Trace Fossils — Introduction to Paleontology (Part VII)

Trace Fossils

Trace fossils preserve evidence of the activities of organisms in the past, such as footprints, scratch marks, bite marks, feeding traces, burrows, and feces. Most trace fossils are not easily recognized. In the early days of paleontology, many elongated trace fossils were mistakenly interpreted as fossilized seaweeds or worms, and numerous dinosaur footprints were once thought to have been left by giant birds.

Modern research on trace fossils largely began with the work of the German paleontologist Adolf Seilacher. He developed a classification system for trace fossils based on behavior and also discovered that, when considered together with the characteristics of sedimentary rocks, trace fossils can reveal the environmental conditions in which they were formed. As research on trace fossils has expanded, scientists have gradually recognized that these fossils can provide substantial information. This has led to the development of a specialized field known as ichnology, the study of trace fossils.

Living organisms are classified according to hierarchical ranks such as kingdom, phylum, class, order, family, genus, and species. Trace fossils have a corresponding classification system with categories such as ichnogenus and ichnospecies. Each trace fossil therefore has its own scientific name. However, the naming and classification of trace fossils are based solely on the characteristics of the trace itself and are not directly connected to the organism that produced it. In many cases, the species responsible for the trace fossil remains unknown.

For example, the U-shaped burrow trace fossil Arenicolites (Figure 1) was named after the annelid genus Arenicola, yet there is no confirmed relationship between the two, and we cannot determine which organism actually produced the trace. In some cases, animal footprints may possibly be linked to their makers. For instance, the ichnogenus Iguanodonichnus has been suggested to have been produced by the dinosaur Iguanodon, and the trace fossil was named accordingly. However, no one can be completely certain that Iguanodon actually produced these tracks, and some researchers have even raised objections. A single species may produce several different kinds of traces; for example, the trace fossils Nereites, Scalarituba, and Neonereites may all have been produced by the same organism. Conversely, a single type of trace may be produced by many species. The trace fossil Rusophycus (Figure 2), for instance, can be created by polychaete worms, notostracans, or trilobites. Because of this high level of uncertainty, most paleontologists believe that trace fossils should not be named after the organisms presumed to have produced them, as doing so may create confusion if later interpretations change. Therefore, the naming and classification of trace fossils must rely only on the appearance, morphology, and structural characteristics of the trace itself.

（Figure 1）Arenicolites（圖片來源：Tatsuo Oji, Stephen Q. Dornbos, Keigo Yada, Hitoshi Hasegawa, Sersmaa Gonchigdorj, Takafumi Mochizuki, Hideko Takayanagi, Yasufumi Iryu，採用 CC BY 4.0 授權）

（Figure 2）Rusophycus（圖片來源：Wilson44691，CC0 1.0 公共領域）

Seilacher divided trace fossils into two preservation types. Those formed within sediment are called endogenic traces, such as burrows or borings created by organisms. Those formed on the surface are called exogenic traces, such as footprints (Figure 3). Well-preserved endogenic traces may show a three-dimensional structure and are referred to as full relief. If only part of the trace is exposed, or if erosion has removed some of it, the fossil is called semirelief. If the upper portion is preserved, it is known as epirelief, whereas preservation of the lower portion is called hyporelief. Hyporelief fossils are commonly found in rock layers where sandstone and mudstone alternate, because mudstone erodes more easily while parts of the trace remain preserved in sandstone. Exogenic traces lie on the surface and are therefore always preserved as semirelief, also divided into epirelief and hyporelief. When an animal steps on mud or sand, it forms not only a depression on the surface but also deformation in the sediment below due to pressure. This subsurface deformation forms what is known as an undertrack, which may reveal information about the weight of the organism or the force applied during walking, making it an important subject in trace fossil studies.

（Figure 3）Endogenic and exogenic trace fossils（水也佑製作）

（Figure 4）Track and undertrack（水也佑製作）

Seilacher classified trace fossils into seven behavioral categories (Figure 5):

1. Repichnia – traces produced when an organism moves from one location to another.

2. Pascichnia – traces formed while an organism feeds as it moves forward, often producing winding or irregular pathways.

3. Agrichnia – traces produced by organisms that remain fixed in one place while capturing food or cultivating symbiotic organisms around them, such as sea anemones or corals.

4. Fodinichnia – traces produced by organisms digging through sediment in search of food, such as earthworms.

5. Domichnia – traces of dwelling structures created by organisms, such as burrows.

6. Fugichnia – escape traces produced when organisms living in sediment rapidly burrow to avoid predators.

7. Cubichnia – resting traces produced when organisms settle temporarily in one position.

（Figure 5）Trace fossils classified by behavioral types（感謝Massimilianogalardi、Falconaumanni、Luke McGuff、Wilson44691提供）

Ichnofacies

The structure and properties of sedimentary rocks themselves can reflect the environmental conditions under which the sediments were formed. When trace fossils are examined together with sedimentary structures, they can reveal the environmental conditions in which organisms once lived. Based on this idea, Seilacher proposed the concept of ichnofacies.

For example, rocks containing the trace fossil genus Scoyenia display characteristics of terrestrial deposition and are primarily composed of sandstone, indicating environments associated with inland or freshwater conditions. Sedimentary rocks containing Psilonichnus consist of sand grains of varying sizes and relatively soft rock, suggesting coastal, bay, or estuarine environments. The table below briefly lists several ichnogenera and the depositional environments associated with their sedimentary rocks.

（水也佑製作）

The relationship between trace fossils and time

In most cases, trace fossils provide very little information about geological time. A single impression or burrow usually cannot indicate a precise age, so trace fossils are rarely used as chronological markers. However, there are notable exceptions. The timing of the Cambrian explosion has long been debated because body fossils and trace fossils suggest different timeframes. Originally, the beginning of the Cambrian period was defined by the first appearance of trilobite body fossils. However, trace fossils attributed to trilobites appear earlier than these body fossils, including ichnogenera such as Monomorphichnus, Rusophycus, Cruziana, and Diplichnites.

Even deeper layers contain a diverse record of trace fossils, including Archaeonassa, Helminthoidichnites, Helminthorhaphe, and Treptichnus. Among these, the trace fossil Treptichnus pedum is considered the most reliable indicator of the Cambrian explosion. Consequently, scientists later defined the earliest occurrence of this trace fossil as the beginning of the Cambrian, pushing the starting point of the Cambrian explosion back by approximately twenty million years.

Trace fossils can sometimes also reveal evolutionary trends. For example, the feeding trace fossil Nereites and the farming trace fossil Paleodictyon tend to become smaller and more regular through time, which may reflect increasing efficiency in feeding behavior.

Although trace fossils often involve considerable uncertainty, ongoing research has demonstrated that they contain a great deal of useful information. In some cases, they preserve key records even more effectively than body fossils. As mentioned earlier, Treptichnus pedum has been used as an indicator of the Cambrian explosion. For this reason, the scientific importance of trace fossils should not be underestimated.

Author: Shui-Ye You