ICHNO FOSSILS

Ichno-fossils are an expression of the alteration of the depositional fabric in sedimentary rocks by living organisms. Often these expressions are part of animal behavior or traces hence called “trace fossils”. Common structures are burrows, boring, trails and tracks. Trace fossils can be common in un-fossiliferous rocks, that can extend the geologists capability to interpret the depositional setting of those rocks.

Assemblages of trace fossils used to determine ancient depositional settings and sedimentary facies are known as “ichnofacies”.

• Most trace fossils have long temporal range – they are not useful as indicators of age
• Most trace fossils are largely facies dependent – they are very useful as indicator of depositional environment
• Biogenic sedimentary structures, where preserved intact, are closely related to the environment in which they formed – they are not transported as some body fossils can be
• Trace fossils may be common in rocks that otherwise are unfossiliferous
• The causative organisms are typically not preserved
• The same individual may produce different structures corresponding with identical behavior but in different substrate – interpreting environments from trace fossils can be difficult
• Identical structures may be produced by the activity of systematically different trace-making organisms where behavior is similar – similar ichnofacies can be formed by organisms from different periods that have similar behaviors.

Some common ichnofossils and their behavior

Domichina	Dwelling
Fodinichnia	Feeders
Pascichnia	Grazers
Cubichnia	Resting
Repichnia	Creeping and Crawling
Agrichnia	Gardening
Fugichnia	Escape

Seilacher’s Concept of Recurring Ichnofacies

Trace fossils are a manifestation of behaviour which can be modified by the environment. The distribution and behaviour of benthic organisms is limited by a number of interrelated ecological controls, including:

1. Sedimentation rate
2. Substrate Coherence
3. Salinity
4. Oxygen level
5. Turbidity
6. Light
7. Temperature
8. Water Energy