First molluscs made appearance in our world 600 million years ago and during Ordovician period, six of the seven classes of molluscs represented today was present. Many of these first molluscs were simple, worm like animals having segments. These first molluscs crawled primitive seas and eating small bits of foods. Because they have soft bodies they were vulnerable in the ocean filled with predators as they are easily eaten, early molluscs developed hard shells to protect themselves from predators they manage to survive because of having shell.

Changes in the environment led to change in shell structure; some grow large while other looses their shell completely, because of adaptation and protection that they want for survival.

They tend to evolve in freshwater and land rapidly today molluscs live in almost all parts of the world from the deepest ocean to high mountains. They have found their niche where they can survive, but for them to survive they have to have moisture because of their soft bodies which need moisture at all times. Those which live in desert environment they retain moisture by curling up in their shell, secreting mucous and stay hold up until next bit of moisture comes along.

Because mollusc they have soft bodies they usual produce an external skeleton, known as exoskeleton. It is formed by calcium carbonate (CaCO3) it is a hard part that cover on the back of molluscs its purpose is to protect and support molluscs. And all molluscs they have common feature which is present, the flesh mantle. Which secretes, modifies and lines the shell with mucous.

Different molluscs have different shell structure depending in the environment they are living in terms of predators, species that are found in the oceans have a dome shape shell that have eight piece plate which overlap other plate behind in a very tight way for protection .this protect it from violet serge of ocean waves this type of shell is found in the primitive molluscs polyplacophora.

Other is bilateral shell with a disk shape valve less than 25mm, this type of shell is thin and fragile. The first part develop a coiled chamber covered with horny periostracum or sheath which have scars inside and it form segmentation this type of shell was found in the class of monoplcophora.

Scaphoda have simplest shell structure than all molluscs it have a shell that resemble a tusk of an elephant, it shell is elongated cyndrical tube open at both end the shell usually heavily ribbed and has small slits at the narrow end with different colours ,it have a she

While other class of molluscs such as Aplocophora do not have shells and there is no fossils that indicate that they were having one. The flesh mantle that they have does not produce shell but it produces calcareous spicules which protect them from predators as it become less palatable.

In all molluscs gastropod underwent a very important change in the evolutionary part, influence the change in shell structure. Before its shell was symmetrical coiled like garden horse but because of changes it change to central axis were coils are laid down, in order to balance out weight of the shell.

In nutshell the importance of the shell in all molluscs is to give protection to the molluscs as they are vulnerable to predators and their body are soft in such away that it cannot stand the disturbance that can involve when it moves or the environment that they can find themselves in. all class of molluscs have different shell structure depending on the place and the protection they want from predators. Molluscs that live in terrestrial have soft shell that protect it from ocean wave, while those in inland have hard shell that protect it from radiation and predators.

References

1. Bourquin.A.2000. The Phylum Mollusc: [internet] [cited 2006-May-4] Available from: http://www.manandmollusc.net/advanced_introduction/advanced_intro_printable.html

2. Wikipedia contributors. Aplacopra [Internet]. Wikipedia, The Free Encyclopedia; 2006 April 20, 20:04 [cited 2006 May 4]. Available from:
http://en.wikipedia.org/wiki/Aplacophora

3. Wikipedia contributors. Polyplacophora [Internet]. Wikipedia, The Free Encyclopedia; 2006 May 04, 15:32 [cited 2006 May 4]. Available from:
http://en.wikipedia.org/wiki/Polyplacophora

4. Wikipedia contributors. Monoplcophora [Internet]. Wikipedia, The Free Encyclopedia; 2006 April 22, 20:03 [cited 2006 May 4]. Available from:
http://en.wikipedia.org/wiki/Monoplacophora

6. Wikipedia contributors. Scaphopoda [Internet]. Wikipedia, The Free Encyclopedia; 2006 May 04, 20:07 [cited 2006 May 4]. Available from:
http://en.wikipedia.org/wiki/Scaphopoda

7. Wikipedia contributors. Gastropoda [Internet]. Wikipedia, The Free Encyclopaedia; 2006 May 04, 20:07 [cited 2006 May 4]. Available from:
http://en.wikipedia.org/wiki/Gastropoda

Mr Elelwani Muanalo
NISL- Ecological Informatics Student
CSIR Pretoria
Tel: +27 12 841 2133
Fax: +27 12 842 7024.
Email: emuanalo@csir.co.za
My bloger URL: http://muanalo.blogspot.com/

Fossilisation can be referred as the set of inorganic processes which allow the remains of formerly living organisms to be preserved after its death and buried under sediment. It happen only in dead organisms, usually the hard parts can be preserved (bones, husk, shell, etc), because they can survive the alteration and disintegration or decomposition caused by atmosphere phenomena while Soft parts are rare to be fossilised this because they are vulnerable to predator and decomposition.

This process of preservation is influenced by natural processes (weathering, floods and volcanic eruption) which bury dead organisms in the surface. Some natural process cause death of the species and some of the process bury them in sediments. Volcanic eruption can cause death to many species and it can bury them on its larva, while erosion erodes all the dead particles and deposited them with other material to form sediments. Weathering which cause breaking down of rocks bury dead organisms in the particles and form sediment or rock.

Environmental condition also plays a part in perseveration of fossils this occur in the maintenance of the morphological structure of the species, if not altered by destructive agent they can keep their morphological structure. Fossilisation that happen in terrestrial environment like marine or fluvial environment are well preserved this because depositing agents act more quickly in marine environments than degrading ones. Morphological structure can be disturbed by physical changes that may involve, like flattened or distorted as sediments is hardened into rock.

Factor that influence the preservation of fossils are:

Carbonization which is known for plants where the organic matter are leached out and reduced to carbon film and the rigid organic walls are the one preserved
Sedimentation; most organisms are preserved in this process as the organism buried in mud where some of the remaining are replaced by sulphide or phosphate mineral.
Recrystalisation; this method of preservation destroy microscopic details of the shell but does not change the overall shape, so it can keep morphological structure of the species.

All this methods of preservation helps to keep evidence of unknown species that live in the past which were influential to evolution as it buries the remaining bones of dead species.

Importance of fossils in evolution is that it helps researchers Archaeologist to identify species that live in the past decades and its origin; they can be used to interpret the process of evolution by using the fossilised material.

The fossil of different species can identify the indigenous species in the area, and they also play a part in tourist attraction because some of people would like to see the indigenous creature of the past. Fossils play a part in interpreting evolution because the remaining bones or shell support the findings of the researcher and what was happening in the previous years about different species that evolve.

Reference:

1. Fossilisation; [Internet] [cited 2006-May-04] Available from: http://www.musei.unina.it/Paleontologia/eng/3.2.4.1.htm.
2. Fossilisation; [internet] [cited 2006-May-03] Available from: http://www.museum.vic.gov.au/dinosaurs/sci-form.html.