Fossils

The study of fossils, known as palaeontology, is key to the understanding of the Earth's history. It is an important branch of geology, as well as one of the most accessible. The work of amateur fossil collectors even today vastly extends the body of knowledge and understanding in a way that is not possible for the more esoteric branches of the Earth's history. Anyone with a hammer and some background knowledge - and respect for the countryside and law - can make discoveries that are useful and expand our knowledge.

The following discussion covers:

• Why study fossils?
• Preservation - How fossils are formed
• Fossil Classifications
• Living and Feeding habits

Why Study Fossils?

The study of fossils can lead to a greater understanding of the Earth's History, and of the life that has lived, and does live, in and on it. Palaeonotology is of particular use in three specific areas:

• Inference of patterns of evolution.
  The findings of palaeontologists have a large and increasingly-important part in zoological and botanical studies, and aid the mapping of a "tree of life". This historical background ('palaeobiology') gives present-day biologists a framework in and on which to work and extend their knowledge. Fossils provide the only direct evidence of life in the past, and are used in the interpretation of anatomical features and for studies of relationships of living organisms.
• Inference of past environments.
  The pattern of evolution with environmental change aids the reconstruction of past environments - and possibly indicates the future as our climate changes. From a more commercial aspect, the fossils that lead to suppositions of past environments, also give an indication of prospective sites in out endless search for minerals, particularly the fossil fuels of coal, oil and gas.
• Earth Age.
  Fossils are also used as an indication of the age of rocks in which they occur, and to provide correlations between rocks of the same age over wide geographical areas - the science of biostratigraphy. Furthermore, they help in the reconstruction of the past arrangement of the continents - palaeogeography - by the correlation of land-based organisms on now-separate continents.

First estimates of rock ages were based solely upon their contained fossils; and were all relative. (Other methods now exist, which are discussed within the Stratigraphy section. )

Fossils suitable for dating rocks must have wide geographical ranges, and must change - evolve - relatively quickly with time.
Ammonites and other molluscs, brachiopods, trilobites and echinoderms fit these 2 requirements, and are particularly useful for stratigraphy.

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Preservation - How fossils are formed

Although most fossils are those of the hard parts of organisms, fossils of soft-bodied organisms are known. Even jellyfish may be fossilised in very special conditions; bit out knowledge of soft-bodied animals is, by necessity, far far less that that of hard-bodied organisms.
Thus, the "Phanerozoic" - the era of visible life - begins at the dawn of the evolution of shelled animals. More recently, evidence of earlier life has been found, but the name of the era is now set in stone (or rock.)

Even the hardest parts of an organism will be broken down or dispersed if they are exposed to scavenging animals, bacterial action or the weather; so for fossilisation to occur, it is essential that these factors be excluded. Rapid burial shortly after the death of an organism is usually a pre-requisite of preservation, whilst a sudden mudslide can lead to "exceptional preservation" - like that of the famous [??] Burgess Shale.

The medium in which the fossils are buried varies (from sand to mud to volcanic ash - and chalk, of course) and this 'substrate' [is this the correct word? Define as tool tip] equally affects the preservation chances of the organisms as fossils.

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Fossil Classifications

Fossils may be categorised in a number of ways.

• A body fossil, (the form of an organism) or
• a trace fossil (the remains of the activities of a fossil - its burrows or borings, bite marks or droppings, nests or eggs, or simply a track of foot prints.)

• an internal mould (formed by infilling of a shell with a fluid sediment of some form, that then lithified to form a mould) or
• an external mould (where the original organism was surrounded by sediment that lithified, and retained the shape when the interior was dissolved. The external mould may thus surround an internal mould, or it may have been infilled with sediment that did not itself form a fossil.)

• original organic material impregnated with chemicals from the surrounding rocks, or be a
• replacement fossil, composed of a different chemical that has taken the form of the original organism.

• a microfossil (best studied under a microscope) or
• a macrofossil (studied with the naked eye, or just a hand lens)

• inaccessible or
• accessible (For the purposes of collection and study, fossils must be available at the surface. This usually implies that they have not only been buried (and lithified), but their surrounding rock must have been uplifted and eroded. There are more fossils within the Earth that we shall never know about - so accessibility is a major factor in the fossils we study)

• A true, stone formation of a past organism, in any of the forms described above,
• A pseudofossil - a rock that may appear to be a fossil, but is not.
  The basement of the Natural History Museum in London is stacked with shelves of such objects, once enthusiastically collected but now thought to be of no significance at all. However, due to regulations of ownership, they cannot be discarded.
• A modern fossil. Whilst the stone representations of life are usually very old, some can be modern. A relatively common fossil found on the north coast of Australia is that of Thalassina, which is a crustacean that lives in burrows on tidal mudflats. Like other crustaceans, it casts it shell in order to grow, and a single organism can produce twelve or more shells in a lifetime. Each shell is cast off at the bottom of the mud burrow, and can be fossilised in a very short time, perhaps less than a year. Some of the fossils are so young, that the animal that shed them may still be alive.
• A fake - history is full of dishonest claims as well as mistaken classifications.

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Living and Feeding

The study of fossils and fossil morphology can tell something of the living conditions and feeding habits of the creatures whose remains have been preserved.
The following is a brief summary of the terms used to describe these different habitats and feeding habits, when discussing the individual species.

Habitats

Epifauna

Live on the sea-floor

Infauna

Buried within the sediment

Sessile

Stay in one place

Vagrant

Frequently move around

Benthic

"Bottom dwellers", including both infaunal and epifaunal creatures.

Pelagic

Living above the sea floor, in the water column.

Planktonic

Pelagic animals, which float or swim at the mercy of currents and winds

Nektonic

Active swimmers

Feeding methods

Predators

Hunters and consumers of live animal prey

Scavengers

Consumers of dead and rotting animal material

Grazers

Those who scrape food (usually algae) from sediment or rock surfaces, or who graze on large plants

Suspension feeders

Those who strain food particles from seawater

Parasites

Those with a long-term attachment to a host, which serves as a food source but who gains nothing in return (unlike symbiotic creatures)

Deposit feeders

Those who eat sediment (swallowers) or select particulate grains (collectors)

For any comments, suggestions or contributions, please e-mail me at: portsdown@bbm.me.uk