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Paleontology

 

Paleontology is the science which deals with studying life of past

geologic ages (fossils).

 

Paleo = ancient onto = life logy = science

 

Fossils: are remains or traces of organisms (animals and plants), which

inhabited the globe since the beginning of life.

 

 

Kinds of fossils:

* Real fossils: are the remnant of an extinct plant or animal.

* Derived (drifted) fossil: are fossils that are washed out from the original beds and re-deposited

in younger strata.

Example: Cretaceous and Eocene fossils deposited in the Miocene basins of the Gulf of Suez.

 

* Pseudo-fossils: are those covered by sediments in recent times and make the impression only of being fossils.

  

Nature of fossil record:

 All fossils should occur in sedimentary rocks being abundant in limestone and limy shale but rare in sandstone. Fossils never occur in igneous rocks except when volcanic ash falls or nearly cooled lava have overcome plants and animals. In Metamorphic rocks they are also absent except when these rocks were originally fossiliferous and subjected to very low grades of metamorphism.

In Nature fossils are found scattered in the rocks, in some cases they are accumulated in layers or patches. Those accumulated in layers or beds are called Biostroms whereas those accumulated without any distinctive layering are called Bioherms.

 

 Conditions of preservation:

 

1-possession of hard skeleton:

In order to be preserved as fossil, the organism must have a hard skeleton. The soft parts decay after death and only the hard parts are preserved.

 

2-Rapid burial:

After death, the organism should be directly covered with sediments to prevent its destruction by waves or winds. On land, rapid burial is not common and hence land organisms have little chance of preservation than marine organisms.

 

 

Chemical and mineralogical composition:

 The hard parts of vertebrates include bones (largely calcium phosphate and carbonate).

Invertebrate skeletons are mainly calcium carbonate (either calcite or aragonite), some skeletons are composed of silica (siliceous). The composition of major invertebrate groups is as follow:

 

Foraminifera: Calcareous (Ca CO3) or agglutinated (sand grains, sponge spicules or mica flakes).

Sponge: calcareous (CaCO3) and siliceous (silica).

Coelenterates: calcareous.

Bryozoa: calcareous.

Brachiopods: Calcareous and chitinophosphatic.

Mollusca: calcareous.

Echinoderms: calcareous.

 

 Modes of preservation:

After death, the organisms are preserved in different forms as follow:

I. Unaltered remains: the hard skeleton of the organism or its soft part or both remains unchanged.

  1. Soft part (organic compounds):

    2. 1- Mammoth: in the Pleistocene glaciers of Siberia.

    2-Insect in Amber: the insects are preserved in the resin (Amber) such as those found in the Oligocene deposits of Baltic province.

  2. Hard skeleton (inorganic compounds):

This is characteristic for Cenozoic shells which underwent little or no alteration of the original mineral substance.

 II. Altered remains: The soft parts decay and the hard skeletons are

completely altered. This takes the following forms:

 

  1. Carbonization: This is the removal of volatile constituents such as oxygen, hydrogen and nitrogen from the organic compound leaving only carbon as a thin black film.

    2. Ex. Graptolites, fishes and plants

  2. Recrystallization: is the alteration of less stable inorganic compounds (e.g. aragonite) into more stable ones (e.g. calcite) without any chemical change.

3- Permineralization: is the deposition of minerals in the interstices of skeleton.

Ex. Bone vertebra

4- Replacement: The original skeleton is removed and replaced by other mineral substances such as silica (silicification), pyrite, iron or carbonates.

Ex. Silicified wood (stone forests)

5-Imprints, casts and Moulds:

 Imprints: are impression made by thin objects such as fish.

Ex. Fish imprint

Cast: is the filling of cavities of shells by minerals or other sediments.

Mould: is the impression of skeletal remains on rocks. The impression may represent the external or internal surface of the organism.

 

6- Evidence of the activity: here we don’t have anything of the body fossil itself but only traces of its movement. This branch of paleontology is called Ichnology, which deals with traces of organisms.

 

Tracks: These are the traces of feet made by quadrepedal or bipedal

vertebrates during moving on soft sediments.

Trails: These are the traces made by animals during crawling on

sediments.

Burrows: are pathways made up by animals in soft sediments as a

normal way of life (worm burrows).

Borings: are holes made by animals in hard rocks and shells either for

protection or as parasites in search for food.

Excrements: these are called coprolites and they indicate the kind of

food, which the organism had eaten.

 

 

Environment

 

As mentioned before, fossils are mostly found in marine rocks rather than continental ones since they had a better chance of preservation.

The marine environment (sea or ocean) is divided into zones, each has its own physical and chemical characteristics.

 

  1. Littoral (Tidal) zone:

    2. This is the zone of water between highst and lowest tide. Living conditions are difficult because of alternate covering and exposure of the bottom materials and organisms due to tidal effect. In spite of that; some organisms adapt themselves to live in these conditions. These are mainly attached or burrowing organisms such as corals, worms, pelecypods, burrowing crustaceans together with lime-secreting algae.

     

  2. Neritic zone:

    3. It is the zone of water between lowest tide and 200 m depth (edge of the continental shelf).

    Organisms are abundant due to excellent light, oxygen and agitated water. This allows plants to grow and produce food by photosynthesis for animals living in this zone.

    The majority of invertebrate fossil assemblages appear to have flourished upon the bottoms of the neritic zone. Also much of invertebrate evolution is thought to have take place upon the continental shelves of ancient seas.

     

  3. Bathyal zone:

    4. It is the zone of water between 200 m and 4000 m.

    Only the upper part of this zone has some light and so there is little or no plant life.

    The inner part of this zone contains remains of neritic and even continental organisms transported by turbidity currents.

     

  4. Abyssal zone:

It is the zone of water between 4000 m and 5000 m.

Water is dark and cold, pressure is very great. There is no green plant life beside

little animal life.

 

Remains of pelagic organisms called oozes are dominantly accumulated on the bottom of this zone. There are two types of these oozes:

      1. Foraminiferal (Globigerina) ooze: This consists of complete or broken tests of planktonic foraminifera.

      2. Radiolarian Ooze: This consists of siliceous tests of Radiolaria beside other remains of siliceous composition such as diatoms.

 

  1. Hadal zone:

It is the zone of water below 5000 m depth (deep sea trenches).

 

 

Habit (mode of life) of marine organisms:

Organisms are classified according to their mode of life into three categories:

    1. Planktonic: these are organisms which have no organs of locomotion and their movement is controlled by waves and currents. Upon death they sink to sea floor or may be washed ashore.

      2. Ex. Diatoms, foraminifera, radiolaria and some ostracods.

    2. Nektonic: are organisms which swim in water by their possession of organs locomotion, so they can control theirmovement.

      3. Ex. Fishes and some mollusca.

       

    3. Benthonic: are organisms which inhabit sea bottom. They are two types:

        1. Epifaunal: living on sea bottom either sessile (fixed) or vagrant (free moving).

        2. Infaunal: living buried within the sediments.