Ichnofossils and Trace Fossils

• Ichnofossils are biogenic sedimentary structures recording biological activities such as burrowing, crawling, boring, and feeding
• Trace fossils are typically preserved in situ, providing a reliable indicator of the depositional environment
• The name given to a trace fossil is not always connected with the specific animal that made it

Ichnological Principles

• Similar ichnotaxa are found in both ancient and modern environments
• Certain traces are closely associated with specific substrates or water depths
• Traces are integral to sedimentary rock fabric and are destroyed by erosion, not reworked like body fossils
• Ichnofossils are helpful in studying environments that are hostile or poorly populated
• Trace fossils can provide the only information about organisms and their behavior otherwise absent in the sedimentary record

Stress Factors

• The intensity of bioturbation reflects colonization duration and relates to bottom-water conditions, salinity, substrate, energy, deposition rate, and geological events
• Oxygen is essential for metazoan life on the sea floor
• Fully aerobic biofacies have high water turbulence, deep bioturbation, diverse endofauna, and well-preserved carbonate shells
• In dysoxic conditions, suspension feeders and large benthos are eliminated, and sediments have a lower diversity fauna

Oxygenation Levels

• No ichnia environments have primary lamination preserved
• Fodinichnia indicate traces of non-vagile deposit feeders.
• Pascichnia indicate deposit feeders or chemosymbiosis
• Domichnia indicate full oxygenated pore and bottom waters, dominated by Skolithos

Sedimentation Rates

• High background sedimentation smothers the benthic community and stops bioturbation
• Rapid sedimentation from turbidite beds kills the benthic community and prevents bioturbation

Ethological Classification

• Cubichnia are resting or hiding traces formed during a temporary pause in locomotion
• Domichnia are dwelling traces formed by burrowing or boring into the substrate
• Fugichnia are escape structures formed when an animal moves rapidly upward to prevent burial
• Equilibrichnia are adjustment traces formed by infaunal animals maintaining their position relative to the sea floor
• Repichnia are locomotion traces representing directed movement from A to B
• Pascichnia traces formed by systematic exploitation of an area of sediment for food
• Agrichnia includes traps and farming traces formed by animals that trap food particles or grow algae
• Fodinichnia traces represent deposit feeding and dwelling

Biostratigraphy

• Trace fossils do not significantly evolve through time so they are not used in biozonation
• The Precambrian-Cambrian boundary is marked by a diversity and complexity increase in trace fossil assemblages
• The Proterozoic-Cambrian boundary is defined by the first appearance of complex arthropod and worm feeding traces

Phanerozoic Evolution

• An increase in diversity at the end of the Ordovician associated with the radiation of metazoans
• The depth of infaunal burrowing increases in the early Carboniferous
• Onshore traces tend to move into the offshore environment

Ichnofacies

• The main control on the marine trace-fossil assemblage is bathymetry (water depth)
• A particular lithofacies relates to the bathymetric gradient from shallow to deep

Softground Facies

• Skolithos ichnofacies indicates deposition with moderate to high-energy wave conditions
• Ichnogener are commonly orientated vertically, are formed by infaunal filter/suspension-feeder organisms
• Nereites ichnofacies are characteristic of abyssal, low-energy, oxygenated marine environments subject to periodic turbidity flows

Cruziana Ichnofacies

• Associated with moderate to low energy, mainly unaffected by storms
• Characterized by a high diversity trace-fossil assemblage with horizontally oriented burrows

Zoophycos ichnofacies

• It is preserved in the bathyal zone, in quiet water conditions
• This is characterized by a low diversity trace-fossil assemblage dominated by grazing traces and shallow feeding structures

Hardground Ichnofacies

• Glossifungites ichnofacies develops in firm but unlithified carbonate or siliciclastic substrates
• The Teredolites ichnofacies is characteristic of borings in mostly marine or marginal marine woody substrates