Palaeos		Precambrian
Geological Timescale		Phanerozoic

The Phanerozoic Eon

The Eon of Multicellular Life

Introduction

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 - "visible" or "revealed life", or "evident 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/Ediacaran) 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.

The following table shows the three eras and eleven geological periods that comprise the Phanerozoic.  Like all geological tables this diagram has to be read from the bottom up; the lowest period in the table, the Cambrian, being the earliest.

Of the three main eras that make up the Phanerozoic, the Paleozoic is the longest and most diverse, spanning the period from very early multicellular life that only inhabited the oceans to quite advanced tetrapods* and reptiles and extensive forests on land.

Early Paleozoic: Age of Invertebrates
Coelomate radiation (Cambrian explosion) - origin of major groups of organisms; nervous system, behavior patterns and simple consciousness (the nascent Noosphere); continents drift apart.

Middle Paleozoic: Age of Fish
Tropical conditions.  Extinction of many "experimental" animal groups; diversification of surviving invertebrate groups, rise of vertebrates (fish).  Life moves on land (rhyniophytes, lycophytes, uniramous arthropods, and proto-amphibians).

Late Paleozoic: Age of Tetrapods* and Reptiles
Ice age.  Coal forests of giant lycopsids, calamites, pteridophytes and ferns cover the tropical landmasses.  Southern landmass of Gondwanaland buried under glaciers; continents drift together.  Reptiles conquer the land.

More on the Paleozoic

The Mesozoic has been called the "age of reptiles", but "age of dinosaurs" would be more appropriate.   There is still controversy over whether dinosaurs really were stupid sluggish ectotherms ("reptiles") or active high-metabolism (endotherm) creatures more like birds.   Even if we define them as "reptiles" the age of reptiles as such begins in the Permian period of the Paleozoic era anyway.

Tropical (Greenhouse) Conditions.  Pangaea continues during the early Triassic; then landmasses begin to drift apart.  Shallow oceans cover much of the continents, breaking the land into large islands.   Mammals remain small, possibly nocturnal.  Most modern groups of organisms appear. Vertebrate animals (mammals, birds, theropod dinosaurs) develop larger brains then their earlier reptilian ancestors.

More on the Mesozoic

Last of all, the Cenozoic - also spelt "Cainozoic" - is the age of mammals.  During this period, following the extinction of the dinosaurs, mammals evolved from small shrew-like types into all the diverse types around today, as well as many different prehistoric forms.

The modern world.  Land masses take their present shape.  "Intelligence race" as herbivores develop larger brains and carnivores do likewise.  The climate, originally tropical, becomes increasingly more seasonal as ice age conditions develop, possibly triggered by the rise of the Himalayan mountain uplift.

More on the Cenozoic

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