Modular skeletons from the Mural Formation (late early Cambrian), Canadian Rocky Mountains: possible hypercalcified sponges

Abstract Macroscopic, modular, morphologically simple skeletons occur in the uppermost Mural Formation (Cambrian, Epoch 2, Bonnia – Olenellus Biozone), west-central Alberta and adjacent east-central British Columbia. They represent organisms that lived almost exclusively in reefal environments dominated by archaeocyaths. Some were attached to archaeocyaths or less commonly other surfaces, and some grew downward, apparently from overhangs or cavities in reefs. Qualitative and quantitative data from a large number of specimens, most of which were serially thin sectioned, indicate that they represent a single, remarkably variable species. The skeletal structure ranges among specimens from entirely cerioid to partially to entirely labyrinthine with irregularly incomplete walls. There is also a wide range of variability in growth form among skeletons, in module size and wall thickness among and within skeletons, in module shape within skeletons, and in number and location of projections extending from the wall into some modules. Module increase occurred by peripheral expansion at the basal surface of the skeleton and longitudinal fission involving projections from the wall as module size increased during vertical growth. Walls of skeletons, now composed of calcite cement, were probably originally aragonite. Modular skeletons from the uppermost Mural Formation are assigned to Rosellatana jamesi Kobluk, 1984a, previously represented only by a few cerioid specimens from correlative strata in the Rosella Formation of north-central British Columbia. The skeletal structure and types of module increase in R. jamesi , and a few similar but less well-known Cambrian taxa from elsewhere in North America, suggest a general biologic affinity with hypercalcified sponges.