Trace fossils or ichnofossils represent the effects of organismal activity upon or in the substrate. Tracks, trails and the like are the most commonly encountered traces. A distinction can therefore be made between body fossils, which are actual remains of organisms, and trace fossils which represent an indication of an organism's behavioral activity.
Trace fossils, though not preserving the body or necessarily the morphology of the original organism, do have certain advantages over body fossils. In general:
Trace fossils may be preserved in a number of reliefs. They may be preserved in actual 3-dimensional relief, within sediment or sometimes the traces become filled in by a more resistant mineral and are subsequently eroded out of the surrounding sediment in full relief. More often, there is partial preservation caused by the movement of the tracemaker in and out of the depositional interface. These semireliefs may occur on the upper surface of a bed (concave epireliefs, or their casts, convex hyporeliefs), or on the underside of a bed (concave hyporelief). What would you call a ridge or hill of sediment, obvious on the surface of a bed, made by an infaunal burrower (see Figure 3.1).
3.1.1 Terminology
Listed below are a number of terms used in the description of trace fossils. Become familiar with them. Also listed are some important ichnogenera. Examples of which will be available in the lab.
Figure 3.1 Types of preservation of trace fossils. The dark stippling indicates mud; the light stippling indicates silt or fine sand (Redrawn form Prothero, 1998; adapted from Seilacher, 1964)
| Terms | Important Ichnogenera | |
| ichnology ichnofossil ichnogenera ichnospecies cubichnia fodichnia repichnia domichnia pascichnia fugichnia | epichnia endichnia hypichnia spreite coprolites gastroliths stromatolites | Arthrophycus Asteriactites Chondrites Cruziana Rusophycus Skolithos Zoophycos |
3.1.2 Types of trace fossils
The major categories of trace fossil ethological classes are described below. If examples are available, you should go through this section while examining the relevant specimens. Also, refer to Table 3.1 and Figure 3.2 for interactive summaries of these categories.
Figure 3.2: Common ichnofacies and examples of the trace fossils that occur in them. 1. Koupichniurn (horseshoe crab tracks); 2. Isopodichnius; 3. borings of Polydora, a polychaete; 4. Entobia, clionid borings; 5. echinoid borings; 6. algal borings; 7. pholadid bivalve borings; 8. Diplocraterion; 9. unlined crab burrow; 10. Skolithos; 11. Thalassinoides; 12. Diplocraterion; 13. Ophiomorpha; 14. Arenicolites; 15. Phycodes; 16. Rhizocorallium; 17. Teichichnus; 18. Diplichnites (trilobite tracks); 19. Cruziana; 20. Rusophycus; 21. Ateriacites; 22. Zoophycos; 23. Lorenzinia; 24. Paleodictyon; 25. Taphrhelminthopsis; 26. Heminthoidia; 27. Spiroraphe; 28. Cosmoraphe. (Redrawn from Prothero, 1998, modified from Ekdale et al., 1984)
Table 3.l. Ethological classification of trace fossils. (Adapted from Frey, 1978)
| Categories of Ichnofossils | Definition | Characteristic morphology |
| Resting traces (Cubichnia) | Shallow depressions made by animals | Troughlike relief, recording to some extent the that temporarily settle onto, or dig lateroventral morphology of the animal; on into, the substrate surface; emphasis structures isolated, ideally, but may intergrade.reclusion with crawling traces or escape structures |
| Crawling traces (Repichnia) | Trackways, surficial trails, and shallow borrows, emphasis on locomotion, | Linear or sinuous overall structures, some traces horizontal structures made by organisms traveling from one place to another; branched; footprints or continuous grooves, commonly annulated; complete form may be preserved or may appear as cleavage reliefs. |
| Grazing traces (Pascichnia) | Grooves, pits and furrows, many of them discontinuous, made by mobile deposit feeders at or near the substrate surface; emphasis on feeding | Unbranched, nonoverlapping, curved to tightly coiled patterns or delicately constructed spreiten dominate; patterns reflect maximum utilization of surficial feeding area; behavior analogous to "strip mining" complete form may be preserved. |
| Feeding traces (Fodichnia) | Temporary burrows constructed by deposit feeders; the structures may also provide shelter for the organisms- emphasis on feeding, behavior analogous to "underground mining" | Single, branched or unbranched, cylindrical to structures sinuous shafts or U-shaped burrows, or complex, parallel to concentric burrow repetitions (spreiten structures); walls not commonly lined, unless by mucus; oriented at various angles with respect to bedding; complete form may be preserved. |
| Dwelling traces (Domichnia) | Burrows or dwelling tubes providing more or less permanent domiciles, mostly for hemisessile suspension feeders, or in some cases, carnivores; emphasis on habitation | Simple, bifurcated, or U-shaped structures structures perpendicular or inclined at various angles to bedding, or branching burrow systems having vertical and horizontal components; walls typically lined; complete form may be preserved |
| Escape traces (Fugichnia) | Lebensspuren of various kinds modified or made anew by animals in direct response to substrate degradation or aggradation; emphasis on readjustment, animals upward or downward with respect to the original substrate surface; complete form may be preserved, especially in aggraded substrates | Vertically repetitive resting traces; biogenic structures laminae either in echelon or as nested funnels or chevrons; U-inU spreiten burrows; and other structures reflecting displacement of or equilibrium between relative substrate position and the configuration of contained traces |
Chapter 3: Questions
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