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.

They were first described in 1920 by Charles Doolittle Walcott, who regarded them as one of the most primitive groups of sponges.

The chancelloriids had bag-like bodies with an orifice at the top, and show no evidence of internal organs. The different species show a variety of shapes and sizes, for example: Chancelloria eros was a slim cone with the narrow end at the bottom, typically 4 to 6 centimetres (1.6 to 2.4 in) long and 1.5 to 2 centimetres (0.59 to 0.79 in) in diameter at its widest point; Allonnia junyani formed a disk or cylinder usually 6 to 7 centimetres (2.4 to 2.8 in) in diameter, and the tallest were about 20 centimetres (7.9 in) long.

Most of the fossils consist of collections of mineralized hard parts called sclerites, and an assembly that is thought to have belonged to one individual is called a scleritome. Many specimens consist only of scattered sclerites, whose form is used to classify them, and some specimens have not yet been assigned to a species or even genus.

Individual sclerites had star-shaped bases that lay flat against the body and one spine projecting outwards at a right angle. The sclerites had internal cavities and in fact many are preserved as castings of the cavities filled with phosphate. It is thought that when the animals were alive these cavities were filled with tissues that secreted the hard outer coverings. It is not clear what the hard substance of the walls was since it has been replaced or converted to a different crystalline form. This suggests it was a slightly unstable material such as aragonite, a form of calcium carbonate. Some sclerites appear to be on top of the skin, other covered by it, and some appear partly covered.

Chancelloriids probably lived on muddy sea-floors, as their sclerites increase in size from the bottom to the top, and all had thickenings at the bases, which are regarded as anchors; they are often preserved in attachment to other organisms or shelly debris. They were very likely filter-feeders.