Small Cruziana, Rusophycus, and related ichnotaxa from eastern Canada: the nomenclatural debate and systematic ichnology

Research by the authors and colleagues in eastern Canada has uncovered, from both marine and nonmarine strata, a variety of trace fossils that have disputed nomenclature. Such are the problems that, elsewhere, other occurrences of each of these trace fossils has often resulted in assignment to Isopodichnus. In several of these cases, assignment has been based on criteria such as stratigraphy, environment, or phylogeny of the producer, although recent ichnological practice has stressed that trace fossils be named exclusively on morphological grounds. Morphological criteria have been utilized in the identification of hopodichnus, for example, the identification of a coffee‐bean‐shaped structure adjoining a bilobate ribbon structure (i.e., a compound specimen), or a one‐dimensional size parameter distinction (i.e., width of less than 5 mm). Unfortunately, such parameters have never been used consistently. Additionally, since compound ichnotaxa are intergradational trace fossils in which one ichnotaxon passes gradually, directly, or occasionally repeatedly into another ichnotaxon, recent ichnological practice again would have compound specimens of Isopodichnus now assigned as compound Cruziana and Rusophycus, or in some other cases as compound Cruziana and ’Diplichnites’ (sensu lato). Although modern practices provide for separate taxonomic treatment for each component of a particular compound form, other forms intermediate between Cruziana, Rusophycus, and related ichnotaxa (e.g., Didymaulichnus, ‘Diplichnites ‘, and Diplopodichnus), also exist To delimit the respective ichnotaxa, two‐dimensional shape criteria are herein set on the basis of width‐Iength and width/depth parameters. Ethological considerations should be subordinate to these morphological ichnotaxobases.The resulting taxonomic revisions include: reaffirmation that hopodichnus has no valid grounds for being retained as a distinctive and separate ichnogenus, emendments to the ichnogeneric diagnosis of Diplopodichnus, and reassignment of the ichnospecies (Beaconichnus) darwinum to Diplopodichnus biformis, (Rusichnites) acadicus to Cruziana acadica, and Isopodichnus stromnessi to Rusophycus stromnessi.