Trace fossils are those details preserved in rocks that are indirect evidence of life. While we are most familiar with relatively spectacular fossil hard part remains such as shells and bones, trace fossils are often less dramatic, but nonetheless very important. Trace fossils include burrows , track marks, coprolites (fossilized feces), stromatolites (fossilized algal mounds), and rhizoliths or rhizocretions (fossil remains of roots).
The study of trace remains is called ichnology, which is divided into paleoichnology, or the study of trace fossil, and neoichnology, the study of modern trace remains. Another name for trace fossils is ichnofossils, taken from the Greek word ichnos, meaning "trace".
The science of ichnology is quite challenging, as many trace remains cannot be positively assigned to a specific organism. Further, trace remains such as burrows can make the work for paleontologists and geologists more difficult as they rework sediments, causing older strata to be mixed with a younger ones. This can cause some confusion in interpretation unless viewed in context.
Trace fossils are generally divided into five groups (by Adolf Seilacher , Yale University):
- Domichnia are dwelling structures,
- Fodichnia are feeding traces left by animals which eat their way through sediment,
- Pascichnia are grazing traces made on the surface of the sediment,
- Cubichnia are resting traces (for example where an animal may have stopped and rested on the sea floor),
- Repichnia are surface traces of creeping and crawling.
Confusion with other types of fossils
Pseudofossils are sometimes confused as being trace fossils. Trace fossils should not be confused with body casts. The Ediacara and Burgess shale fauna remains for instance primarily consist of the casts of these organisms in the sediment.
Early geologists who studied the markings found on the bedding planes of sedimentary rocks gave them the name 'Fucoid', and they applied this name to a wide variety of markings and interpreted them as being the fossilized remains of seaweed. However, in more recent years these markings have been studied with greater thoroughness and it is now apparent that the 'Fucoids' and other markings have in fact been caused by a variety of organisms, and are now termed trace fossils.
Information provided by ichnofossils
Trace fossils provide us with indirect evidence of life in the past, such as the footprints, tracks, burrows, borings, and droppings 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. They 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 worms. Some of these worm castings are the only fossil record we have of these creatures.
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.
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 to determine the maker of the trace conclusively 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 genus or species of the organisms which produced them.
Inherent bias and principle of actualism
Most trace fossils are known from marine deposits. 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.
Trace fossils are found in abundance even in rocks from the upper part of the Vendian period, some 550 million years ago and even in Proterozoic (more than 1 billion years ago). 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 is Skolithos, the tubes 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 onwards.
Other common type of trace fossil made by invertebrates are Chondrites, Cruziana, Thalassinoides, and Asteriacites. 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 onwards and are especially common in sediments which were deposited in reduced oxygen environments.
Cruziana are a excavation trace marks made on the sea floor which have a two-lobed structure with a central groove. The lobes are covered with the 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. Cruziana are most common in marine sediments formed during the Palaeozoic 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, and also 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.
The oldest types of tetrapod trace fossils date back to the Upper Devonian period and are found in Scotland, Pennsylvania, and Australia.
Important hominid trace fossils are the Laetoli footprints, imprinted in volcanic ash.