Sclerite ray canals in the Cambrian coeloscleritophoran Chancelloria from North Greenland (Laurentia)

A sclerite of Chancelloria from the Henson Gletscher Formation (Cambrian Stage 4) of North Greenland preserves phosphatised central ray canals within the sclerite rays, confirming the extension of epithelial tissue into the sclerite interior. The canals fill the basal foramina and extend as thin central tubes within the lumen towards the apex of the individual rays, developing longitudinal ridges distally. In general terms they resemble the robust central canal of Sinosachites (= Thambetolepis), although the leaf-like sclerites of this halkieriid develop an extensive pattern of lateral canals and tubules not seen in the conical rays of Chancelloria. This internal similarity between chancelloriid and halkieriid calcareous sclerites adds support to the notion that coeloscleritophores are a paraphyletic group in which parallel episodes of calcareous mineralisation of pre-bilaterian and bilaterian coeloscleritophoran stocks took place in the early Cambrian from a sclerotised but non-mineralised, chancelloriid-like, eumetazoan ancestor.