Fossils are found in the Sedimentary Rock layer which is the result of the deposition of mud, silt, sand etc then compressed by the extremely high pressures caused by the increasing density of the layer, as more and more sediment built up over millions of years. (see my page on The Earth's Crust). These fossils are found by Paleontologists in exposed areas of sedimentary rock which may be found along river banks, or cliff faces

Fossil Hunting

as depicted above, or where uncovered by man, as the result of the excavation of quarries, road building, or as the result of the preparation of large builing developments etc.

Relative time ("chronostratic") -- subdivisions of the Earth's geology in a specific order based upon relative age relationships (most commonly, vertical/stratigraphic position). These subdivisions are given names, most of which can be recognized globally, usually on the basis of fossils.

Absolute time ("chronometric") -- numerical ages in "millions of years" or some other measurement. These are most commonly obtained via radiometric dating methods performed on appropriate rock types.
Think of relative time as physical subdivisions of the rock found in the Earth's stratigraphy, and absolute time as the measurements taken upon those to determine the actual time which has expired. Absolute time measurements can be used to calibrate the relative time scale, producing an integrated geologic or "geochronologic" time scale.

The geologic time scale is mainly the result of work done in Europe during the eighteenth and nineteenth centuries. The history of the Earth was sub-divided into eons, then, eras, periods, and epochs, as shown in the above table. The names of these sub-divisions may look a bit daunting to the uninitiated, but geologists know that paleo means ancient, meso means middle, ceno means recent, and zoic means animal life,

Therefore Paleozoic is the oldest, then comes Mesozoic, and finally Cenozoic the latest era.

Dinosaurs

Solely based upon the discovery so far of complete skeletal remains, the smallest dinosaur was Compsognathus this dinosaur was 1m (3ft) long and an estimated weight of 2.5kg (6.5lb).

I have seen this small dinosaur of the jurassic period, described in many places as being "the size of a chicken". Although small, this dinosaur was a carnivore, and one fossil shows the remains of a lizard, it's last meal, in the stomach area.

Also of the same jurassic period this dinosaur as so often found in large animals, was a herbivore. With it's head 15m above ground level, and it's close cropping teeth, it was able to graze conifer leaves and fruit, well above the reach of any other species.

Dating of fossils

I acknowledge that there is not one single valid scientific method of determining the date of a fossil, but I will not enter into the arguments of the accuracy or otherwise, of the various methods of dating, other than to point out that an error of 1 percent, on age determination of 100 million years, would give a million year error up or down.

You may find that the radiometric dating methods with half-life and parent, daughter references a little difficult to get your head around. I offer this analogy, in the hope that it may make it a little easier for you to understand the basic theory:

Imagine filling a glass with lemonade. You watch the bubbles fizz (no puns please) to the top and after some time they decrease in number, until they almost stop. Now if you could count those bubbles at the beginning, and at timed stages until the end, you would be able to calculate how long ago a similar glass of lemonade had been poured, simply by counting the bubbles in the glass. Replace the bubble count, with a radioactivity count, and you have it.

As an example, let us take Carbon-14 . Almost everyone has heard of 'Carbon dating' from films, TV or reports of some artifact having been aged by that method. The half-life of carbon-14 is 5,568 years. This means that half of the bubbles - sorry, C-14 decays (into nitrogen-14) in 5,568 years. Half of the remaining C-14 decays in the next 5,568 years, and so on. This method is quite ok for dating ice-age animals, neanderthal man, organisms etc, but when it comes to dating dinosaur fossils, this is no good, as we need to go back further than the 50,000 year limit of C-14. Potassium-Argon and Rubidium-Strontium have isotopes with slower decay rates (longer half-lives). These isotopes are rarely found in fossils themselves, but they do occur in the surrounding rock strata. Scientists are therefore able to arrive at a reasonably accurate dating, by the 'above and below' method. That is, they date the igneous layer immediately below and above the fossil, giving a time span of the dinosaur's life.

Scientists are well aware of the others factors that can cause false readings, so they always try to use more than one method in an attempt to increase the accuracy of the dating. One very useful aid is the existence of index fossils. These are fossils with a short, well defined life span, and are widely distributed throughout the world.