Redefining the Treptichnus pedum zone at the Global Boundary Stratotype Section and Point (GSSP): A critical reassessment of the Ediacaran-Cambrian boundary.

Abstract

The Cambrian explosion bisects the history of life, separating relatively simple pre-Cambrian life from a complex and diverse Cambrian fauna. Due to the paucity of the body-fossil record, trace fossils often offer the only available insight into these evolutionary milestones, representing a continuous record through the late Ediacaran-early Cambrian. The Chapel Island Formation (CIF) of the Burin Peninsula, Newfoundland, provides an exceptional record of these innovations. Currently, the Global Boundary Stratotype Section and Point (GSSP) for the basal Cambrian boundary is located 2.4 m above the base of member 2 of the CIF, delineated by the lowest observed appearance of the Treptichnus pedum Ichnofossil Assemblage Zone (IAZ). Currently, researchers are facing difficulties when attempting to correlate with a few sections worldwide, and a formal revision of the boundary has been proposed. We hypothesize that a revision of the ichnotaxonomy of the GSSP with an emphasis on trace fossil functional morphology may better illustrate evolutionary innovations at the Ediacaran-Cambrian boundary. In turn, this revised ichnotaxonomy may provide further support for the position of the Ediacaran-Cambrian GSSP. Through a bed-by-bed study, the ichnotaxonomy of the T. pedum IAZ was revised, and a stratigraphic section was measured. Twenty ichnospecies comprising thirteen ichnogenera were observed (Figure 0.1). The ichnospecies were grouped into five ichnoguilds, which were used to conduct an ecospace analysis of the section. The ichnofauna reveled a more gradual appearance of ichnofossil diversity at the boundary, and a more protracted transition between Ediacaran and Cambrian ecosystems than previously envisioned. The T. pedum IAZ in the CIF marks the appearance of novel methods of interacting with the substrate as documented by sub-horizontal branching burrows (treptichnids), equilibrium structures (Bergaueria isp.), and complex vertical burrows (Gyrolithes scintillus). Additionally, it marks the evolution of novel body plans, as revealed by the presence of arthropod scratch marks (Dimorphichnus cf. obliquus). However, remnants of Ediacaran matground ecology are also present. Farming feeding styles are utilized by the Gyrolithes scintillus ichnoguild, and mat grazing remains a common feeding style. These ichnotaxa provide valuable insight into the very beginnings of vertical, penetrative burrowing. They further illuminate the depth and rate at which this new lifestyle evolved, and shed light on the evolution of three-dimensional burrowing.