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What is Ichnology?Authored by:Anthony J.
Martin |
Ichnology is the study of plant and animal traces. Implicit to this definition is that the traces made by plants and animals reflect some sort of behavior.
Ichnology can be divided into two major subdivisions: paleoichnology (the study of ancient traces) and neoichnology (the study of modern traces). Most ichnologists are involved in paleoichnology but a considerable number also study neoichnology for the comparison of modern equivalents (and their trace makers) to ancient traces. Technically speaking, wildlife biologists or ecologists who study tracking (identification of animals and their behavior on the basis of their tracks and feces) are neoichnologists, although they probably would not recognize such an designation if you told them.
This introduction emphasizes paleoichnology, but examples from neoichnology are provided in some cases for clarification of concepts. Indeed, some fossil traces were well studied long before modern equivalents were found at all, causing a reverse form of uniformitarianism: "the past is the key to the present."
Biogenic Structures
A biogenic structure is a feature caused by an organism while it was still living. This definition effectively separates a biogenic structure from a structure made by the body of a dead organism (such as a drag mark, cast, or mold). The following outline, modified after Frey and Pemberton (1984), provides a breakdown of different categories of biogenic structures and examples of each category:
Biogenic structures, once described, can be classified on the basis of their behavioral association. These behavioral modes represent basic biological functions that are nearly universal to multicellular organisms, such as feeding, dwelling, and locomotion. Of course, multiple or overlapping types of behavior can be interpreted from one trace, but the classification is generally applied to the predominant motive of the organism. For example, a snail might be moving across a surface, hence you would label its behavioral mode as locomotion. However, if the surface has a wonderfully delicious film of organic scum that the snail is consuming as it moves, then the primary purpose for movement was feeding.
Different behavioral modes for biogenic structures were assigned categories by Seilacher (1953). These categories reflect Latinized names that were meant to standardize the categories; all have the suffix "ichnia" to indicate that they represent traces of the behavior. The following diagram shows the interrelationships of the different behavioral modes, their Latinized names, and explanations of each mode. This diagram is modified after Pemberton et al. (1992).
Paleontology and Paleoichnology: What's the Difference (Besides Spelling)?
A fossil is any evidence of ancient life. Two major types of fossils are trace fossils and body fossils. A trace fossil (also known as ichnofossil) is any indirect evidence of ancient life that reflects some sort of behavior. In contrast, a body fossil is an actual body part of an ancient organism, which includes any casts or molds that were made of the dead body. Paleontology is the study of ancient life, which means that it can include both the study of trace fossils and body fossils. The study of trace fossils is specifically called paleoichnology.
Trace fossils have many useful advantages over body fossils for a paleontologist who wishes to learn more about ancient life. Among these advantages are:
- Long Time Ranges - Certain trace fossils, because they were made by similar organisms with similar behavior, have much longer time ranges than most body fossils. Because these trace fossils are useful as environmental indicators for broader periods of time, they are more likely encountered than some body fossils.
- Abundance - One animal, especially if mobile, can make many traces during its lifetime, whereas it may or may not have its body preserved in the fossil record. Trace fossils made by most organisms should be more abundant than body fossils of the organisms themselves.
- Common In-situ Occurrence - Unlike body fossils, trace fossils are very rarely transported out of their original substrate. In the majority of cases, trace fossils represent behavior that occurred exactly where you find them.
- Preservation Potential - Trace fossils are common in clastic sedimentary rocks that are normally lacking body fossils. This phenomenon is largely a result of poor preservation potential for body fossils in such a medium, whereas trace fossils are preserved or (in some cases) enhanced in these rocks.
- Excellent Environmental Indicators - Because behavior is often influenced by environmental factors, trace fossils provide important clues to the original conditions of ancient environments. Environmental factors reflected by trace fossils include salinity, oxygen levels, energy, organismal interactions, and food supplies.
Disadvantages of trace fossils are relatively fewer than their advantages but mainly center on the fact that one trace maker can make many different traces or many different trace makers can make the same trace. Because of these considerations, most trace fossils have limited value for biostratigraphy.
Trace fossils are most commonly represented in the geologic record by features formed through animal activity in a substrate of some sort, such as sediment, rock, or wood. These features include burrows, tracks, trails, and borings. Burrows are excavations made into an unconsolidated substrate. Tracks are imprints on a sediment surface by an animal with legs. Trails are imprints on a sediment by a legless animal dragging its body across a surface. Borings are excavations made into a consolidated substrate, which could include rock or wood. Burrows, tracks, and trails are examples of biogenic sedimentary structures (structures made in an unconsolidated substrate); borings are examples of bioerosion structures.
Another type of trace fossil is biostratification, which can be represented by stromatolites or biogenic graded bedding. Stratification refers to the layering of sediments, hence biostratification is layering caused by organisms. Stromatolites are biogenic sedimentary structures formed by sediment binding and trapping in cyanobacterial or algal colonies. Biogenic graded bedding is bedding where organisms mixed the larger particles to the bottom of a sedimentary profile, causing a gradual decrease in particle size upward.
Fossilized feces are called coprolites; these are also trace fossils. Coprolites are valuable clues to the paleodiet of extinct organisms and provide additional information to paleontologists interested in reconstructing ecosystem relationships of fossil plants and animals.
Lastly, eggs and nests are indirect evidence of reproductive behavior and their fossilized equivalents are also considered as trace fossils. Although some dinosaur eggs have preserved remains of embryos (which are body fossils), the egg itself is a trace fossil. Nests may or may not include eggs but are structures that were primarily made for facilitating the development of younger animals. For example, some dinosaur nest sites containing eggs have been discovered, but termite nests without any eggs or body fossils also have been preserved in the fossil record.
Classification of Trace Fossils
Because trace fossils are not actual organisms or parts of organisms, they cannot be given Linnaean names recognized by the ICZN (International Committee on Zoological Nomenclature) as organisms. Nevertheless, morphologically distinctive trace fossils are given genus and species names by ichnologists for the sake of international communication. For example, a simple, unbranched, unlined, horizontal burrow might be given the name Planolites, and varieties of that same basic morphology can be identified as "species" under that same "genus" name (e.g.,Planolites montanus). To avoid confusion with binomial nomenclature used in naming body fossils, trace fossils are named as ichnogenera (plural of ichnogenus) and ichnospecies.
Part of the confusion regarding this nomenclature arises from the common lack of connection between the trace fossil name and the name of its original trace maker. Some trace fossils were made by unknown, extinct, or poorly preserved organisms, hence the ichnogenus name can not be expected to reflect a specific trace maker. Consequently, a trilobite trackway may have been made by a species of the trilobiteIsotelus, but the trilobite trackway itself might be called Cruziana. When you recall that one of the disadvantages of trace fossils is that the same trace fossil could have been made by many organisms, think of the difficulty of trying to match everyCruziana with each of the hundreds of trilobite genera. Even dinosaur tracks can rarely be matched with a specific dinosaur; most ichnologists are satisfied enough to say that certain trackways were made by sauropods or theropods.
ICHNOFACIES
Basic Definition of Ichnofacies
Assemblages of trace fossils, in association with body fossils and lithologic information, provide excellent clues to parameters of ancient environments. This use of all preserved aspects of an ancient sedimentary deposit for interpreting its original environment of deposition is called facies analysis. If dealing simply with the trace fossils in a sedimentary deposit, you would be interested in describing its environmentally related and contemporaneous trace fossils; in a modern environment, this assemblage is an ichnocoenose. The preserved record of the original ichnocoenose is an ichnofacies.
The ichnofacies concept, since its inception in geology by Adolph Seilacher, has become perhaps the best paleontological tool for interpreting ancient environments. Ichnofacies were originally defined as archetypal and recurring assemblages with reference to a bathymetric profile, but subsequent work has shown that water depth is only one facet of ichnofacies. Ichnofacies are named after one distinctive ichnogenus that is commonly (but not necessarily) present in the assemblage.
Nine ichnofacies have been named so far. These ichnofacies, their general environmental association, and representative ichnogenera are (from Pemberton et al., 1992):
- Trypanites - lithified
marine substrates.
- Entobia
- Gastrochaenolites
- Trypanites
- Teredolites - marine or marginal marine woody substrates.
- Scoyenia - nonmarine substrates.
- Skolithos - high energy, shallow marine substrates.
- Cruziana - lower energy,
circalittoral marine substrates.
- Arenicolites
- Aulichnites
- Cruziana
- Planolites
- Teichichnus
- Thalassinoides
- Glossifungites - firm (but unlithified) marine
substrates.
- Gastrochaenolites
- Psilonichnus
- Rhizocorallium
- Skolithos
- Thalassinoides
- Psilonichnus -
supralittoral, moderate to low energy (beach to backshore).
- Psilonichnus
- Macanopsis
- Zoophycos - circalittoral
to bathyal, low energy, marine substrates.
- Phycosiphon
- Spirophyton
- Zoophycos
- Nereites - bathyal to
abyssal, low energy, marine substrates.
- Cosmoraphe
- Lorenzinia
- Nereites
- Paleodictyon
Development and refinement of the ichnofacies concept is still ongoing, especially with increased recognition of the diversity of nonmarine trace fossil assemblages (which will necessitate subdivision of theScoyenia ichnofacies).
Applications of Ichnofacies
The ichnofacies concept has been applied to petroleum exploration as an aid to interpreting depositional environments. The interpretation of sedimentary environments helps to assess potential petroleum reservoirs and source rocks on the basis of stratigraphic architecture; laterally adjacent facies succeeding one another vertically (Walther's Law) can be better discerned if the facies are distinctive and identifiable. Trace fossil assemblages provide an excellent supplementary tool for facies analysis, especially when body fossils are lacking.
A promising area for future applications of ichnofacies in facies analysis is in hydrogeology and other aspects of environmental geology. Hydrogeologists often must approximate many of the same parameters (porosity, permeability, facies architecture) sought by petroleum geologists, hence ichnofacies present another important set of data for these geoscientists.
Ichnofacies also indicate the evolution of paleocommunities throughout geologic time. Trace fossils are evident in rocks from the Proterozoic Eon to the Pleistocene Epoch, hence their assemblages record organismal behavior and the evolution of behavior. This information is particularly valuable for interpreting the behavior of organisms that rarely have body parts fossilized, adding another dimension to the paleontological data set for evolutionary theory.
Quantification of Bioturbation and Use in Ichnofacies
Many workers have attempted to quantify the amount of biogenic disturbance of sediment in a given stratigraphic sequence. Most schemes for classification of amounts of bioturbation involve semiquantitative categories, such as those used by Reineck (1963), Howard and Frey (1972), Droser and Bottjer (1986), and Martin (1993). These categories are divisions between two extremes: 0% bioturbation (no evidence of biogenic mixing of sediments) and 100% bioturbation (complete biogenic mixing of sediments). Thus, categories between these end members in a continuous spectrum of bioturbation could be ranges of biogenic disturbance, such as 1-10% bioturbation, 10-40%, 60-90%, and so on. Names have been given to such categories reflecting some ordinal scale ("ichnofabric index 2," "ichnofabric index 3") or general description that shows the relative progression of bioturbation ("very slightly bioturbated,"slightly bioturbated").
The ichnofacies concept has incorporated concepts regarding amounts of bioturbation, although only in an imprecise way. This imprecision is probably more representative of the great variety of expression of bioturbation in different environments rather than faulty work by ichnologists. Nevertheless, some ichnofacies have been noted as containing relatively more or less evidence of bioturbation in addition to qualitative information regarding trace fossil assemblages. For example, theSkolithos ichnofacies typically shows less evidence of bioturbation than theCruziana ichnofacies, although some exceptions occur.
The "ichnofabric" approach has been advocated in recent years as an alternative or modification to the ichnofacies concept and has been the main topic of discussion at the last three International Ichnofabric Workshops (1991, 1993, 1995). Additionally, traditional ichnofacies have been regarded by some workers as taphonomically biased and are thus not representative of original ichnocoenoses (Bromley and Asgaard, 1990). Further testing of the ichnofacies concept should resolve its utility as a tool for the interpretation of ancient environments.
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HISTORY OFICHNOLOGY |
Ichnology as a science actually began in prehistory, when hunter-gatherer societies used the signs of animals (tracks, feces, and other indirect evidence) for acquiring their prey. Ichnology (or, in this case, neoichnology) is thus probably one of the first applied sciences in human history, born out of necessity for survival.
Other aspects of ichnology, such as paleoichnology, became known later in human history, including the observation of dinosaur tracks in Mesozoic strata by Native Americans and other aboriginal peoples and the first recognition of some coprolites as feces in the 17th century. Dinosaur tracks were first documented in 1802 by Pliny Moody in the eastern U.S. but were not studied in any detail until more than 30 years later by Edward Hitchcock.
By name, ichnology is a relatively young science and has only reached widespread international recognition in the second half of the 20th century. Probably the first use of the term "ichnology" in the title of a scientific publication was in 1858, Ichnology of New England, by Edward Hitchcock, regarding the vertebrate tracks of Mesozoic strata in the Connecticut Valley, U.S. The naming of invertebrate trace fossils began early in the 19th century, when paleontologists mistook some trace fossils for fossil algae ("fucoids"), reflected in the suffix of many an ichnogenus (i.e., Zoophycos, Palaeophycus, Spirophycus). In 1873, a Swedish paleobotanist, Alfred Nathorst, refuted the algal origin for most of these trace fossils and demonstrated their affinity to modern biogenic sedimentary structures, but his arguments were not readily accepted until later.
Neoichnology of invertebrate organisms in modern times may have actually started with Charles Darwin, who observed the effects of bioturbation by earthworms in his yard during the 19th century. Darwin measured the amounts and rates of soil overturn in his yard by using "wormstones," experiments that took Darwin years to gather data.
With regard to marine environments, the pioneering efforts of Rudolf Richter in the 1920's, where he studied modern traces left in the North Sea tidal flats, lead to a much better understanding of the uniformitarian principles required for understanding trace fossils. One of the most comprehensive works ever done on invertebrate and vertebrate trace fossils, Vorzeitliche Lebensspuren, was published by Othenio Abel in 1935, soon after Richter began his work. Meanwhile, in vertebrate ichnology, the discovery of Protoceratops eggs and nests in Mongolia by the Roy Chapman Andrews expeditions was one of the most significant finds of the 1920's.
Other than works done by mainly German authors and a few scientists in other countries, relatively little was published in invertebrate ichnology from the 1930's nearly through the 1950's. Vertebrate ichnology was represented during that time by the work of Roland Bird in dinosaur track studies. The works of Adolph Seilacher in the 1950's marked some new innovations in invertebrate ichnology and the beginning of the ichnofacies concept of ichnology, as well as the beginnings of behavioral (ethological) and preservational classification methods for trace fossils.
The difficult task of classifying invertebrate trace fossils became one of the most important life works of Walter Häntzschel, who published the first concise catalog of described and illustrated invertebrate trace fossils in 1962, in a volume of the Treatise on Invertebrate Paleontology. Quantification of bioturbation, a subject originally approached by Darwin, was revisited by H.-E. Reineck in 1963, who proposed semiquantitative categories for amounts of bioturbation in a vertical sequence of sediment. Work in ichnology increased throughout the 1960's and 1970's and the efforts of British paleontologists Peter Crimes and James Harper, as well as American paleontologist Robert Frey, resulted in several trace fossil books that attempted to popularize the subject more amongst geologists.
Ichnology in the 1980's was represented by a renaissance in vertebrate ichnology, especially with regard to dinosaur tracks and dinosaur eggs. Martin Lockley began much of the documentation of the numerous dinosaur tracksites in the western U.S. and other areas of the world. John Horner, through his description of dinosaur nests and associated body fossils in Montana, U.S., demonstrated for the first time parental care and nurturing behavior in dinosaurs. Guiseppe Leonardi also made a major contribution to vertebrate ichnology through his 1987 publication of the Glossary and Manual of Tetrapod Footprint Palaeoichnology. Invertebrate ichnology had matured enough that the ichnofacies concept became a commonly taught part of sedimentology and paleontology courses at universities worldwide. The applicability of ichnology to the interpretation of depositional environments lead to its use in petroleum exploration, which has been exemplified by the continuing work of George Pemberton . Quantification of bioturbation was revisted again by the work of Mary Droser and David Bottjer in the mid- to late-1980's and other marine invertebrate ichnologists, such as Richard Bromley and Tony Ekdale, began to incorporate this work into their concepts of ichnofabric analysis through the 1990's. Gatherings and discussion of ichnofabric and other aspects of trace fossils have been facilitated by the Ichnofabric Workshops , which started in 1991 and have now become a biannual international event.
Probably the most neglected aspect of invertebrate ichnology, continental invertebrate ichnology, has received much more attention in recent years through the work of Luis Buatois, Stephen Hasiotis, Molly Miller, and Gabriela Mángano. This aspect of ichnology will continue to be one of the more exciting fields for discovering more about the evolution of terrestrial ecosystems. Vertebrate ichnology was most recently boosted by Jerry MacDonald, who discovered extensive trackways in Permian strata of New Mexico, U.S. Vertebrate ichnology will undoubtedly still be represented by more dinosaur-related studies, but hopefully the literature will include more representation of fossil tracks and other traces from nondinosaurians, such as those made by birds, mammals, amphibians, reptiles, and fish.
BIBLIOGRAPHY
ABEL, O. 1935. Vorzeitliche Lebensspuren. Jena, Gustav Fischer, 644 p.
BIRD, R. T. 1985. Bones for Barnum Brown: Adventures of a Dinosaur Hunter. (Edited posthumously by V. T. Schreiber.) Fort Worth, Texas, Texas Christian University Press, 225 p.
BROMLEY, R. G., 1990, Trace fossils, biology and taphonomy: Special Topics in Palaeontology 3: London, Unwin Hyman, 280 p.
BUATOIS, L. A., and MÁNGANO, M. G. 1993. Ecospace utilization, paleoenvironmental trends, and the evolution of early nonmarine biotas. Geology, 21:595-598.
CRIMES, T. P., and HARPER, J. C. 1970. Trace Fossils. Liverpool, Seel House Press, 547 p.
CRIMES, T. P., and HARPER, J. C. 1977. Trace Fossils 2. Liverpool, Seel House Press, 351 p.
DROSER, M. L., and BOTTJER, D. J. 1986. A semiquantitative field classification of ichnofabric. Journal of Sedimentary Petrology, 56:558-559.
EKDALE, A. A., BROMLEY, R. G., and PEMBERTON, S. G. 1984. Ichnology: Trace Fossils in Sedimentology and Stratigraphy. Society of Economic Paleontologists and Mineralogists Short Course No. 15, 317 p.
FREY, R. W. 1975. The Study of Trace Fossils. New York, Springer-Verlag, 562 p.
HÄNTZSCHEL, W. 1962. Trace fossils and problematica. In R. C. Moore (ed.), Treatise on Invertebrate Paleontology, Part W, Miscellanea. Geological Society of America and Univ. of Kansas Press, p. W177-W245.
HASIOTIS, S. T., and BOWN, T. M. 1992. Invertebrate trace fossils: the backbone of continental ichnology. In C. G. Maples and R. R. West (eds.), Trace Fossils. Short Courses in Paleontology No. 5, Knoxville, Tennessee, Paleontological Society, p. 64-104.
HITCHCOCK, E. 1858. Ichnology of New England. A Report of the Sandstone of the Connecticut Valley Especially Its Footprints. Boston, W. White, 220 p.
HORNER, J. R, and Gorman, J. 1988. Digging Dinosaurs. New York, Workman Publishing, 210 p.
LEONARDI, G. 1987. Glossary and Manual of Tetrapod Footprint Palaeoichnology. Departmento Nacional da Produção Mineral, Brasilia, 75 p.
LOCKLEY, M. 1991. Tracking Dinosaurs: A New Look at an Ancient World. Cambridge, Cambridge University Press, 238 p.
MACDONALD, J. 1994. Earth's First Steps. Boulder, Colorado, Johnson Books, 290 p.
MILLER, M. F. 1984. Distribution of bioenic structures in Paleozoic nonmarine and marine-margin sequences: an actualistic model. Journal of Paleontology, 58:550-570.
NATHORST, A. G. 1873. Om några förmodade växtfossilier. Öfversigt af Kgl. Vetensk. Akad. Förhandl. 1873, 9:25-52 (1874).
OSGOOD, R. G., Jr. 1975. The history of invertebrate ichnology. In R. W. Frey, (ed.), The Study of Trace fossils. New York, Springer-Verlag, p. 3-12.
REINECK, H.-E. 1963. Sedimentgefüge im Bereich der südlichen Nordsee. Abhandlungen der Senckenbergischen Naturforschenden Gesellschaft 505:138 p.
RICHTER, R. 1927. Die fossilien Fährten und Bauten der Würmer, ein Überblick über ihre biologischen Grundformen und deren geologische Bedeutung. Paläont. Zeitschr., 9:193-240.
SEILACHER, A. 1953. Studien zur palichnologie. I. über die methoden der palichnologie. Neues Jahrb. Geologie Paläontologie Abhandlungen 96:421-452.
Links to Related Resources in Ichnology and Paleontology
The Introduction to Ichnology page, put online in early August, 1995, was the first one on the World-Wide Web related specifically to ichnology. Web resources related directly to ichnology are still sparse but I will be adding those as they undoubtedly manifest themselves through the efforts of other online ichnologists. A Dinosaur Ichnology Links section, which can also be found at the Dinosaur Trace Fossils site, provides sites specifically related to dinosaurs.
I have also added some links to general paleontological resources, listed in alphabetical order, for those of you interested in looking at the rest of the fossil record. Most of these resources relate to body fossils, so these would be good for finding out more about what organisms made trace fossils.
General Ichnology Links
Dinosaur Tracks
Andrew MacRae's Dinosaur Footprints in Coal.Dinosaur Ridge, Colorado, USA.
Dinosaur Tracks in Rocky Hill, Connecticut, USA.
Field Trip to Dinosaur Tracks in Central Texas, USA.
Fossils and Formations of Grand Junction, Colorado, USA.
Glen Kuban's Paluxy Dinosaur/"Mantrack" Page".
New Mexico Museum of Natural History Dinosaur Footprint Page.
Overview of Dinosaur Tracking.
Terry Acomb's Dinosaur Tracksite Page.
Triassic-Jurassic Dinosaur Footprint Project, Pratt Museum, Amherst, Massachusetts, USA.
Virtuzo on the Tracks of Dinosaurs.
Dinosaur Eggs
Dinosaur Egg Project.National Geographic Special Report on Dinosaur
Eggs.