Treptichnus pedumand the Ediacaran–Cambrian boundary: significance and caveats

AbstractThe Ediacaran–Cambrian (E-C) boundary is based on the first appearance of the ichnofossilTreptichnus 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 thatTreptichnus pedumwas originally referred to asPhycodes pedumand, more recently, some authors have placed it inTrichophycusorManykodes. However, the overall geometry of these burrows indicates they belong inTreptichnus.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. pedumis 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 ofT. pedumin most E-C successions comprising shallow-marine deposits. Fourth, although treptichnids have been documented below the E-C boundary,T. pedumis not known from Ediacaran rocks. The delayed appearance ofT. pedumin E-C successions should be analysed on a case-by-case basis.