Trace Fossils
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Fossils can be defined as the preserved remains of the plants, animals or any living organism belonging to prehistoric period. But the Trace fossils can be defined as a mark left by the organisms at the location as fossils, in the form of  footprints, tunnels or burrows or feces, but not the fossil of the organism itself.  The Trace fossils give us evidences of living organism in the geological period. Many organisms like bacteria, plants, Protists (eukaryotic microorganisms,) and animals have left these  evidences. Each and every Trace fossil is different and it is a bit difficult to estimate the origin of the animal that made the particular trace. Usually the trace fossils are found in the soft sediments, which are undisturbed for years and has gradually turned into a rock. The Trace fossils have been found, dated back to the end of the Precambrian era, and it is believed to be the first trace fossil occurrence. 

From the Ediacaran or Vendian era, the first fossil of burrowing trace has been found. These burrow fossils are approximately 560 million years old. The burrows and other traces found during this era were basically horizontal, and were found on or just below the surface of the sea. Hence it is believed that such burrows are created by single-celled organisms that are capable of motion, which is also referred to as motile organisms. These organisms can create such traces by their heads, and can be probably from the Bilateria group. From this type of traces, it was concluded that the organisms probably feed above the earth's surface, and they burrow themselves to protect themselves from the predators. Many new types of Trace fossils were discovered, belonging to the end of the Cambrian period, which included vertical type of burrows and new traces. Those new traces pointed towards the Arthopods. The oldest trackways of the arthropods have been dated back to the ancient Cambro- Ordovician era.

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Classification of Trace fossils
Practically, it is very difficult to recognize the makers of the traces. Many similar looking Trace fossils have been confused to be created by the same species, where the actual organisms belong to two different genes. Only a very few tracks could be properly linked to their makers, hence 5 different behavioral models are used to classify the Trace fossils

Repichnia is a mode of behavior in which the organisms create traces of single footprints, traces of crawling; several tracks created from one end of the path to the other. This mode also includes the traces formed by slithering organisms.

This mode is referred to the traces formed while grazing over the sediments. When the herbivorous organisms dig through the sediments or soil for their food, they leave behind different patterns of trace. Generally, these fossils are seen near the intersection area of water and sediment.

Domichnia mode is seen where the organisms make big tunnels or holes with an intention to create a shelter for temporary or permanent period, depending on the nature of the species. They also make a shelter, with an intention to create a safe place while dwelling or grazing. These traces are usually made in a U-shape and can be seen in a vertical position.

Cubichnia traces appear in  locations where the organisms simply rest in one position for a long time. These traces are usually formed on the soft sediments, which turns into a fossil after many years.

Fodinichnia mode of trace is created by the deposit feeders or the organisms feeding through the sediments. Usually a three dimensional shape is left by those organisms.

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