The first question to be asked in this context is usually: is this a trace fossil at all? Nature often deceives and can produce features that, at first glance, look uncannily like the work of something living, but which are, in fact, just tricks of sediment erosion, transport and deposition. The example of Dinocochlea ingens (image, top right) serves a warning. Long thought of as a giant gastropod, this feature in the Cretaceous rocks near Hastings, England, was later reclassified as an ichnofossil. More recently, still, however, detailed analysis of the material showed that it is, in fact, a series of concretions, formed post-depositionally (but pre-compaction) by crystallisation of calcite. Even the experts can be fooled!
To avoid such errors, we need to study the specimens carefully, and use all the clues we can. Key characteristics that can be used for this purpose include not only the size, shape and fine detail of the morphology of the fossil, but also the setting in which it occurs (e.g. the type of rock, sediment size, bedding). Does it conform to the sedimentary structure and patterns in the rock (in which case it is probably a depositional feature) or is it in some way imposed upon them? Likewise, is the trace fossil integral - i.e. does it all fit together coherently - or is it just a random assemblage of features? Repetition is also a good clue. Find one in isolation, and doubts might legitimately remain. Find a number, in an organised group, and we can be more certain that it is real. If we are sure that it is an ichnofossil, the next step is to try and give it a name, so that it can be related to other trace fossils and something about its origin inferred. As with true fossils, the names assigned comprise a Latin binomial with the 'genus' first (the ichnogenus), followed by the 'species' (the ichnospecies). The names, of course, do not relate directly to the actual organisms that created the trace fossils, but to the features thremselves. Classification can be done in different ways, but probably the most widespread approach is the Seilacher system, developed by a palantaeologist of that name. In its earliest form, this recognised five main groups of ichnofossil, based on their inferred behaviour (ethology):
The figures to the right give examples of several of these categories. Since this classification was first proposed, however, it has been extended and refined with a dozen or more additional classes of ichnofossil. A more complete glossary of trace fossil classifications is given here. It needs to be acknowledged that applying this classification isn't always easy: it requires inferences to be made about the behaviour of the organism involved, and this requires care. What makes the process even harder is the lack of a hierarchical basis to the classification, like the evolutionary one used in biological taxonomy. As a result the relationships between individual 'taxa' is not clear, and the basis for linking one find to another is uncertain. The consequence has been a proliferation of new names every time a new find is made. Faced with a find, the amateur geologist has a huge task ahaead to match it to previously described fossils and find an appropriate name. Comprehensive guides to trace fossil classification have been written which might help in this endeavour, but unfortunately most are extremely expensive or no longer in print. For now, the best available guide is probably Karalinska University's ichnology website: this mammoth effort (excuse the pun) provides some wonderful material though sadly it is, as yet, far from complete. A useful introduction to ichnology is also available here. |
Dinocochlea ingens: an assumed fossil or ichnofossil that has now been identified as a series of concretions
Some features remain enigmatic: a trace fossil (Zoophycos) or a sedimentary feature?
Fodinichnia: worm burrows (note the characteristic menisci in each burrow)
Repichnia: Ignotornis sp.: footprints of an assumed shorebird
Domichnia: Amphorichnus papillatus, an ichnofossil of a limestone filled boring
Repichnia: The track of a trilobite across the surface, with the fossil of the organism at the end (left)
|