Paper No. 6
Presentation Time: 10:00 AM
A REEXAMINATION OF YUKNESSIA WALCOTT, 1919 FROM THE CAMBRIAN OF BRITISH COLUMBIA AND UTAH
Charles Walcott described Yuknessia simplex in 1919 as a monotypic genus of possible algal affinity based on three specimens from the middle Cambrian (Series 3, Stage 5) Burgess Shale Formation. Additional specimens have been assigned to this taxon from a number of Cambrian Burgess Shale-type deposits, but because of the lack of detailed knowledge of the type material, such identifications remain tentative. Here, we report the results of a comprehensive morphological and taphonomic restudy of Walcott’s type material of Y. simplex and specimens previously referred to this taxon from the middle Cambrian of Utah, details of which provide new insights into the phylogenetic affinity, ecology, and taphonomy of this enigmatic taxon. Scanning electron microscopic (SEM) investigation of the holotype specimen, which comes from the Trilobite Beds on Mount Stephen, and all Utah material reveals banding interpreted as fusellar structure, including oblique sutures for the Utah material. On this basis, the taxon is reinterpreted as a benthic pterobranch. Considered as such, Y. simplex is similar to several taxa described as rhabdopleurids from the middle Cambrian, and is the second pterobranch species recorded from the Burgess Shale Formation. The two paratypes, which come from the Walcott Quarry on Fossil Ridge, are reinterpreted respectively as Yuknessia? sp. Walcott, 1919, and Dalyia racemata Walcott, a putative red alga. While zooids or other definitive aspects of soft tissue anatomy have not been preserved in any the material observed, other finely-preserved features of the Utah specimens, expressed along the kerogenization-aluminosilicification-pyritization taphonomic gradient, provide critical new details about coenecium ultrastructure and morphology.