A New Stalked Filter Feeder From The S7 Burgess Shale Community, Mount Stephen  

The S7 locality on Mount Stephen (Campsite Cliff Member of the Burgess Shale Formation) was discovered by the Royal Ontario Museum in the early 1980s and is currently thought to be comparable in age to the Trilobite Beds.  The S7 community is dominated by an undescribed, gregarious, stalked, epibenthic organism nicknamed the “Tulip animal”. The name refers to its shape; a large cup-shaped calyx connects to a long, narrow stem.  This study is the first detailed description of this new organism.  It is based on observations of over 700 specimens, mostly laterally preserved, found both as isolated individuals and as large clusters (65+) on single slabs.  The abundance of specimens on single surfaces, and the absence of extensive evidence of decay, suggest preservation by the smothering of live communities with little transportation. 

Specimens range in length from 50 to 250 mm (including the stem) and 16 to 59 mm in width (at the calyx). Internally, the calyx contains a sac-like stomach, followed by an intestinal tract that extends to the top of the calyx, terminating in a central opening interpreted to represent the anus.  The stomach is arranged over a rigid conical structure, which gives a triangular shape to the base of the calyx in lateral specimens.  Surrounding the stomach are six radially symmetrical comb-like elements, each consisting of a central spine, from which two rows of tooth-like projections extend towards the centre of the calyx.  These comb-like elements extend away and taper towards the top of the calyx.  They are interpreted to represent a filter feeding apparatus, which captured food particles and moved them downwards into the stomach, presumably via a central mouth.  The calyx is enclosed by a flexible outer sheath which folds in around the comb-like elements.  The stem has a uniform width and is generally preserved straight.  The stem is composed of two layers: an inner rigid core surrounded by a soft deformable outer sheath.  The stem terminates in a small holdfast, which varies in shape from a flattened disc to a bulb; it often has granular elements at its base. 

This animal occupies one of the highest epibenthic tiers of all Burgess Shale-type deposits and provides evidence of a complex tiering structure in the S7 community.  The biological affinities of the “Tulip animal” remain unclear.  Due to the morphological limitations imposed by their mode of life, convergence in form is common among sessile filter feeders (e.g., Cnidaria, Entoprocta, Ectoprocta). The body of the “Tulip animal” does not compare well with extant stalked organisms.  The stalk and calyx is reminiscent of Dinomischus, a small, solitary species known from several Burgess Shale-type localities in Canada, including S7, as well as China; but the internal morphology is not comparable, and these animals are probably not directly related.  A comparison with stalked Ediacaran organisms is worth considering, despite inherent difficulties in interpreting morphologies preserved in two different taphonomic modes.