Trace fossils

Key points

• Trace fossils represent the activities of organisms.

• Trace fossils may be treated as fossilized behavior, or as biogenic sedimentary structures.

• Trace fossils include tracks and trails, burrows and borings, fecal pellets and coprolites, root penetration structures and other kinds of pellets.

• Trace fossils are named on the basis of shape and ornamentation, not on the basis of the supposed maker, environment or stratigraphy.

• One animal may produce many different kinds of trace fossils, and one trace fossil type can be produced by many different kinds of animals.

• Trace fossils may be produced within a sedimentary layer, or on the surface; trace fossils may be preserved in the round, and may be seen as molds and casts on the bottoms and tops of beds.

• Trace fossils may be classified according to the mode of behavior represented: movement, feeding, farming, dwelling, escape and resting.

• Certain trace fossil assemblages (ichnofacies) appear to repeat through time, and may give clues about the environment of deposition.

• Trace fossils often occupy particular levels (tiers) in the sediment column, and the depth of tiering has apparently increased through time.

• Trace fossils are of limited use in stratigraphy, except in some special cases.

"But one false statement was made by Barrymore at the inquest. He said that there were no traces upon the ground round the body. He did not observe any. But I did - some little distance off, but fresh and clear." "Footprints?" "Footprints." "A man's or a woman's?"

Dr. Mortimer looked strangely at us for an instant, and his voice sank almost to a whisper as he answered:

"Mr. Holmes, they were the footprints of a gigantic hound!"

Arthur Conan Doyle (1901) The Hound of the Baskervilles

All the classic detective stories hinge on a footprint on the flowerbed, a used cigarette end, a crumpled scrap of paper. These are traces of what happened, and the skilled detective has an uncanny ability to read clues from them. Sherlock Holmes astounded his colleague Mr. Watson by being able to estimate the height of a felon from his footprint; but is that really so difficult?

Trace fossils are the preserved remains of the activity and behavioral patterns of organisms. Common examples are burrows of bivalves and worms that live in estuaries and shallow seas, complex feeding traces of deep-sea animals on the ocean floor, and the footprints of dinosaurs and other land animals preserved in mud and sand beside rivers and lakes. At first sight, these remains might seem rather obscure, but they can tell some remarkable stories (Box 19.1).

Every trace fossil offers us a vignette of ancient life, both the life of the organism that made the marking, as well as the environment in which it lived. Trace fossils give evidence about:

• the behavior of organisms - and so are part of the organisms' paleobiology;

• sedimentary environments - and so are like sedimentary structures.

For example, a trackway of dinosaur footprints may tell us about the shape of the soft parts of the feet of the dinosaur that made them, the pattern of scales on the skin, the running speed and the environment in which the animal lived. The dinosaur tracks can equally be used to show that the sediments were deposited on land or in shallow water, and that the climate was probably warm (appropriate for dinosaurs).

Trace fossils are common in many sedimentary rocks, and they have been observed by geologists for centuries. Indeed, many trace fossils were given zoological and botanical names from early in the 19 th century, since they were thought to be fossilized seaweeds or worms. The only trace fossils that were correctly interpreted from the start were dinosaur footprints, although many of these were interpreted at first as the products of flocks of huge birds.

The modern era of trace fossil studies began in the 1950s with the work of the German paleontologist Adolf Seilacher. He established a classification of trace fossils based on behavior, and discovered that certain assemblages of trace fossils indicate particular water depths in the sea. In addition, trace fossils have been used widely by exploration geologists since the 1960s and 1970s when the study of depositional environments revolutionized understanding of the sedimentary rock record. These contributions gave a strong scientific basis to the study of trace fossils, often called ichnology (from the Greek ichnos, a trace).

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