Burgess Shale Chancelloriids – A Prickly Problem  

Chancelloriids were discovered by Walcott and described in his 1920 monograph on Burgess Shale sponges.  The cactus-like organisms, with their characteristic bundles of sharp protruding spines, were likely candidates for a sponge affinity, but later research showed that the spines themselves were formed as dermal sclerites, rather than as spicules, by enveloping sclerocytes.  This important homology with sponges thus appears to be lacking.  Currently there is no consensus on the affinity of chancelloriids – the “sponge school” views them as calcareous sponges, in which the spines may possibly be homologous to sponge spicules, while the “coeloscleritophoran school” suggests that the sclerites represent a metazoan plesiomorphy also found in bilaterally-symmetrical early animals, such as halkieriids. 

Our restudy of Walcott’s material and of substantial new collections of Burgess Shale chancelloriids shows that three genera, Chancelloria, Allonnia and Archiasterella, are present among the material originally described by Walcott as a single species, Chancelloria eros. Walcott interpreted the varying ray arrangements of the sclerites as a taphonomic artefact, caused by different degrees of burial within a three-layered body wall.  However, this interpretation is flawed – as shown by numerous sclerites, both isolated and in position in the body.  Chancelloria has the highest variability of sclerites, whereas the sclerites of Allonnia and Archiasterella typically vary only in size, not in basic morphology.  An apical tuft of slender spines is present in all taxa.  It is particularly conspicuous in Allonnia, where the spines form a palisade around the central opening.  No other openings in the body wall have been observed; the integument between the sclerites has a rhombic texture of imbricating scales, recurring at 60–100 µm intervals.  No internal organs have been observed.  The presence of circular contractile tissue is indicated by an infrequent body contraction, which appears to have had the function of emptying the internal cavity.  The chancelloriid body was basally anchored to other organisms, usually sponges or other chancelloriids, or to shell debris in the soft sediment.  We explore alternative interpretations of chancelloriid biology and affinities, from stem-group metazoans to bilaterians.