These include trails and burrows and other fossil evidence of the activities of once-living creatures. A tidal mud flat at low tide is an amazing sight, covered with innumerable trails left by clams, crabs, and worms and speckled with holes that are the tops of burrows of a multitude of worms and clams. Should this area dry sufficiently to harden, the next tide might not destroy these markings but instead gently cover them with a layer of mud. When turned into rock and split apart, these layers would reveal the trails and burrows of extinct organisms. Some of the oldest known fossils are burrows and trails, evidence that something crawled in Pre-Cambrian seas but never was preserved as a fossil.
Occasionally the nature of such a trail becomes clear through the lucky discovery of its fossilized creator, too. Jurassic horseshoe crabs have been found in Germany in this situation at the end of their last crawl.
In the early days of paleontology, ridgelike and tubelike swellings in rocks that showed definite indication of having been formed by something living were classified as marine algae. Because many showed definite
branching, this marine-algae theory gained support. Many generic names still end in -phycus, such as Arthrophycus or Dolatophycus, signifying algal origin. In the same way, such names as Fucoides, Algacites, and Chondrites cling to remains not now believed to have any connection with the plant kingdom.
Gradually these trace fossils became accepted as burrows and trails after their resemblance to those made by modern worms and pelecypods was noticed. Many are believed to be subsurface burrows that filled with a different sediment. This sediment now appears in relief on the surface of a slab. A thin, buried layer of sediment of different texture often created a plane of weakness that offered an easy path for the burrowing creatures. It also offered an easy parting plane when the rock was split some millions of years later by a fossil collector.
A single individual may have made a series of quite different markings in the sea bottom, depending on whether it was crawling, running, feeding, or burrowing. Thus it is difficult to relate these problematical fossils to their creator. The system proposed by Adolf Seilacher for classification of these fossils seems to be the most reasonable. He has grouped trace fossils by their ecological similarity whether their proprietors were similar or not. A tubelike burrow made by a trilobite is similar to one made by a worm. They cannot be told apart unless a very dead worm or trilobite is found at the end of one. So this system has merit. His five basic groups are:
1. Dwelling burrows: Tunnels made as living quarters, usually at right angles to the bedding of the layers, originally opening out on the surface.
2. Feeding burrows: Tunnel systems, originally dug below the surface, excavated by sediment-eaters while searching for food.
3. Feeding trails: Tunnels and bands that are extremely winding and very numerous, probably made on the surface, rarely crossing each other, made by organisms also in pursuit of food or on their way somewhere.
4. Resting trails: Isolated impressions with a vague outline of their producer, probably representing resting spots or nap sites.
5. Crawling trails: Showing variable direction and imprints of legs, usually made on the surface.
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