TREPTICHNUS AND THE NEED FOR A BIOLOGICALLY INFORMED CLASSIFICATION OF TRACE FOSSILS

Treptichnus is based on zigzag burrows that were originally described by S. A. Miller (1889) from Lower Pennsylvanian freshwater tidal flat deposits in Indiana. But in part W of the Treatise on Invertebrate Paleontology, a posthumous work, Häntzschel (1975) provided a different zigzag burrow from marine Ordovician strata as the search image for the ichnogenus. The ichnogenus was extended further by applying it to Lower Cambrian marine “Phycodes” or “Treptichnus” pedum. As this ichnospecies is now used to define the base of the Cambrian, its taxonomy, paleoecology, and stratigraphy are matters of broad interest.

The classification of trace fossils must be based on their morphology, because trace fossils consist of sediment and other abiologic materials, and tracemakers usually cannot be unambiguously deduced. However, deciding which morphologic criteria to choose for diagnoses is often a matter of debate among ichnologists. For the past 30 years, all zigzag burrows have been placed in Treptichnus. This geometrically based criterion makes Treptichnus easy to identify at the ichnogeneric level, but obscures the fact that different ichnospecies of Treptichnus occur in strata of very different ages and paleoenvironmental settings.

If the overall zigzag shape is set aside, it becomes clear that the forms attributed to Treptichnus fall into three groups that should be assigned to separate ichnogenera: (1) “T.” pedum and similar forms that branch irregularly and have relatively thick, bulging branches (latest Precambrian to Cambrian, shallow marine); (2) “feather-stitch” burrows that branch regularly deep in the sediment and whose branches are relatively thin and constant in diameter (Phanerozoic, shallow to deep marine); and (3) T. bifurcus and similar forms that branch irregularly at or near branch tips just below the sediment-water interface and have relatively thin branches of fairly constant diameter (Carboniferous, freshwater). Groups 1 and 2 have unknown makers but group 3 is made by insect larvae today. In such cases, it is preferable to select morphologic criteria that shed light on the maker, paleoenvironment, or stratigraphy of the trace fossils, rather than adhere to a strictly geometric approach.