What is Ichnology?
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, 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).
Trace fossils have many useful advantages over body fossils for a paleontologist who wishes to learn more about ancient life. Among these advantages are:
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.
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 trilobite Isotelus, 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 every Cruziana 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.
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