Trace fossil classification

Trace fossils are classified in various ways for different purposes. Traces can be classified taxonomically (by morphology), ethologically (by behavior), and toponomically, that is, according to their relationship to the surrounding sedimentary layers. Outside of special cases, phylogenetic classification of trace fossils is unsatisfactory because the makers of most trace fossils are unknown.

Taxonomic Classification of Trace Fossils

The taxonomic classification of trace fossils parallels the taxonomic classification of organisms under the International Code of Zoological Nomenclature. In trace fossil nomenclature a Latin binominal name is used, just as in animal and plant taxonomy, with a genus and specific epithet. When talking about trace fossils the genus is called an "ichnogenus", and the species is an "ichnospecies", the "ichno-" prefix coming from the Greek "ιχνος" (transliterated "ikhnos"), meaning "trace". The name is similarly italicized, and in full reference the author plus the year of publication should be cited. "Ichnogenus" and "ichnospecies" are commonly abbreviated as "igen." and "isp.", respectively.

Most researchers classify trace fossils only as far as the ichnogenus rank, based upon trace fossils that resemble each other in morphology but have subtle differences. Some authors have constructed detailed hierarchies up to "ichnosuperclass", recognizing such fine detail as to identify "ichnosuperorder" and "ichnoinfraclass" (e.g. [http://www.paleofile.com/Demo/Localities/Africademo/Algeria.htm] ), but such attempts are controversial.

The most promising cases of phylogenetic classification are those in which similar trace fossils show details complex enough to deduce the makers, such as bryozoan borings, large trilobite trace fossils such as Cruziana, and vertebrate footprints. However, most trace fossils lack sufficiently complex details to allow such classification.

Ethologic Classification of Trace Fossils

The Seilacherian System

Adolf Seilacher was the first to propose a broadly accepted ethological basis for trace fossil classification [Seilacher, A (1953) Studien zur paläontologie: 1. Über die methoden der palichnologie. Neues Jahrbuch fur Geologie und Paläontologie, Abhandlungen 96: 421-452.] [Seilacher, A (1964) Sedimentological classification and nomenclature of trace fossils. Sedimentology 3: 253-256.] . He recognized that most trace fossils are created by animals in one of five main behavioural activities, and named them accordingly:

* "Cubichnia" are the traces of organisms left on the surface of a soft sediment. This behaviour may simply be resting as in the case of a starfish, but might also evidence the hiding place of prey, or even the ambush position of a predator.

* "Domichnia" are dwelling structures that reflect the life positions of organisms, for example the subsurface burrows or borings of suspension feeders, and are perhaps the most common of the established ethological classes.

* "Fodinichnia" are feeding traces which are formed as a result of organisms disturbing the sediment in their search for food. They are normally created by deposit feeders as they tunnel through soft sediments, usually producing a 3D structure.

* "Pascichnia" are a different type of feeding trace for which the trophic guild responsible are grazers. They create 2D features as they scour the surface of a hard or soft substrate in order to obtain nutriment.

* "Repichnia" are locomotory tracks that show evidence of organisms moving from one station to another, usually in a near-straight to slightly curved line. Most of the very few traces to be verifiably assigned to a specific organism are in this category, such as various arthropod and vertebrate trackways [Seilacher, A (1967) Bathymetry of trace fossils. Marine Geology 5: 413-428.] .

Other Ethological Classes

Since the inception of behavioural categorization, several other ethological classes have been suggested and accepted, as follows:

* "Aedificichnia" [Bown, TM; Ratcliffe, BC (1988) The origin of "Chubutolithes" Ihering, ichnofossils from the Eocene and Oligocene of Chubut province, Argentina. Journal of Paleontology 62: 163-167.] : evidence of organisms building structures outside of the infaunal realm, such as termite mounds or wasp nests.

* "Agrichnia" [Ekdale, AA; Bromley, RG; Pemberton, SG (1984) Ichnology: Trace fossils in sedimentology and stratigraphy. Society of Economic Paleontologists and Mineralogists Short Course, no 15, 317 pp.] : so called "gardening traces", which are systematic burrow networks designed to capture migrating meiofauna or perhaps even to culture bacteria. The organism would have continually inspected this burrow system to prey on any smaller organisms that strayed into it.

* "Calichnia" [Genise, JF & Bown, TM (1991) New Miocene scarabaeid and hymenopterous nests and Early Miocene (Santacrucian) palaeoenvironments, Patagonian Argentina. Ichnos, 3: 107–117.] : structures that were created by organisms specifically for breeding purposes, e.g. bee cells.

* "Equilibrichnia" [Bromley, RG (1990) Trace fossils: biology and taphonomy. Unwin Hyman Ltd, London, 280 pp.] : burrows within the sediment that show evidence for organisms' responses to variations in sedimentation rate (i.e. the burrow moves upwards to avoid burial, or downwards to avoid exposure). Typically this evidence will be in the form of spreiten, which are small laminations in the sediment that reflect previous positions the organisms were in.

* "Fugichnia" [Simpson, S (1975) The morphological classification of trace fossils. In Frey, RW (ed.) The study of trace fossils. New York, Springer-Verlag, pp 39-54.] : "escape traces" that are formed as a result of organisms' attempts to escape burial in sudden high-sedimentation events like turbidity currents. The burrows are often marked with chevron patterns showing the upward direction the organisms were tunnelling.

* "Praedichnia" [Ekdale, AA (1985) Palaeoecology of the marine endobenthos. Palaeogeography, Palaeoecology, Palaeoclimatology 50: 63-81.] : trace fossils that show evidence of predatory behaviour, such as the drill holes (borings) left in shells by carnivorous gastropods, or more dramatically, the bite marks found on some vertebrate bones.

Over the years several other behavioural groups have been proposed, but in general they have been quickly discarded by the ichnological community. Some of the failed proposals are listed below, with a brief description.

* "Chemichnia": a type of agrichnia applied specifically to those instances of bacterial harvesting.

* "Cecidoichnia": a plant trace in which a gall is left on the plant as a result of interaction with animals, bacteria, or other plants.

* "Corrosichnia": traces that are left by plant roots as a result of their corrosive action on the sediments.

* "Cursichnia": a subgroup of the repichnia, created by a crawling or walking habit.

* "Fixichnia": traces left by sessile organisms that anchored themselves to a hard substrate.

* "Mordichnia": a praedichnial subgroup that shows evidence of the prey's death as a result of the attack.

* "Natichnia": a type of repichnia caused by disturbances to a soft sediment by a swimming organism, e.g. a benthic fish.

* "Polychresichnia": traces that show an origin in the combination of two or more established trace-producing behaviours, e.g. domichnia that served as the feeding position of the organisms.

* "Sphenoichnia": a plant trace created by the bioturbational action of roots.

* "Taphichnia": fugichnia in which the organism failed to escape and was buried, often resulting in its body fossil being found in association with the trace.

* "Volichnia": traces that show the position a flying organism (usually an insect) landed on a soft sediment.

Fixichnia [Gibert, JM de, Domènech, R & Martinell, J (2004) An ethological framework for animal bioerosion trace fossils upon mineral substrates with proposal of new class, fixichnia. Lethaia 37 (4): 429-437.] is perhaps the group with the most weight as a candidate for the next accepted ethological class, being not fully described by any of the eleven currently accepted categories. There is also potential for the three plant traces (cecidoichnia, corrosichnia and sphenoichnia) to gain recognition in coming years, with little attention having been paid to them since their proposal [Mikuláš, R (1999) Notes on the concept of plant trace fossils related to plant-generated sedimentary structures. Vestník Veskeho geologickéhoústavu 74: 39-42.] .

Toponomic Classification of Trace Fossils

Another way to classify trace fossils is to look at their relation to the sediment of origin. Martinsson [Martinsson, A (1970) Toponomy of trace fossils. In Crimes, TP & Harper, JC (eds.) (1970) Trace fossils. Geological Journal, Special Issue 3: 323-330.] has provided the most widely accepted of such systems, identifying four distinct classes for traces to be separated in this regard:

* "Endichnia" are those traces that are found wholly within the casting medium, and therefore can only have been made by an infaunal organism.

* "Epichnia" are found on the tops of the strata of origin, being those ridges and grooves that were formed by benthic organisms or infaunal burrows that have been exposed by erosion.

* "Exichnia" are traces that are made of material that is different from the surrounding medium, having either been actively filled by an organism or eroded out and re-covered by an alien sediment.

* "Hypichnia" are ridges and grooves found on the soles of the beds of origin at their interfaces with other strata, representing the opposite of epichnia.

Other classifications have been proposed [Seilacher, A (1964) Sedimentological classification and nomenclature of trace fossils. Sedimentology 3: 253-256.] [Chamberlain, CK (1971) Morphology and ethology of trace fossils from the Ouachita Mountains, southeast Oklahoma. Journal of Paleontology, 45: 212-246.] [Simpson, S (1957) On the trace fossil "Chondrites". Quarterly Journal, Geological Society of London 112: 475-99.] , but none stray far from the above.

References

ee also

* Ichnology

* Trace fossil

External links

* http://www.peripatus.gen.nz/paleontology/trafos.html

* http://www.envs.emory.edu/ichnology/

* http://www.wooster.edu/geology/Bioerosion/Bioerosion.html