DISCOVERY OF A NEW ‘PHYLLOPOD BED’ -LIKE ASSEMBLAGE FROM THE BURGESS SHALE, CANADIAN ROCKIES

The fossils of the Burgess Shale have transformed our understanding of the Cambrian explosion. While Burgess Shale-type deposits are known from Cambrian strata worldwide, the abundance and diversity of soft-bodied faunas from Walcott’s Quarry, and its famous “phyllopod bed,” remain unmatched.

Here, we report the discovery of a new phyllopod bed-like assemblage from Kootenay National Park, ca. 40 km southeast of the Walcott Quarry. The assemblage, discovered in 2012, occurs at the top of the Burgess Shale Formation and is significantly younger than the localities of the type area. In situ excavation and talus collections from a two-meter thick interval have so far yielded 3053 specimens representing at least 52 taxa. Among these, half are known from the Walcott Quarry and at least 15 are new. The abundance and diversity of soft-bodied organisms, in particular the arthropods, is matched only by the Walcott Quarry. Arthropods representing 34 taxa include, Liangshanella, Marrella, Misszhouia, Molaria, Naraoia, Sidneyia, a new leanchoiliid, and at least two new bivalved taxa. Several taxa previously unique to the Walcott Quarry, such as the rare chordate Metaspriggina and the putative hemichordate Oesia, are common in the new locality and preserve as yet undocumented features. Motile organisms are dominant but benthic sessile organisms are very rare. Rarefaction curves for arthropods show remarkable similarity to those of the Walcott Quarry, and their steepness suggests that further collecting should yield many additional species.

Regional stratigraphic evidence confirms that the depositional environment lay in close proximity to a prominent submarine escarpment. Together, ecological, taphonomic and sedimentologic evidence indicate that the biota was entombed by periodic high energy depositional events under fluctuating benthic redox conditions. Ichnofossils dominated by shallow-tier forms occur in a few horizons within the studied interval, and suggests episodes of increased oxygenation.

This discovery not only complements our understanding of the diversity and ecology of the Burgess Shale fauna and the affinities and range distributions of many taxa, but also provides new information on variation in community structure and the paleoenvironmental settings of the Burgess Shale.