The evolutionary significance of a Lower Cambrian trace-fossil assemblage from the Meguma terrane, Nova Scotia

We report a diverse assemblage of trace fossils from the Lower(?) Cambrian High Head member of the Church Point Formation near Yarmouth, Nova Scotia. Based on the presence of sharp-based beds, load casts, and flute casts, along with other sedimentological criteria, the strata are interpreted to have a turbidite origin. Other characteristics, including wrinkle structures and high fidelity of trace-fossil preservation, suggest that the inter-episode biotope was influenced by the presence of biomats. The trace-fossil assemblage includes (in approximate order of abundance) Planolites, Helminthopsis, Oldhamia, Chondrites, Gordia, Cladichnus, Psammichnites gigas, Treptichnus, Phycodes, Lorenzinia, Palaeophycus, and Teichichnus. Trace fossils are increasingly abundant upwards in the section, but they are ultimately mitigated by an increase in (inferred) sedimentation rates. The trace-fossil assemblage, which bears many similarities to other Lower Cambrian locales (e.g., Puncoviscana Formation of northwest Argentina, Chapel Island Formation of southeastern Newfoundland), and the distribution of ichnofossils strongly support an earliest Cambrian age for the studied strata. More importantly, evolutionary trends can be interpreted from the trace-fossil assemblage, namely (1) an increase in size and diversity of animals in deep water; (2) an expansion of motile strategies used to coexist with biomats; and, (3) increasingly complex mining strategies below the sediment-biomat interface. Nous signalons l'existence d'un assemblage varie; d'ichnofossiles dans le membre cambrien infeerieur (?) de High Head de la Formation de Church Point, pres de Yarmouth, en Nouvelle-Ecosse. A la lumiere de la presence de lits dont la base est nettement definie, d'empreintes de charge et de flutes, ainsi que d'autres criteres sedimentologiques, les strates sont interpretees comme etant d'origine turbiditique. D'autres caracteeristiques, dont la presence de rides et le degre eleve de detail preserve des ichnofossiles, suggerent que le biotope inter-episode etait influence par la presence de tapis biologiques. L'assemblage d'ichnofossiles comprend (en ordre approximatif d'abondance) Planolites, Helminthopsis, Oldhamia, Chondrites, Gordia, Cladichnus, Psammichnites gigas, Treptichnus, Phycodes, Lorenzinia, Palaeophycus et Teichichnus. L'abondance des ichnofossiles augmente vers le haut de la section, bien que cette augmentation soit ultimement contrecarree par une augmentation des taux de sedimentation (presumes). L'assemblage, qui ressemble a plusieurs egards aux assemblages d'ichnofossiles d'autres localues du Cambrien inferieur (p. ex. la Formation de Puncoviscana, du nordouest de l'Argentine, la Formation de Chapel Island du sud-est de Terre-Neuve), et la distribution des ichnofossiles appuient fortement la these d'un age du tout debut du Cambrien pour les strates etudieees. Fait encore plus important, l'assemblage d'ichnofossiles permet de deduire diffeerentes tendances evolutionnaires, a savoir : (1) l'augmentation de la taille et de la diversite des animaux en eau profonde, (2) la mise au point de nouvelles strategies motiles permettant de coexister avec les tapis biologiques et (3) l'augmentation de la complexite des strategies de forage sous l'interface sediments- tapis biologique.
Introduction Because of the paucity of body fossils in the Lower Cambrian, the biostratigraphic indicator of the start of the Lower Cambrian System is placed at the base of the [...]