What Kind of Information Do Trace Fossils Provide?

Trace and Non-Trace Fossils

• The technical term for trace fossils is ichnofossils. These are the non-bodily remains of dead organisms that paleontologists find in rock strata dating back tens of millions of years. It is sometimes unclear what a trace fossil is because some of the non-bodily remains of an animal can be quite well-preserved. For instance, egg shells and coprolites (fossilized feces) are both trace fossils because these are solid objects not belonging to the animal's body.

  Conversely, bodily remains can be indirect or wholly external to the animal. For instance, the shell of a sea creature or a body cast without any actual bones (such as the ashen outlines seen at Pompeii) are both considered body fossils; they are indirect remains of the organism's body. Other indirect traces such as burrows, footprints, and nests are more obviously ichnofossils.

Fossil Age and Location

• An ichnofossil conveys the date and location at which an animal was present. The trace fossil can only be formed when the earth was soft and pliable and is later fossilized when heat and pressure solidify the rock. Therefore, if the age of the rock layer can be verified, paleontologists can infer when the animal was alive.

  Likewise, the location of the fossil can also be inferred. For instance, if nearby body fossils belong to aquatic animals, the trace fossils must have been on the bottom of a sea or lake. Sometimes, the consistency of the soil (such as wet mud versus dry ash) can be determined based on the depth and displacement made by footprints. This can provide valuable information about the ecosystem in which the animal lived in addition to the approximate geographic distribution of the animal.

Size, Speed, and Motion

• Ichnofossils are the only direct evidence we have of what extinct animals did while they were alive. The tracks left behind by various species of dinosaur reveal quite a bit about the way the animal moved. Tracks indicate whether it was swimming, wading, or on dry land, whether it walked with splayed or straight legs, and whether it walked or ran.

  An excellent example of such trace fossils are the Laetoli footprints in Tanzania. Before the discovery of these footprints, it was hotly debated whether Australopithecines, a genus of ancient hominids predating modern humans, could walk fully upright. The discovery of these 3.6 million year old fossil tracks demonstrated that these organisms were capable of fully upright locomotion.

Burrows, Tracks, and the First Animal Life

• Before the evolution of hard body parts in the Cambrian era around 530 million years ago, animals were small and soft-bodied. Consequently, almost no body fossils are found in the Precambrian strata. Nevertheless, paleontologists know a bit about the organisms that populated the sea floor at that time.

  The first burrows are found in rock layers over a billion years old. They were made by organisms that apparently fed on the detritus beneath microbial mats on the seabed. Later tracks and burrows more definitively belong to animals, showing telltale signs of crawling and sinusoidal slithering motions. The sudden appearance of distinct types of animal burrows beginning in the late Precambrian around 570 million years ago is so distinctive that rock strata can often be dated based upon the trace fossils they contain.