Treptichnus pedum and the Ediacaran-Cambrian boundary: significance and caveats

The Ediacaran-Cambrian (E-C) boundary is based on the first appearance of the ichnofossil Treptichnus pedum. Investing an ichnotaxon with such biostratigraphic pre-eminence has been the focus of criticism. Points of contention have revolved around four main issues: (1) ichnotaxonomy, (2) behavioural significance, (3) facies controls and (4) stratigraphic occurrence. First, confusion results from the fact that Treptichnus pedum was originally referred to as Phycodes pedum and, more recently, some authors have placed it in Trichophycus or Manykodes. However, the overall geometry of these burrows indicates they belong in Treptichnus. Second, regardless of its precise mode of feeding, the behaviour involved is iconic of the Cambrian explosion. Third, objections are based on the idea that trace fossils show a closer link to facies than body fossils. Notably, in contrast to common assumptions, T. pedum is not only present in the low-energy offshore of wave-dominated marine settings, but it occurs at considerably shallower water in intertidal and shallow-subtidal zones of tide-dominated systems, as well as in mouth bars of deltaic systems and lower shoreface to offshore transition zones of wave-dominated marine settings. Its broad environmental tolerance supports evolutionary innovations rather than facies controls as the main mechanism underlying the observed vertical pattern of distribution of T. pedum in most E-C successions comprising shallow-marine deposits. Fourth, although treptichnids have been documented below the E-C boundary, T. pedum is not known from Ediacaran rocks. The delayed appearance of T. pedum in E-C successions should be analysed on a case-by-case basis.