it was difficult to say whether chancellorids were members of the Eumetazoa, "true animals" whose tissues are organized into Germ layers: chancellorids' lack of internal organs would seem to exclude them from the Eumetazoa; but possibly chancellorids descended from Eumetazoans that lost these features after becoming sessile filter-feeders. There are intriguing hints that the Ediacaran genus Ausia may represent a halkieriid ancestor with strong similarity to the chancelloriids. The coelosclerites ("hollow sclerites") of halkieriids and chancelloriids resemble each other at all levels: both have an internal "pulp cavity" and a thin external organic layer; the walls are made of the same material, aragonite; the arrangement of the aragonite fibers is in each is the same, running mainly from base to tip but with each being closer to the surface at the end nearest the tip. It is extremely improbable that totally unrelated organisms could have developed such similar sclerites independently, but the huge difference in the structures of their bodies makes it hard to see how they could be closely related. This dilemma may be resolved in various ways: One possibility is that chancelloriids evolved from bilaterian ancestors but then adopted a sessile lifestyle and rapidly lost all unnecessary features. However the gut and other internal organs have not been lost in other bilaterians that lost their external bilateral symmetry, such as echinoderms, priapulids, and kinorhynchs. On the other hand, perhaps chancelloriids are similar to the organisms from which bilaterians evolved. That would imply that the earliest bilaterians had similar coelosclerites. However, there are no fossils of such sclerites before , while Kimberella from was almost certainly a bilaterian, but shows no evidence of sclerites. One solution to this dilemma may be that preservation of small shelly fossils by coatings of phosphate was common only for a relatively short time, during the Early Cambrian, and that coelosclerite-bearing organisms were alive several million years before and after the time of phosphatic preservation. In fact there are over 25 cases of phosphatic preservation between and , but only one between and . Alternatively, perhaps the common ancestor of both chancelloriids and halkieriids had very similar but unmineralized coelosclerites, and some intermediate groups independently incorporated aragonite into these very similar structures. See also Coeloscleritophoran Notes External links Palaeos' article on Halkieria & H. evangelista Pharyngula entry on Orthrozanclus reburrus Cambrian molluscs Cambrian invertebrates Cambrian animals of Africa Cambrian animals of Asia Cambrian animals