Fossils and Fossilisation

Fossils are the recognisable remains or traces of activity of prehistoric life, typically defined as >10 000 years old. Pseudofossils are nonorganic objects that bear false resemblance to organism remains. The fossil record is strongly biased toward organisms with hard parts, such as mineralised skeletons of calcite, aragonite, phosphate, silica or refractory organic materials such as wood, that live in areas prone to pulses of sediment accumulation. Hence, preservation is not only particularly favoured in shallow offshore, storm-affected and marine environments but also to a lesser extent, in the deep sea, lakes and river point bars. Occasionally, rapid burial in anoxic setting coupled with early mineralisation leads to extraordinarily preserved fossil Lagerstatten. The study of fossil preservation – taphonomy – is subdivided into biostratinomy and fossil diagenesis. Biostratinomic processes affect potential fossil remains between death and final burial, including decay of organic parts, disarticulation, fragmentation, abrasion, bioerosion and dissolution. Fossil diagenesis constitutes processes that affect organic remains subsequent to burial such as dissolution, compaction and early and late mineralisation. Taphonomy reveals biases of the fossil record and also provides insights into depositional rates and processes. Key Concepts: Fossils are prehistoric (>10 000 years), discrete remains or traces of behaviour of once-living organisms. Preservation in the fossil record is a rare event that generally requires organisms with ‘hard parts’ (mineralised or resistant organic skeletons) and entombment within sediments. Extraordinary preservation of articulated skeletons and even soft parts requires very rapid burial, often associated with mass mortalities, in low oxygen sediments and various types of early diagenetic mineralisation. A variety of settings may favour exceptional preservation, including storm-influenced continental shelves, deeper marine environments and stagnant lagoons, freshwater lakes, including maars or volcanic lakes, caves, tar pits and permafrost. The study of fossil preservation, taphonomy, is subdivided into biostratinomy and fossil diagenesis. Biostratinomy comprises the study of all processes that affect potential fossils from the time of death, or production of a trace or sheddable structures (leaves, seed and exoskeletal parts), to final burial. Biostratinomic processes include decay of soft parts, infilling by disarticulation of bivalved or multielement skeletons, breakage, bioerosion, abrasion, transport and chemical corrosion. Fossil diagenesis includes compaction, early mineralisation of various sorts including infillings and/or replacements of pyrite, phosphate, and silica and late-stage mineralisation including overgrowth of earlier-formed minerals and major episodes of (sometimes selective) dissolution. A combination of biostratinomic and diagenetic characteristics can aid in identifying taphonomically defined assemblages or taphofacies that may provide a ‘fingerprint’ of particular depositional environments. Keywords: taphonomy; preservation; fossils; depositional environment; biostratinomy; burial; diagenesis; time-averaging; palaeoecology