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A trace fossil primer

Fossilized structures, which result from the activity of animals and plants in the sediment, are called trace fossils or sometimes ichnofossils. Main groups of trace fossils include burrows, tracks, trails, borings, superficial excavations, coprolites, and bite traces. The study of trace fossils and bioturbate textures, called ichnology, is traditionally subdivided into invertebrate and vertebrate ichnology, depending on the assumed trace makers.

For almost 200 years, trace fossils have been described with scientific names (ichnogenus and ichnospecies), lately following the International Code of Zoological Nomenclature (iczn.org). In the beginning, many trace fossils were described as plants (e.g. Chondrites), sponges (e.g. Spongeliomorpha), or corals (e.g. Rhizocorallium), because of their morphological similarities. Today, more than 600 invertebrate ichnogenera, and probably even more vertebrate ichnogenera, are known.

The producers of trace fossils often remain unknown or can only be assigned to higher taxonomical groups of organisms. Such interpretations are based on modern analogs, functional morphology, or experiments. Neoichnology, the study of modern traces, has shown that the same organism is capable of producing different traces — e.g. a crab can burrow into the substrate, create a superficial trackway, or leave faecal pellets. The same trace can result from the activity of different trace makers — e.g. the simple vertical shaft Skolithos, known to be produced by insects, spiders, crustaceans, worms, and even plant roots.

Ichnologists recognize groups of trace fossils according to the assumed behaviour of their producers (Seilacher 2007). This behavioural, or ethological, classification is interpretative and often remains uncertain, because the purpose of the producer can hardly be verified and may have been manifold. Ethological categories include domichnia (dwelling), fodinichnia (feeding), repichnia (locomotion), and pascichnia (grazing); there are many more. A more descriptive classification of trace fossils uses their characteristic morphological features (such as orientation, branching, lining, fill, symmetry, number of imprints, etc.) in a hierarchical manner (Knaust 2012).

Trace fossils and bioturbate textures have importance for palaeoenvironmental and sequence-stratigraphic interpretations, and are known to influence the quality of hydrocarbon reservoirs and aquifers (Buatois & Mángano 2011, Knaust & Bromley 2012, Knaust 2014). There are two complementing concepts in ichnology: ichnofacies and ichnofabrics. Ichnofacies are broader depositional environments — such as shores, shelf, continental slope, and deep sea — which are characterized by particular trace-fossil associations (e.g. dominantly vertical, horizontal, or spreiten burrows). Identifying specific ichnofacies can be useful in cases where little data is available (e.g. new exploration areas). Ichnofabrics, on the other hand, contain all ichnological components of a rock (e.g. the combination of trace fossils resulting from various colonization events) and reflect their relationship with the sedimentological features (such as bedding). Thus, ichnofabric analysis becomes useful in the detailed description and interpretation of bioturbated sediments and sedimentary rocks.

Trace fossils on a bedding plane. Scale bars = 1 cm. Left: Rhizocorallium commune Schmid, 1876, probably a sediment-feeding trace of a polychaete. Middle Triassic (Anisian) limestone (Muschelkalk Group), Weimar, Germany. Right: Chondrites bollensis (Zieten) Schimper, 1869, maybe formed by a chemosymbiotic worm-like organism. Lower Jurassic (Toarcian) black shale of Holzmaden, Germany.

Trace fossils on a bedding plane. Scale bars = 1 cm. Left: Rhizocorallium commune Schmid, 1876, probably a sediment-feeding trace of a polychaete. Middle Triassic (Anisian) limestone (Muschelkalk Group), Weimar, Germany. Right: Chondrites bollensis (Zieten) Schimper, 1869, maybe formed by a chemosymbiotic worm-like organism. Lower Jurassic (Toarcian) black shale of Holzmaden, Germany.


References

Buatois, L A and M Mángano (2011). Ichnology: Organism–Substrate Interactions in Space and Time. Cambridge University Press, Cambridge, 366 p.

Knaust, D (2012). Trace-fossil systematics. In: Knaust, D and Bromley, R G (eds), Trace Fossils as Indicators of Sedimentary Environments. Developments in Sedimentology 64, 79–101.

Knaust, D (2014). Classification of bioturbation-related reservoir quality in the Khuff Formation (Middle East): towards a genetic approach. In: Pöppelreiter, M C (ed.), Permo-Triassic Sequence of the Arabian Plate. EAGE, 247–267. 

Knaust, D and R Bromley (2012). Trace Fossils as Indicators of Sedimentary Environments. Developments in Sedimentology 64. Elsevier, Oxford, 960 p.

Seilacher, A (2007). Trace Fossil Analysis. Springer, Berlin, 226 p.

 

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