SKELETAL MICROSTRUCTURE INDICATES A CLOSE RELATIONSHIP BETWEEN CHANCELLORIIDS AND OTHER COELOSCLERITOPHORANS

Chancelloriids were Cambrian animals that possessed a radially symmetric, sac-like body covered with an array of star-shaped sclerites. Their phylogenetic affinities are controversial; first interpreted as sponges, they were later united with halkieriids, wiwaxiids, sachitids, and siphogonuchitids to form the Coeloscleritophora, a group characterized by the possession of mineralized sclerites with a prominent internal cavity and restricted basal foramen. Because the chancelloriid body plan is so different from that of other Coeloscleritophora, however, their affinities with this group have never been well accepted.

A detailed study of skeletal microstructure in chancelloriids provides new support for their inclusion in the Coeloscleritophora. Halkieriid, sachitid, and siphogonuchitid sclerites exhibit a microstructure consisting of the following four characters: 1) a thin (<1 µm) organic sheet coating the entire sclerite; 2) an inner layer of longitudinally-oriented aragonitic fibers that are either parallel to the sclerite surface or inclined toward its distal tip; 3) bundles of aragonitic fibers that form projections -- lamellae, platelets, tubercles or spines – that are commonly inclined toward the sclerite's distal tip and give the sclerite's upper surface a ‘scaly' appearance; and 4) a lower sclerite surface on which the projections are absent. (Wiwaxiid sclerites are unmineralized, making comparison difficult, but their microstructure is broadly similar.) Sclerites of the chancelloriid Archiasterella hirundo possess all four of these characters; other chancelloriid species possess the first and second characters but lack projections (characters 3 and 4). With the possible exception of problematic Cambrian shells like Ernogia, Aegides, and Porcauricula, this microstructural theme appears to be unique to the Coeloscleritophora. Its restricted phylogenetic distribution and congruence with other sclerite characters suggest that skeletal microstructure is homologous in chancelloriids, halkieriids, sachitids, and siphogonuchitids, supporting a close relationship among these groups and suggesting that microstructure can be useful for determining relationships of Cambrian problematica.