Redescription of the trace fossil Gyrolithes and taxonomic evaluation of Thalassinoides, Ophiomorpha, and Spongeliomorpha

The branching burrow systems Thalassinoides and Ophiomorpha vary widely in morphological detail. Individual burrow components otherwise characteristic of these two genera may in fact occur together in the same system, and may further intergrade with the spiral burrow Gyrolithes (redescribed herein). Nevertheless, the predominant morphological traits within each ichnogenus tend to be distinctive, and most specimens can be identified ichnogenerically with relative ease. Thus, despite a recent suggestion to the contrary, these ichnogenera are not strict synonyms; such variations are to be expected among trace fossil taxa. Furthermore, extensive taxonomic revision would undermine the present stability and proven usefullness of these ichnogeneric concepts in field application and environmental reconstruction. The ichnogenus Spongeliomorpha is so ill-defined that the name should be abandoned, regardless of whether one wishes to "split" or "lump" the related burrow ichnogenera.