Types of Fossils ! Biological Sciencess !!

Fossils

 Any body, body parts (traces) of animals and vegetables buried and preserved by natural causes. Fossils are remains, traces or other direct evidence of past life forms.

Most fossils form from burial of plants and animals in sediment; soft parts are more often consumed or decomposed but may leave imprints if buried rapidly. Most fossils are embedded in sedimentary rock,
weathered particles that provide strata from lower older layers to upper newer layers.

Paleontologists study the fossil record based on boundaries between strata, where one mix of fossils gives way to another. Transitional links are intermediate between major groups.

The fossil record allows us to trace the history of the modern-day horse Equus. The earliest fossils in this lineage is Hyracotherium , which was the size of a dog, with cusped low-crowned molars,
four toes on each front foot, three on each hind foot—all adaptations for forest living.

When forests were replaced by grasslands, the intermediates were selected for durable grinding teeth, speed, etc.with an increase in size and decrease in toes.

Types of fossils

(i) Body fossils: hard part of organism such as shell, tooth, bone etc.e.g. bones of
dinosaurs.

(ii) Subfossils: Remains of animals and plants preserved in rocks less than 10,000 years e.g. vision in frozen ice. Sub fossils were formed after the last ice age during Holocene epoch.

(iii) Microfossils: microscopic remains less than 0.5 mm or 1/50th inch.

(iv) Macrofossils: Larger than 1 cm in size.

(v) Unusual fossils: Sudden preservation of entire organism e.g. Solenhofen Limestone of Southern Germany – containing fossils Archaeopteryx.

(vi) Coprolities: fossils of droppings of animal faecal matter. Large coprolities of
crocodiles, Dinosaurs etc.

(vii) Bioclast: Fossils of fragments of fossils enclosed in sediments. The term is usually
applied to thin sections of fossils under microscope.

(viii) Gastroliths: These are found in abundance in the body cavities of certain reptiles.

(ix) Pseudofossils: Many objects of in organic origin closely resemble the forms of organic origin and are found in sedimentary rock.

Examples

• Preservation in ice: Woolly mammoths from Siberia. The flesh is so well preserved
that it can be fed to dogs several thousand years. Discovered from Lena Delta in 1790
and Siberia in 1901.

• Fossils in petroleum springs and asphalts: Rancho La Brea now in Los Angeles.

• Fossils in resins and ambers: e.g. Fossil fly in amber from Baltic forests of Europe
during Oligocene period.

• Pterification of hard parts: Fossils are found in sedimentary rocks. Soft parts
disintegrate leaving the fossil porous. Water seeps into fossil and replaces the hard
part particle by silica or iron pyrite. The process of so gradual and slow that even the outlines of cellular structure are preserved.

• This process of fossilization to preserve the finer details is known as histometabasis.

• Moulds and casts: The material surrounding the fossil hardens and preserves the outer details. The actual bodies disintegrate and are removed by slippage of the ground leaving the harden cavities called moulds. When moulds are filled with natural deposits. They are called as casts e.g. fossils of Pompeii city buried in volcanic ash of mount Vesuvious in A.D.79.

• Impressions: Impressions of leaves of plants, feathers of extinct birds, wing
membranes of flying reptiles, skin of dinosaur.

• Tracks and trails: The footprints or tracks left in the soft moist mud gets hardened
up e.g. tracks of amphibians discovered near Pittsburg, Germany from Pennsylvanian period.

• Mummies: Bodies of dead animals or plants become dehydrated in the deserts and are preserved as mummies.

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