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Handout 9 of 14 (Topic 3.3)

The Geological Time-scale

The Grand Canyon, eroded by the Colorado River, viewed from the South Rim (). The canyon is less than six million years old, but exposes Proterozoic, Palaeozoic and Mesozoic sedimentary strata.

The History of the Earth

The Geological Time-Scale

Key Ideas

Intended Student Learning

Fossil evidence was used to develop the geological time-scale.

Isotopic dating is a means of assigning absolute ages to rocks.

Explain the relationship between the fossil record and the eras in the geological time-scale.

Explain why the fossil record is inevitably incomplete, especially for organisms that lived more than 600 million years ago.

Identify each of the following fossils and explain its significance within the geological time-scale:

Ediacaran fauna Trilobites Dinosaurs Ammonites

Archaeocyatha Graptolites Mammals.

Explain, in terms of parent/daughter elements and half-life, the concept of radioactive decay.

Interpret decay curves.

Topic 3.3

The Geological Time-scale

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The Geological Time-Scale

This is the revised Geological Time-Scale and will be provided to you in the 2006 examination.

Eon Era

Period Neogene (previously Quaternary)

Old Tertiary-Quaternary boundary

Cainozoic

Phanerozoic Mesozoic

Palaeogene (previously Tertiary)

Cretaceous Jurassic Triassic Permian

Carboniferous Devonian Silurian Ordovician Cambrian Ediacaran

Palaeozoic

Epoch Holocene Pleistocene Pliocene Miocene Oligocene Eocene Palaeocene

Date at Boundary (Ma = million years ago)

0.01 1.5 5 24 35 55 65 145 210 250 300 350 400 440 500 540 600

Proterozoic

2500

Archaean

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The Geological Time-scale

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3.3 ? The Geological Time-Scale

THE PALAEONTOLOGICAL RECORD The history of life on Earth is recorded in rock strata which may be compared to the pages of a book. Unfortunately, the earliest pages of the book are illegible, and a number of other pages are missing...

Fossil Evidence The study of fossils (palaeontology), together with other geological and biological evidence, provides information on the history of Earth and the evolution of life. Fossils provide evidence about the relative ages of rock strata, palaeoenvironments and evolution of life. The fossil record has been used to develop the worldwide geological time-scale.

Organisms may leave traces of their existence in the sediments formed during, or shortly after their lifetimes.

FOSSILS are preserved remains of living organisms. (Minerals have replaced the carbon-based structure within the mould, or the organism has left an impression in the rock.)

They may be preserved in rocks formed from sediments that were deposited in a wide variety of environments.

TRACE FOSSILS are disruptions of sediments caused by the normal activities of animals.

Examples are footprints, feeding traces, worm burrows or coprolites (fossilised faeces).

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The Geological Time-scale

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Examples

are

footprints, feeding

traces,

worm

burrows

or

coprolites (fossilised

faeces).

Features of the Fossil Record Below is a list of some of the significant features of the fossil record as we know it today:

1. The oldest known fossils, of single-celled organisms, are from about 3800-3500 Ma.

2. Evolution proceeded very slowly at first. The oldest known fossils of multicellular organisms are the Ediacaran fauna (580 to 550 Ma) of the Flinders Ranges, South Australia.

The first organisms with hard parts evolved during the Cambrian era, when there was a 'sudden' increase in the number and diversity of living organisms -- the Cambrian Explosion

3. The rate of evolution has been ever-increasing. More organisms have evolved in the 60 million years of the Cainozoic era than in the whole of geological time before the beginning of the Cainozoic.

4. Evolution has not proceeded at a uniform rate. There have been intervals during which a large number of new life forms has evolved (e.g. the Cambrian Explosion), and periods of 'sudden' extinction of many life forms.

5. Life forms have evolved from the simple to more complex: from singlecelled organisms to humans.

Within a group of organisms, such as ammonites, the same tendency has been noted. The earliest members of the group to evolve were much simpler in form and structure than those that evolved later.

6. Increasing diversity -- from a few species of single-celled organisms to the enormous variety of life on earth today.

7. Organisms have succeeded each other in a sequence that is the same in all parts of the world.

e.g. Trilobite fossils are always older than ammonite fossils, no matter where these fossils are found.

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Once an organism disappears from the fossil record, it never reappears in younger strata. It has gone forever!

A group became which extinct, may have been replaced by another, more 'modern' group of organisms.

The Geological Time-scale

The ideas of superposition and evolution provide the basis of the geological time-scale, which was developed in a somewhat random fashion (mostly in Europe) during the 19th century. It is a worldwide scale developed by correlation of fossils from all around the world. It is based on the features of the fossil record discussed above.

The geological time-scale divides the history of Earth is divided into eons, eras, periods and epochs.

Eons are the largest intervals of geologic time. A single eon covers a period of several hundred million years. The history of Earth has been divided into three eons: Archaean, Proterozoic and Phanerozoic. Life began to evolve during the Archaean (at least by 3700 Ma), but multicellular organisms did not appear until about 580 Ma (in the middle of the Ediacaran period), some 30 million years before the end of the Proterozoic. Nearly all the evolution of life has occurred during the Phanerozoic (in which we live).

NB: The time between Earth's formation and the beginning of the Palaeozoic era (i.e. the Proterozoic and Archaean eons together) are often collectively called the Precambrian.

The Phanerozoic eon has therefore been divided into three eras -- the Palaeozoic (early life) , Mesozoic (middle life), and Cainozoic (recent life). A significant change in the dominant life forms marks the transition from one era to the next one. The changes that mark the transitions between eras are summarised in the table below.

Name of era Cainozoic (recent life)

Mesozoic (middle life)

Palaeozoic (ancient life)

Transition events

Ma

Extinction of dinosaurs and many other organisms

65

Extinction of over 90% of organisms, including trilobites

living

250

First appearance of organisms with hard parts i.e. the Cambrian Explosion

540

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The Cambrian period -- the first period of the Palaeozoic era -- is of major significance in the history of evolution. The earliest organisms with hard parts evolved during this era. In addition, so many organisms first appeared in the fossil record during this period that it has been called the time of the Cambrian Explosion.

All the eras named in the above table are divided into periods, which are generally named after the places in Europe where the rocks of that period were first studied. For example, rocks of the Jurassic age were first studied in the Jura Mountains, in Switzerland and southern Germany, and the Cambrian is named after the Cambrian Mountains in Wales.

The periods of the Cainozoic era are further subdivided into epochs.

An Incomplete Record

Fossils provide all the evidence we have about evolution of life on earth, but this record is far from complete. Special conditions are necessary if an organism is to be fossilised, rather than decay, after death. The everincreasing rate at which palaeontologists are finding fossils, of hitherto unknown organisms, indicates that fossil evidence of many life-forms has not yet been found -- and will never be found! Some reasons why the fossil record is inevitably incomplete are:

1. Most organisms either decay or are eaten by predators soon after death. Special conditions must exist for a dead organism to be preserved and become a fossil. Some of these conditions include: ? extreme cold -- woolly mammoths in Siberia. ? extreme dryness -- mummification in desert sand. ? anaerobic conditions (exclusion of oxygen). ? rapid burial in sediment deposited in water (hence fossils of marine organisms are significantly more common than those of terrestrial organisms). ? burial in volcanic ash (Pompeii), or tar (La Brea tar pits) ? covering of amber (i.e. the film Jurassic Park).

2. Even if an organism does become fossilised, it is highly probable that no palaeontologist will ever study it. It may remain buried, or it may become exposed at the surface in a remote and unexplored area.

The diagram below shows that only a very small proportion of the fossils present in an extensive layer of fossiliferous rock may be available for collection.

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Layer of fossiliferous rock

3. Organisms that lived more than 600 Ma (i.e. before the Cambrian Explosion) are even less likely to appear in the fossil record for the following reasons:

i. There were very few organisms, compared with today's numbers.

ii. During the Ediacaran period, organisms did not contain any hard parts, and it is usually hard parts -- such as bones and teeth -- which become fossilised.

iii. Any rocks in which these very ancient fossils were formed may have been eroded or metamorphosed in the intervening 600 Ma.

Significant Time-scale Fossils

1. Ediacaran Fauna In 1947, Reg Sprigg discovered the Ediacaran fossil assemblage, in the Ediacara Hills, on the western edge of the Flinders Ranges. When he reported his find to the ANZAAS conference, fellow geologists scorned him, and his fossils were described as `fortuitous markings on the rocks'. However, the assemblage is now accepted as being international significance. These fossils represent some of the earliest known multicellular organisms, which lived from about 580 to 560 Ma (and a small number beyond). All the fossils are of soft-bodied animals, many of which cannot be matched with living species. However, some resemble modern annelid worms, jellyfish and other Coelenterates. The reconstructed Ediacaran seafloor (below) shows some examples of the Ediacaran Fauna. The heights of the vertical leaf-like organisms are approximately 35 cm.

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The Geological Time-scale

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