Trace fossil
[3] For example, burrows, borings (bioerosion), urolites (erosion caused by evacuation of liquid wastes), footprints, feeding marks, and root cavities may all be trace fossils.
Accordingly, researchers classify trace fossils into form genera based on their appearance and on the implied behaviour, or ethology, of their makers.
Examples include: Trace fossils are important paleoecological and paleoenvironmental indicators, because they are preserved in situ, or in the life position of the organism that made them.
[7] Trace fossils are formed by organisms performing the functions of their everyday life, such as walking, crawling, burrowing, boring, or feeding.
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.
[12] Palaeontologist Adolf Seilacher pioneered the concept of ichnofacies, whereby geologists infer the state of a sedimentary system at its time of deposition by noting the fossils in association with one another.
[4] The principal ichnofacies recognized in the literature are Skolithos, Cruziana, Zoophycos, Nereites, Glossifungites, Scoyenia, Trypanites, Teredolites, and Psilonichus.
[13] Water depth, salinity, hardness of the substrate, dissolved oxygen, and many other environmental conditions control which organisms can inhabit particular areas.
Surface trails on sediment in shallow marine environments stand less chance of fossilization because they are subjected to wave and current action.
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.
[17] Putative "burrows" dating as far back as 1,100 million years may have been made by animals which fed on the undersides of microbial mats, which would have shielded them from a chemically unpleasant ocean;[18] however their uneven width and tapering ends make a biological origin so difficult to defend[19] that even the original author no longer believes they are authentic.
[22] The traces observed imply simple behaviour, and point to organisms feeding above the surface and burrowing for protection from predators.
[24] The producers of burrows Skolithos declinatus from the Vendian (Ediacaran) beds in Russia with date 555.3 million years ago have not been identified; they might have been filter feeders subsisting on the nutrients from the suspension.
[36] 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.
[40] Bioerosion through time has produced a magnificent record of borings, gnawings, scratchings and scrapings on hard substrates.
A sandstone slab containing the track of tetrapod, dated to 400 million years, is amongst the oldest evidence of a vertebrate walking on land.
[45] Important human trace fossils are the Laetoli (Tanzania) footprints, imprinted in volcanic ash 3.7 Ma (million years ago) – probably by an early Australopithecus.
Early paleobotanists misidentified a wide variety of structures they found on the bedding planes of sedimentary rocks as fucoids (Fucales, a kind of brown algae or seaweed).
Studies in the 1880s by A. G. Nathorst and Joseph F. James comparing 'fucoids' to modern traces made it increasingly clear that most of the specimens identified as fossil fucoids were animal trails and burrows.
Charles Darwin's The Formation of Vegetable Mould through the Action of Worms[a] is an example of a very early work on ichnology, describing bioturbation and, in particular, the burrowing of earthworms.
