Biology - 1. Fossils

Fossil

Preserved evidence of an organism that lived in the past.

General steps of fossil formation

An organism dies, is rapidly buried by sediment, remains are preserved, minerals replace or fill the remains, and the sediment becomes rock.

Conditions that favour fossilisation

Rapid burial, low oxygen, presence of hard body parts, sediment accumulation, mineral-rich groundwater, and limited disturbance.

Why hard body parts are more likely to fossilise

Bones, teeth, and shells resist decay and are more easily preserved than soft tissues.

Importance of rapid burial in fossilisation

It protects remains from scavengers, decomposition, and physical disturbance.

Reasons why the fossil record is incomplete

Preservation is rare, soft tissues usually decay, remains can be destroyed, and many specimens remain undiscovered.

Index fossil

A specimen used to determine the relative age of rock layers.

Characteristics of a good index fossil

Short geological time range, wide geographical distribution, high abundance, easy identification, and good preservation potential.

Transitional fossil

A specimen showing characteristics associated with both an ancestral and descendant group.

Example of a transitional fossil

Archaeopteryx, which possessed both bird-like and non-avian dinosaur characteristics.

How transitional fossils support evolution

They show combinations of traits that provide evidence of evolutionary change between groups.