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age or period dates notes and main features    
Precambrian era comprising the Archaean and Proterozoic eons 4600 - 595 mya Origins of life, oxygen atmosphere forms.  
Archaean eon (Archean eon), also called Archaeozoic or Archeozoic 4600 - 2500 mya Early part of the Precambrian, stromatolites formed, the name means "ancient life" or "archaic life".  
Proterozoic eon 2500 - 595 mya Ediacaran fauna is from this time: the name means "first life".  
Phanerozoic time 595 mya - present All ages since the Precambrian, once thought to be the only period with fossils.  
Palaeozoic era (Paleozoic era) comprising 595 - 250 mya The name means "old life", the dates, using other criteria, can also be found as 570 - 245 mya (million years ago).
Lower Palaeozoic era 595 - 408 mya Lower here in the sense of lower in the ground than later rocks.  
Cambrian period 595 - 505 mya Many invertebrate groups with "hard" parts that fossilise well, named for Cambria (Roman name for Wales), where these rocks were first noted.
Ordovician period 505 - 438 mya Trilobites at their peak, all life marine, first fish appear (vertebrates), named after the Ordovices, a tribe who once lived in north Wales.
Silurian period 435 - 408 mya True jawed fish, first land plants, scorpions the first air-breathing animals, named after the Silures, a tribe who inhabited the part of Wales where these rocks are found (among other places).
Upper Palaeozoic era 408 - 250 mya Upper here reminding us that these are the top beds of this sequence.  
Devonian period 408 - 360 mya First amphibians, first forests, named for Devon (southern England) where rocks of this age occur.  
Carboniferous period (Divided in north America into the next two groups) 360 - 286 mya Coral reefs form limestone, swamps with conifers and ferns form most modern coal, reptiles emerge. Named for the carbon of the coal deposits.
Mississippian period (in the USA) 360 - 330 (?) The lower part of the Carboniferous in north America: large primitive trees, flooding of N. America, limestone laid down: uplift and folding of the Appalachian mountains across the Mississippian and Pennsylvanian, named for the rocks of the Mississippi Valley in the USA.
Pennsylvanian period (in the USA) 330 (?) - 286 mya The upper part of the Carboniferous in north America: abundant insects, first reptiles, modern USA had a tropical climate, widespread swamps, providing today's US coal sources, named for the rocks of Pennsylvania, USA.
Permian period 286 - 250 mya Many invertebrate groups (including trilobites) go extinct, mammal-like reptiles appear, reptiles diversify, named after the Perm province of Russia.
Mesozoic era comprising 250 - 65 mya Middle life period, may also be dated 245 - 66 mya by other authors.  
Triassic period 250 - 213 mya First dinosaurs, small mammals, named after a threefold division of rocks of this age in Germany.  
Jurassic period 213 - 144 mya Large reptiles, Archaeopteryx, first birds, primitive mammals, named for the Jura mountains of Switzerland.  
Cretaceous period 144 - 65 mya Dinosaurs dominate and then go extinct, flowering plants appear. Named for the chalk deposits of France and England.
Cenozoic era comprising 65 mya - present Mammals and flowering plants become dominant. The name means "recent life".  
Tertiary period comprising 65 - 2 mya The third major period, when the world's major mountain ranges developed: the old terms "primary" and "secondary" are no longer used.
Palaeocene epoch   65 - 55 mya The name means "ancient recent"(these next few names came from the similarities of fossil molluscs to modern species!).
Eocene epoch 55 - 37 mya The name means "dawn recent".  
Oligocene epoch 37 - 25 mya The name means "few recent" (molluscs).  
Miocene epoch 25 - 5 mya Alps and the Himalayas formed, the name means "lesser recent".  
Pliocene epoch 5 - 2 mya The name means "more recent", and Australopithecines are found, right through it.  
Quaternary period comprising 2 mya - present Modern humans arise around this time, most probably with genus Homo at 2.5 mya and humans like us in about the last 100 000 years.
Pleistocene epoch 2 mya - 10 kya The name means "most recent".  
Holocene epoch 10 kya - present The name means "whole recent" (that is, all fossil molluscs from this time are still known as modern species).