Delving into the early Cambrian Cranbrook Lagerstätte of western Canada: an ichnologic analysis

The lower Cambrian Cranbrook Burgess Shale-type Lagerstätte (Series 2, Stage 4) occurs within the Eager Formation of southeastern British Columbia, Canada. This deposit consists of claystone intercalated with normally graded mudstone, which has medium silt to very fine-grained sand and disarticulated skeletal debris near the bed bases, recording mostly accumulation from turbidity currents. Well known for its olenelloid trilobites in particular, the site also contains rare soft-bodied fossils, including Tuzoia carapaces and Anomalocaris claws, and low-diversity and low-density suites of ichnofossils. A 2015 systematic field investigation of a 2.65 m interval recovered 12 ichnotaxa and three types of trace fossils left in open nomenclature. The most abundant are Helminthoidichnites tenuis Fitch, 1850, Palaeophycus tubularis Hall, 1847, Diplocraterion isp., and finger-like structures (FLS). These trace fossils form two ichnocoenoses recurring through the studied interval: (1) the Helminthoidichnites tenuis ichnocoenosis, consisting of small trails and shallow burrows, and (2) the Diplocraterion isp. ichnocoenosis, consisting of paucispecific suites of U-shaped burrows, commonly associated with FLS. The FLS contain a diverse range of disarticulated or broken trilobite sclerites and grains coarser than the host sediment and are interpreted as passively infilled burrows, suggesting significant sediment bypass and trapping of transported grains. Overall, the Cranbrook ichnocoenoses do not display a well-defined tiering structure. The trace fossils overall record the activities of a surficial epifauna and a shallow-tier infauna that colonized the sea bottoms during short windows between episodic flows and inhabited dysoxic (Helminthodichnites tenuis ichnocoenosis) to relatively well-oxygenated (Diplocraterion isp. ichnocoenosis) outer shelf environments.