Trace fossil
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Trace fossils, also called ichnofossils (IPA: /ˈɪknəʊˌfɒsəl/, Ancient Greek: ιχνος or ikhnos, meaning "trace" or "track") are structures preserved in sedimentary rocks that record biological activity. While we are most familiar with relatively spectacular, fossilized hard-part remains such as shells and bones (known as body fossils), trace fossils are often less dramatic, but nonetheless very important.
Strictly defined, trace fossils must reflect both the anatomy of their maker in some way, and be the result of behaviour. Sedimentary structures made by empty shells rolling along the sea floor are thus excluded (as "death marks"), as are structures such as stromatolites that, although the result of behaviour, do not reflect the anatomy of their maker. However, spun coccoons and spiders webs are considered to be trace fossils, as they are manipulated by their makers after secretion; egg cases, on the other hand, are not.
Trace fossils include burrows (such as Chondrites), borings, ichnites (footprints and track marks), trails (such as Cruziana scratched by trilobites), coprolites (fossilized droppings) and other gut-derived objects, and rhizoliths or rhizocretions (the fossil remains of roots).
The study of trace remains is called ichnology, which is divided into paleoichnology, or the study of trace fossils, and neoichnology, the study of modern trace remains.
The science of ichnology is quite challenging, as most trace remains cannot be positively assigned to a specific organism or even to a specific class of organisms. Furthermore, trace remains such as burrows can make the work for paleontologists and paleobiologists more difficult as they rework sediments, causing older strata to be mixed with younger ones. This can cause some confusion in interpretation, unless viewed in geologic context.
Adolf Seilacher divided trace fossils into five main behavioral groups:
- Domichnia are dwelling structures that reflect the life position of the organism that created it;
- Fodinichnia are three-dimensional structures left by animals which eat their way through sediment, such as deposit feeders;
- Pascichnia are another type of feeding trace, left by grazers on the surface of a soft sediment or a mineral substrate;
- Cubichnia are resting traces, in the form of an impression left by an organism on a soft sediment;
- Repichnia are surface traces of creeping and crawling, as an organism moved from one station to another.
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[edit] Confusion with other types of fossils
Trace fossils should not be confused with body casts. The Ediacaran biota, for instance, primarily consist of the casts of these organisms in the sediment.
Early geologists gave the name 'fucoid' to a wide variety of markings they found on the bedding planes of sedimentary rocks. The earth scientists frequently misinterpreted these 'fucoid' marks as being the fossilized remains of seaweed. However, in more recent years, these markings have been studied with greater thoroughness. It is now apparent that the 'fucoids' and other markings have in fact been caused by a variety of plants and animals. As a result, these 'fucoid' markings are now termed trace fossils.
Pseudofossils, which are not true fossils, should also not be confused with ichnofossils, which are true indications of prehistoric life.
[edit] Information provided by ichnofossils
[edit] Paleoenvironments and Paleoecology
Trace fossils provide us with indirect evidence of life in the past, such as the footprints, tracks, burrows, borings, and feces left behind by animals, rather than the preserved remains of the body of the actual animal itself. Unlike most other fossils, which are produced only after the death of the organism concerned, trace fossils provide us with a record of the activity of an organism during its lifetime.
Trace fossils are formed by organisms performing the functions of their everyday life, such as walking, crawling, burrowing, boring, or feeding. Tetrapod footprints, worm trails and the burrows made by clams are all trace fossils.
Fossil footprints made by tetrapod vertebrates are difficult to identify to a particular species of animal, but they can provide us with valuable information -- such as the speed, weight, and behavior of the organism which made them. Such trace fossils are formed when amphibians, reptiles, mammals or birds walked across soft (probably wet) mud or sand which later hardened sufficiently to retain the impressions before the next layer of sediment was deposited.
Perhaps the most spectacular trace fossils are the huge, three-toed footprints produced by dinosaurs and related archosaurs. These ground impressions give scientists clues how these animals lived. Although the skeletons of dinosaurs can be reconstructed, only their fossilized footprints can determine exactly how they stood and walked. Such tracks can tell us a lot about the gait of the animal which made them, what its stride was, and whether or not the front limbs touched the ground.
However, most trace fossils are rather less conspicuous, such as the trails made by segmented worms or nematodes. Some of these worm castings are the only fossil record we have of these soft-bodied creatures. Within the trace and microfossil record, the most common worm remnants are those produced by scolecodonts, otherwise known as serpulid annelids or polychaetes.
[edit] Use as index fossils
Some trace fossils can be used as local index fossils, to date the rocks in which they are found, such as the burrow Arenicolites franconicus which occurs in a 4 cm (1.6") layer of the Triassic Muschelkalk epoch, throughout wide areas in southern Germany[citation needed].
[edit] Identification of the trackmaker
The organisms which produce trace fossils are usually not preserved with their markings and, although it may be possible to deduce what the animal was doing at the time, it is usually impossible conclusively to determine the maker of the trace and to assign it to a given species of animal. Since different types of organisms are able to make the same types of markings, trace fossils are usually classified by their shape and their cause (such as feeding, dwelling, or crawling), rather than by the types of organisms which made them.
The usual classifications for trace fossils are ichnogenera for genera and ichnospecies for species. It should be emphasized that ichnogenera and ichnospecies are artificial classifications that apply only to the trace fossils themselves and do not relate to the genus or species of the organisms which produced them.
[edit] Inherent bias and principle of actualism
Most trace fossils are known from marine deposits[citation needed]. Essentially, there are two types of traces, either exogenic ones, which are on the surface of the sediment (such as tracks) or endogenic ones, which are within the layers of sediment (such as burrows).
Surface trails on sediment, in shallow marine environments, stand less chance of fossilization, because they are subjected to wave and current action. Conditions in quiet, deep-water environments tend to be more favourable for preserving fine trace structures.
Most trace fossils are usually readily identified by reference to similar phenomena in modern environments. This method is known as the principle of actualism. However, the structures made by organisms in recent sediment have only been studied in a limited range of environments, mostly in coastal areas, including tidal flats. Many trace fossils were formed within the sediment itself, by infaunal species rather than just at the surface, so it is more difficult to compare them to modern forms.
[edit] Examples
No generally accepted trace fossils are found in rocks older than the latter part of the Ediacaran period (580 to 542 million years ago) of the Neoproterozoic era, with the earliest undoubted occurrences perhaps only 570 My old or even younger. During the succeeding Cambrian period, trace fossils greatly diversify in all ways. One well-known occurrence of trace fossils from this period is the famous 'Pipe Rock' of northwest Scotland. The 'pipes', which give the rock its name, are closely packed straight tubes, which were presumably made by some kind of worm-like organism. The name given to this type of tube or burrow is Skolithos, which may be 30 cm (12") in length and between 2 to 4 cm (0.8 to 1.6") in diameter. Such traces are known worldwide from sands and sandstones deposited in shallow water environments, from the Cambrian period (542 to 488 m.y.a) onwards.
Other common types of trace fossil made by invertebrates are Chondrites, Cruziana, Thalassinoides, Asteriacites, Rhizocorallium, and Teichichnus. These are all ichnogenera:
- Chondrites are small branching burrows of the same diameter, which superficially resemble the roots of a plant. The most likely candidate for having constructed these burrows is a nematode (roundworm). Chondrites are found in marine sediments from the Cambrian period of the Paleozoic onwards. They are especially common in sediments which were deposited in reduced-oxygen environments.
- Cruziana are excavation trace marks made on the sea floor which have a two-lobed structure with a central groove. The lobes are covered with scratch marks made by the legs of the excavating organism, usually a trilobite or allied arthropod and, in fact, several different types of trilobite have been discovered at the end of Cruziana trails[citation needed]. Cruziana are most common in marine sediments formed during the Paleozoic era, particularly in rocks from the Cambrian and Ordovician periods. Over 30 species of Cruziana have been identified.
- Thalassinoides are burrows which occur parallel to the bedding plane of the rock and are extremely abundant in rocks, worldwide, from the Jurassic period onwards. They are repeatedly branched, with a slight swelling present at the junctions of the tubes. The burrows are cylindrical and vary from 2 to 5 cm (0.8" to 2") in diameter. Thalassinoides sometimes contain scratch marks, droppings or the bodily remains of the crustaceans which made them.
- Asteriacites is the name given to the five-rayed fossils found in rocks and they record the resting place of starfish on the sea floor. Asteriacites are found in European and American rocks, from the Ordovician period onwards; and are numerous in rocks from the Jurassic period of Germany.
- Rhizocorallium is a type of burrow, the inclination of which is typically within 10° of the bedding planes of the sediment. These burrows can be very large, over a meter long in sediments that show good preservation, e.g. Jurassic rocks of the Yorkshire Coast (eastern United Kingdom), but the width is usually only up to 2 cm, restricted by the size of the organisms producing it. It is thought that they represent fodinichnia as the animal (probably a nematode) scoured the sediment for food.
- Teichichnus has a distinctive form produced by the stacking of thin 'tongues' of sediment, atop one another. They are again believed to be fodinichnia, with the organism adopting the habit of retracing the same route through varying heights of the sediment, which would allow it to avoid going over the same area. These 'tongues' are often quite sinuous, reflecting perhaps a more nutrient-poor environment in which the feeding animals had to cover a greater area of sediment, in order to acquire sufficient nourishment.
Less ambiguous than the above ichnogenera, are the traces left behind by invertebrates such as Hibbertopterus, a giant "sea scorpion" or eurypterid of the early Paleozoic era. This marine arthropod produced a spectacular hibbertopteroid track preserved in Scotland.[1]
The oldest types of tetrapod tail-and-foot prints date back to the latter Devonian period. These vertebrate impressions have been found in Scotland, Pennsylvania, and Australia.
Important trace fossils of human evolution are the Laetoli (Tanzania) footprints, imprinted in volcanic ash 3.7 million years ago (mya) -- probably by an early Australopithecus.
[edit] See also
[edit] References
- ^ Whyte, MA (2005) Palaeoecology: A gigantic fossil arthropod trackway. Nature, 438: 576.
