Markus Bertling and Richard G. Bromley convened the Workshop on Ichnotaxonomy on 4 to 7 August 1998, so that current problems in trace-fossil classification could be resolved. Seven researchers attended the meeting, including Markus Bertling, Simon Braddy, Richard G. Bromley, Georges Demathieu, Kurt Nielsen, Andrew K. Rindsberg, and Alfred Uchman. In addition, several others participated by email. The venue was the Limensgade Mølle ("Lemon Street Mill"), a pension run by Ulla Asgaard near the geologic research station of the University of Copenhagen on Bornholm, an isolated island in the Baltic Sea. Many hours were spent in the well-appointed conference room, with occasional interruptions for delicious, wholesome meals and field trips to sites on the island.

A surprising degree of unanimity was reached on important problems, and the results are outlined here. We collaborated on an abstract of the solutions to these problems and published it on the PaleoNet and BioCode electronic listservers in fall 1998, With Markus Bertlingís permission, I have modified that abstract here.

Among the questions addressed at the Workshop were: What is a trace? Should ichnotaxa above the genus group be used, and if so, how? What criteria should be used to diagnose ichnotaxa? What should be done in cases where two ichnotaxa intergrade? How should poorly preserved material be dealt with? And finally, what recommendations should be made for revision of the International Code of Zoological Nomenclature (ICZN) and the BioCode? Our answers, although unanimous, are not the last word on the subject, and the participants welcome further comments and suggestions.

The current definition of "trace fossil" in the ICZN leaves it unclear whether some groups of objects are included. The WIT group discussed not only these groups, but also the criteria used for our decisions. Criteria included such considerations as:

• Does the structure fit the body of the producer precisely? Except for some borings, structures that fit the body so well as to act like an external skeleton might as well be considered as a body part.
• Is there evidence of mobility of the structure? Some animals construct a tube and then move it, suggesting, again, that these are body parts, not traces.
• Was the structure actively secreted? Skeletons are actively secreted; traces are generally made of sediment.
• Does the structure actively disturb the substrate? Surely disturbance of the substrate is one of the prime determinants of a trace.

These criteria obviously do not apply evenly to different kinds of structures, and in many cases the answers point in opposite directions. In the end, we had to make compromises based on practicability in several instances. In brief, we proposed, "Ichnotaxa are names for structures resulting from life activities of organisms. Examples are (among others) burrows, trails, tracks, nests, coprolites, galls, and bioerosion structures, but not parts or secretions of organisms (e.g., eggs, pearls, agglutinated body cases, pupal cases, gallstones) or the results of the activity of whole communities, e.g., stromatolites, palaeosols."

All agreed that it is time to utilize ichnofamilies and other taxonomic ranks higher than the ichnogenus, as the Code allows. A few have already been validly named. We further agreed that it is desirable to apply uniform taxonomic principles to all ranks of ichnotaxa. In order to be widely accepted, they must be based on the same principles as ichnospecies and ichnogenera. An ichnofamily of "traces made by crabs" is not acceptable, for instance.

The WIT group recommends that the Code be revised to allow the type principle to be applied to all ichnotaxa. Ichnospecies must have type specimens; ichnogenera must have type ichnospecies; and ichnofamilies must have type ichnogenera.

No new ichnotaxa should be based on distal (indistinct) transmitted traces such as undertracks, or on badly (penecontemporaneously) scoured material. Of course, ichnotaxa that already have been based on poor material are still valid. In cases where much better preserved material is discovered for previously named undertracks, the new material should be given a new name and the old ichnotaxon allowed to fall into disuse as a nomen dubium. Where the new material is only slightly better, it is preferable to emend the older ichnotaxon.

Information about undertraces and physical haloes or deformation caused by the tracemaker should be included in the description of a new ichnotaxon, but not in the diagnosis.

The Code currently allows only "fossil" type specimens for ichnospecies, and in general we agree with the exclusion of "modern" traces. We cannot define "fossil" and "modern" precisely, but offer some guidelines to determine which material is fossil. These include whether the host rock is lithified or unlithified, Pleistocene or Holocene, and occurring within the "historical layer" or within the actively bioturbated zone. The fossilization barrier is important, but is not easily pinned down!

Borings in lithified substrates are a special case, because they may be considered as fossilized immediately following the death of their makers. Insisting on a fossil holotype, which can hardly be better preserved than a modern specimen, may impede the pursuit of knowledge. Should the choice of a recent type specimen be allowed in erecting a new ichnospecies? Just for borings, or even for all traces? Comment is especially welcome on this vexing issue.

The most important ichnotaxobases (attributes, taxonomic criteria) are morphology and, with much less applicability, substrate. We thus strongly encourage the use of morphometric analysis to distinguish ichnospecies.

Substrate is important to distinguish burrows and borings of identical morphology, e.g., burrow Skolithos and boring Trypanites.

We recognize that borings in lithic substrates are fundamentally different from burrows in softgrounds and firmgrounds. Despite some possible uncertainty in few rare cases when the substrate cannot be identified, borings and burrows should be given different names even if they are identical morphologically. Xylic borings (borings in wood) are similarly distinct from lithic borings.

Coprolite ichnotaxa can depend on gross composition, e.g., wood, sediment, or fish scales. However, composition is not always a good determining criterion, as in the case of crocodile dung, which can include bird or fish remains.

Recognized ichnotaxobases are thus:

Shape (morphology), including:

• Overall shape (general form): course (path), cross-sectional outline, pattern, branching (presence/absence and type), etc.
• Internal structure: fabric (including sorting), texture, lamination, active vs. passive fill, type of active fill, spreite, lining
• Orientation
• Surface features (sculpture), e.g. for gastroliths

Substrate: applicable only in the case of xylic borings vs. lithic borings vs. others

Gross composition (lithology, constituents): mainly applicable to coprolites

Not useful as ichnotaxobases are:

• Producer biology: biological taxon, ontogeny, sex of producer, phylogeny (however, inferences about producers can help to select proper ichnotaxobases!)
• Diagenetic features
• Stratigraphic age
• Geographic location
• Type of passive fill, as opposed to presence/absence
• Taphonomic effects, including collapse and erosion features
• Substrate consistency (but not type; see discussion of borings above)

We also considered the problem of intergradation and contact of ichnotaxa. Ichnospecies of the same ichnogenus should be synonymized if one form grades into the other. The junior ichnospecific name may be lowered in rank to ichnosubspecies and assigned to the senior ichnospecies. However, intergradation or contact of traces belonging to different ichnogenera is not grounds for synonymy. Rather, in these cases, a notation such as "Thalassinoides suevicus X Ophiomorpha nodosa" should be used. Uniform principles of ichnotaxonomy should be applied to all fossil traces. In any case, the difference between mere contact and true intergradation should be emphasized, e.g. in the case of superimposed borings.

The Workshop on Ichnotaxonomy made considerable progress in dealing with the ambiguities remaining after the last revision of the Zoological Code in 1985. Although it is too late to have these suggestions incorporated into the impending fourth edition of this Code, it is not too late to consider our position for later revisions of the Zoological Code and/or BioCode. Ichnotaxonomists are strongly encouraged to make their voices heard as part of an ongoing consensus.

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