoic, Mesozoic and Cenozoic eras; the relatively short period during which the Earth has been inhabited by multicellular organisms that leave fossil traces in the rocks. This is in contrast to the "Precambrian", which lasted for a very much longer time, but was characterized only by micro-organisms that generally do not leave fossils. With the discovery of a complex late Precambrian (Vendian/Edicarian) biotas the term Phanerozoic has lost much of its meaning, but can still be used perhaps to define the period of the development and evolution of higher groups of organisms like arthropods, molluscs, vertebrates etc that are still alive and predominant today. For although primitive algae existed throughout much of the Precambrian, this was not the case with multicellular animals (metazoa), which only appeared during the very earliest Cambrian. This eon can also be considered (as suggested by Dr James Lovelock in his book Ages of Gaia) as the modern period in the life of Gaia (following the Archean and the Proterozoic), the maturity or third age of Gaia so to speak, and is characterized as much, if not more, by the presence of abundant free oxygen as by the existence of multicellular organisms or fossil-bearing rock strata.
| Phanerozoic eon|
| Paleozoic era|
| Mesozoic era|
| Cenozoic era|
Paleozoic Era: 542-252 Mya
The Cambrian explosion heralds the beginning of the era of 'Old Life'. The simultaneous appearance of most major groups of animals has been made to seem less explosive by the continuing finds of precambrian multicellular life in British Columbia, the Ediacara hills and elsewhere, but this is nonetheless the beginning of complex life as we know it. As the first nervous systems and complex behaviour were evolving the continents were drifting apart.
More tropical conditions prevailed through the middle Paleozoic, and as some 'experimental models' became extinct, other groups of animals diversified. Fish and other, stranger, swimming creatures were ascendant, and arthropods, proto-amphibians and early plants made the first steps on to land.
The giant coal forests of lycopods, pteridophytes, calamites and ferns covered the warmer tropical landmasses in the later Palaeozoic, while glaciers covering Gondwanaland hint at the generally icier conditions prevailing. Reptiles emerge as the continents drift back towards each other again.
Mesozoic Era: 251-65.5 Mya
The Mesozoic - or 'Middle Life' - has been called the age of the reptiles, but age of the dinosaurs is much more appropriate. The reptiles seemed to rule the world in the Permian, at the end of the Palaeozoic, but by the Mesozoic the dinosaurs (sens.lat.) were the flashest model going. Contrary to long held assumptions that the dinosaurs were lumbering, stupid, cold-blooded lizard relatives, there is a growing consensus that many were agile, intelligent, warm-blooded bird relatives. This is supported by the spectacular feathered dinosaurs found in China, as well as by details of bone structure and a body of other evidence from many different lines of enquiry. The supercontinent Pangea breaks up in the late Triassic, and the continents drift slowly apart again, forming the Atlantic, Indian and Southern Oceans. Mammals and birds appear.
Cenozoic Era: 65.5-0 Mya
The Cenozoic began with a bang - literally. The 'Cretaceous-Tertiary boundary event', marking the beginning of the Cenozoic Era, was the famous iridium-rich meteorite which hit the Earth where the Gulf Of Mexico now lies, leading to environmental disaster and the extinction of all dinosaurs (as usually understood). It is not, of course quite so clear cut: there may have been a series of impacts, the dinosaurs may have already been stressed by climate change or other factors, or the dinosaurs may not be extinct.
That last one isn't quite as silly as it sounds. If we regard birds as descendants of the same lineage that produced dinosaurs, they can be thought of as microdinosaurs who survived the Cretacious-Tertiary event. There is some evidence to suggest that the atmosphere didn't mix across the equator as much then as it does now. Perhaps these microdinosaurs survived the global catastrophe in the Southern Hemisphere, where the atmosphere was less severely affected, and subsequently repopulated the globe. They may have accompanied India in its sprint across the nascent Indian Ocean in the early Cenozoic.
This era is held to be 'The Age of Mammals', in typically anthropocentric fashion. If science was a democracy I would vote for 'The Age of the Microdinosaurs'. Despite my aviophile prejudices, the mammals did get an awful lot done in a short period of time (geologically speaking). Tropical conditions prevailing in the earlier Cenozoic were replaced by more seasonal climates and ice ages, possibly a result of the Himalayan uplift. From humble, shrew-like beginnings the versatile mammals diversified into most habitats with the development of efficient reproductive and metabolic strategies, advanced dentition and attendant musculature, and most importantly, the mammalian brain. The competition between mammalian herbivores and predators may have fuelled the development of the brain in a sort of 'intelligence arms race', as well as encouraging the development of special senses and cooperative social behaviour.
Palaeos com Phanerozoic
credits: content by MAK and ATW Palaeos com; most recent update ATW030218, checked ATW050726, transferred Palaeos org and fiddled with MM060920