Geological Eons, Eras, Periods and Epochs, and How Fossils Are Used

Chart of major geological eons, eras, periods, epochs and events. How fossils are used by scientists to determine its relationship with other specimens, with objective to un-ravel evolutionary patterns and origins.

Index fossils are sometimes used by Geologists to determine stratigraphic data about Earth's surface. This is true in the case of spores and pollen grains, the reproductive materials found in plants. By doing so provides the palyntologist means to determine relative age and position of rock, containing the spores. When studying drill cores, this kind of information can be useful to miners searching for fossil fuels.

Palynologists have used index fossils for practical applications, including understanding relationship between major groups of plants, specifically, gymnosperms and angiosperms.
A good index fossil is considered to be one that is easily identifiable, with wide horizontal distribution, and vertical range of approximately one million years. Traditionally, due to scarcity and difficulty in identification, plant megafossils were rarely used as index fossils. Though possessing wide geographical distribution in various sedimentary rocks, the vertical range spans millions of years. Assemblages of megafossils used as indices (or, indexes), accompanied by palynological information, scientists have been able to characterize restrictive stratigraphic units in rock units containing megafossils.

EON	ERA	PERIOD		EPOCH	Characteristic organisms and major geological events		Duration in millions of years	Began (m.y.a.)
	C E N O Z O I C E R A	QUARTERNARY PERIOD		HOLOCENE EPOCH			Last 5,000 years	0-10,000 years ago
				PLEISTO- CENE EPOCH			2.5 million years	10,000 to 1.6 million years ago
		TERTIARY PERIOD Late Tertiary Middle Tertiary Early Tertiary KT Times		PLIOCENE EPOCH			4.5 million years	2.5-7 million years ago
				MIOCENE EPOCH			19 million years	7-26 million years ago
				OLIGOCENE EPOCH			12 million years	26-38 million years ago
				EOCENE EPOCH			16 million years duration	38-54 million years ago
				PALEOCENE EPOCH			11 million years duration	54-65 million years ago
	M E S O Z O I C E R A	CRETACEOUS PERIOD Later Cretaceous Early Cretaceous		UPPER Maastrichtian Campanian Santonian Coniacian Turonian Cenomanian		76 million years duration		65-141 million years ago
				LOWER Albian Aptian Barremian Hauterivian Valanginian Berriasian
		JURASSIC PERIOD		UPPER Tithonian Kimmeridgian Oxfordian		54 million years duration		141-195 million years ago
				MIDDLE Callovian Bathonian Bajocian Aalenian
				LOWER Toarcian Pliensbachian Sinemurian Hettangian
		TRIASSIC PERIOD		UPPER Rhaetian Norian Carnian		30 million years duration		208-245 million years ago
				MIDDLE Ladinian Anisian
				LOWER (Scythian) Olenekian Induan
	P A L E O Z O I C E R A	PERMIAN PERIOD		UPPER Lopingian - Changhsingian - Wuchiapingian MIDDLE Guadalupian - Capitanian - Wordian - Roadian		55 million years duration		245-280 million years ago
				LOWER Cisuralian - Kungurian - Artinskian - Sakmarian - Asselian
		C A R B O N I F E R O U S	PENNSYLVANIAN	UPPER Kasimovian | Gzhelian		45 million years duration		280-325 million years ago
				MIDDLE Moscovian
				LOWER Bashkirian
			MISSISSIPPIAN Carboniferous	UPPER Serpukhovian MIDDLE Vis?an		20 million years duration		286-360 million years ago
				LOWER Tournaisian
		DEVONIAN PERIOD		UPPER Fammenian | Fransnian		50 million years duration		360-408 million years ago
				MIDDLE Givetian | Eifelian
				LOWER Praghian | Lockhovian
		SILURIAN PERIOD		UPPER Pridolian | Ludlovian		40 million years duration		408-438 million years ago
				LOWER Wenlockian | Llandoverian
		ORDOVICIAN PERIOD		UPPER Ashgillian | Caradocian MIDDLE Llandeilian | Llanvirnian		65 million years duration		438-505 million years ago
				LOWER Arenigian | Tremadocian
		CAMBRIAN PERIOD Late Cambrian Early Cambrian		UPPER Furongian		70 million years duration		505-540 million years ago
				MIDDLE * table below
				LOWER * table below
	P R O T E R O Z O I C E R A	VENDIAN PERIOD				--		540-650 million years ago
		PRE-VENDIAN PERIOD						650-2,500 million years ago
	ARCHAEAN 3.8 to 2.5 billion years ago The atmosphere of Earth was very different from what we breathe today; at that time, it was likely a reducing atmosphere of methane, ammonia, and other toxic gases. During this time, the Earth's crust cooled enough that rocks and continental plates began to form. Early in the Archaean that life first appeared on Earth. Our oldest fossils date to roughly 3.5 billion years ago, and consist of bacteria microfossils. All life during the more than one billion years of the Archaean was bacterial, stromatolites, colonies of photosynthetic bacteria which have been found as fossils in Early Archaean rocks of South Africa and Western Australia. Stromatolites increased in abundance throughout the Archaean, but began to decline during the Proterozoic.							2,500-3800 million years ago
	HADEAN 4.5 to 3.8 billion years ago Hadean time is not a geological period as such. During Hadean time, the Solar System was forming. Sometime during the first 800 million or so years of its history, the surface of the Earth changed from liquid to solid. Once solid rock formed on the Earth, its geological history began. This most likely happened prior to 3.8 billion years, but hard evidence for this is lacking. Erosion and plate tectonics has probably destroyed all of the solid rocks that were older than 3.8 billion years.							3,800-4,600 million years ago

CAMBRIAN SUBDIVISIONS
Cambrian is divided into three epochs ? the Early Cambrian (Lower Cambrian, Caerfai or Waucoban), Middle Cambrian (St Davids or Albertian) and Furongian (a.k.a. Late/Upper Cambrian, Merioneth or Croixan). Each of the epochs are divided into two faunal stages. Only one, Paibian has been recognized by the International Commission on Stratigraphy. Others are still unnamed. However, the Cambrian is divided into several regional faunal stages.
Source: Answers.com - Cambrian Subdivisions

	Chinese	North American	Russian-Kazakhian	Australian	Regional
Furongian		Ibexian	Ayusokkanian	Idamean	Dolgellian
		Sunwaptan	Sakian	Mindyallan	Festiniogian
		Steptoan	Aksayan	Payntonian	Maentwrogian
		Marjuman	Batyrbayan
Middle Cambrian	Maozhangian		Mayan	Boomerangian
	Zuzhuangian	Delamaran	Amgan	Undillian
	Zhungxian			Florian
				Templetonian
		Dyeran		Ordian
Early Cambrian	Longwangmioan		Toyonian		Lenian
	Changlangpuan	Montezuman	Botomian
	Qungzusian		Atdabanian
	Meishuchuan		Tommotian
			Nemakit-Daldynian

Extant organisms in their structure and distribution, reflect the composition of their environments. We assume extinct organisms also adapted to their environment in the same way. If this assumption is true, then it is possible to determine seasonal variations using growth rings from petrified wood, including paleo-environmental availability of water and temperature changes. Fossilized wood which reflects lack of growth rings, indicate a continuous supply of water and uniform temperature, just as thickened cuticles and sunken stomata of fossilized leaves indicate a lack of water, while roots and spongy stem tissue suggest a swampy or aquatic paleo-environment. With such information extracted from morphology and anatomy of fossil plants, provides in part, the basis for paleoecology and paleoclimatology. Further studies are taken into consideration, such as those on sedimentary materials which naturally occur with the fossils and, how the fossil became preserved, all play into better understanding the paleoenvironment. Specialists caution however, conclusions should never be based entirely upon extant organisms and how they interact with present environments. What we observe occuring today, is not necessarily the key to the past. (Paleobotany and the Evolution of Plants, Stewart and Rothwell, 1993; DiMichele & Wing, 1987.)

Paleofloristics, which specializes in assemblages of fossil plants, provides insight into Earth's restricted and widescale climate. On the worldwide scale, such studies have provided scientists with insight into plate tectonics and continental drift. These studies can also provide further insight into plant distribution, population, migration and significant changes in early environments. Studies on succession of plants in the geological column have became popular among paleoecologists, and also useful in studies on extant organisms and plant successions in natural history.

Successional changes in organisms throughout natural history are the basis of studies related to the evolution of life. Most paleontologists focus more on relative stratigraphic positioning of fossils, than absolute ages. It remains more important to most in the field, to determine how a fossil relates to other specimen with objective to un-ravel evolutionary patterns and origins.

REFERENCES
Bringing Fossils to Life, An Introduction to Paleobiology, McGraw Hill Publishers, Donald R. Prothero

Paleobotany and the Evolution of Plants, by Cambridge University Press; 2 edition, Wilson N. Stewart, Gar W. Rothwell

Atlas of the Prehistoric World, by Discovery Channel Books, Douglas Palmer

Kingfisher Illustrated Dinosaur Encyclopedia, Kingfisher Publishers, David Burnie

FURTHER SUGGESTED READING
Adrienne Mayor's books
1) The First Fossil Hunters (Princeton 2000) explains how ancient Greek and Roman discoveries of mysterious petrifed bones of extinct dinosaurs and mastodons led to myths about griffins, giants, and monsters. Watch for "Ancient Monster Hunters" on the History Channel.
2) Fossil Legends of the First Americans (Princeton 2005) gathers exciting Native American discoveries and myths about fossils, from tiny shells to enormous dinosaur bones, with stories from more than 45 different tribes, beginning with the Aztecs & Incas.

Stephen Meyer's article, "Are Dinosaurs Mentioned in the Bible?"