Chancelloriidae

The Chancelloriids are an extinct family of superficially sponge-like animals common in sediments from the Early Cambrian to the early Late Cambrian. Many of these fossils consists only of spines and other fragments, and it is not certain that they belong to the same type of organism. Other specimens appear to be more complete and to represent sessile, radially symmetrical hollow bag-like organisms with a soft skin armored with star-shaped calcareous sclerites from which radiate sharp spines. Classifying the chancelloriids is difficult. Some paleontologists classify them as sponges, an idea which chancelloriids' sessile lifestyle and simple structure make plausible. Other proposals suggest that they were more advanced, or at least originated from more advanced ancestors; for example chancelloriids' skins appear to be much more complex than those of any sponge. It has been suggested that chancelloriids were related to the "chain mail" armored slug-like halkieriids, which are typically considered to be stem-group molluscs. While the sclerites of the two groups are very similar right down to the microscopic level, the large dissimilarity in the body plans of the two groups is difficult to reconcile with this hypothesis. The proposed clade containing the two groups, "Coeloscleritophora", is generally not thought to be monophyletic. Recent research has suggested that chancellorids represent an independent group of basal eumetazoans. Chancelloriid fossils have been found in many parts of the world, including various parts of Asia (e.g. Siberia, China, Mongolia), Australia's Georgina Basin, Canada's Burgess Shale, and the United States. The earliest known fossils come from the small shelly fossil assemblage of the Anabarites trisulcatus Zone of the Lower Nemakit-Daldynian Stage, Siberia and its analog in China is the Anabarites trisulcatus-Protohertzina anabarica Zone of the basal Meishucunian Stage. The fossil record suggests that chancelloriids declined rapidly during the Late Cambrian, and they were probably extinct by the end of the Cambrian.