Trace fossils have the privilege of being two things at once: sedimentary structures, and fossils. They occur in sediment, are made of sediment, but represent the activities of creeping, crawling or burrowing critters, mostly at or immediately below the sediment water interface (marine, lacustrine, estuarine, swamp), or subaerial environments such as dune fields. As such, trace fossils represent the range of activities that critters are normally occupied with – grazing or foraging for food, home construction and house-keeping, predating or escaping predators, wandering aimlessly, or taking a nap after an exhausting day. Some critters like to rough it, preferring the tumble of waves or strong currents, while others like the peace and quiet of deeper realms. Lives are frequently interrupted by storms or violent, turbulent flows of sand and mud; their traces, or lack of them, also reflect these events.
Most animals produce more than one kind of trace depending on what they are doing, which means that in most cases, traces reflect animal activity and biometrics, rather than the specific critter species. Most traces do not contain any remnants of the animal that made them (there are a few exceptions); finding a trilobite body fossil at the end of its scampering trail is pretty rare.
Trace fossils provide valuable information on benthic communities, environmental conditions such as wave or current energy, redox conditions, rates of sedimentation, or periods of time when sedimentation slowed (e.g. hiatuses, disconformities, omission surfaces).
Intense bioturbation can also obliterate other kinds of sedimentary structures; for a geologist, this may be an annoyance or a happy circumstance. Most Precambrian successions are free of trace fossils and bioturbation; this changed during the Ediacaran, the period that appears to have been a kind of precursor to the Cambrian invertebrate explosion. Most Phanerozoic sedimentary successions (since 540 million years ago) have enjoyed the munching-burrowing efforts of a myriad nameless critters.
Trace fossils, like regular body fossils, are part of the rock.
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Radiating spreite of Zoophycus, organized into a 3-dimension cork-screw like structure, probably representing systematic feeding by a marine worm or similar creature. Mid-Miocene Mohakatino Fm. North Taranaki
Two examples of Scolicia. Left: basin floor fan, Mid-Miocene Mohakatino Fm, N Taranaki. Right: shelf, Iceberg Bay Fm. Axel Heibe4rg Island.
Thalassinoides, Left from submarine fan Mohakatino Fm. N Taranaki. Right: shallow shelf, Expedition Fm. Axel Heiberg Island.
Rhizocorallium, both images from Grizzly Gorge, Yukon
Rippled bedding, with numerous small feeding traces, one Paleophycus, and the slightly raised mounds of abundant vertical Skolithis burrows. Left Paleocene Expedition Fm. Right Eocene Iceberg Bay Fm, Axel Heiberg Island.
Ophiomorpha, with typical ornamented burrow lining. Both images from the Paleocene Wangaloa Fm, Otago coast, NZ.
Ray jetting holes (feeding), in basal Waitemata Basin shallow shelf deposits (Lower Miocene). The holes are filled with poorly sorted grit and shell fragments. Some holes overlap. The right image shows the density of holes over several square metres. Matheson’s Bay, Auckland.
Left: recent Pholad borings into indurated greywacke. Fragments of the bivalve shells remain. Diameter of the coin (top left) is about 25mm. Right: Lower Miocene Pholad borings into a greywacke sea-stack (eventually blanketed by turbidites) in basal Waitemata Basin shelf-beach deposits. The borings are mostly filled with mudstone. Matheson’s Bay, Auckland.
Exhumed, calcite-cemented burrows in Permian calcarenite, Canon Fiord Fm, Vesle Fiord, Ellesmere Island. These critters enjoyed fairly high energy wave-current activity, indicated by grain-size and cross bedding.
Pseudo-burrows, formed as reduction spots in Late Pleistocene dune sands. Reduction commonly occurrs around small plant fragments. The host sands are a brownish colour from iron mineral oxidation. Note the primary dune laminae that pass through the spots. Late. Great Exhibition Bay, NZ.