The Coelosclerithophorans are a

polyphyletic A polyphyletic group is an assemblage that includes organisms with mixed evolutionary origin but does not include their most recent common ancestor. The term is often applied to groups that share similar features known as Homoplasy, homoplasies ...

group of organisms bearing hollow

sclerites A sclerite (Greek , ', meaning " hard") is a hardened body part. In various branches of biology the term is applied to various structures, but not as a rule to vertebrate anatomical features such as bones and teeth. Instead it refers most commonly ...

made of

aragonite Aragonite is a carbonate mineral and one of the three most common naturally occurring crystal forms of calcium carbonate (), the others being calcite and vaterite. It is formed by biological and physical processes, including precipitation fr ...

, and with a supposedly distinctive microstructure. Their skeletons may be homologous to those of the molluscs, and ''

Halkieria The halkieriids are a group of fossil organisms from the Lower to Middle Cambrian. Their eponymous genus is ''Halkieria'' , which has been found on almost every continent in Lower to Mid Cambrian deposits, forming a large component of the smal ...

'' looks very much like a

polyplacophoran Chitons () are marine molluscs of varying size in the class Polyplacophora ( ), formerly known as Amphineura. About 940 extant and 430 fossil species are recognized. They are also sometimes known as sea cradles or coat-of-mail shells or suck ...

. The shells of this group and the molluscs appear to have originated in the same fashion. However, there are several reasons to doubt this relationship, many of which may not pass muster. There are two groups within the coeloscleritophorans: the Sachitids, to which ''

'' and ''

Wiwaxia ''Wiwaxia'' is a genus of soft-bodied animals that were covered in carbonaceous scales and spines that protected it from predators. ''Wiwaxia'' fossils—mainly isolated scales, but sometimes complete, articulated fossils—are known from early C ...

'' belong, and the

Chancellorids 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 t ...

. The Ediacaran fossil '' Ausia'' has been touted as an ancestral 'intermediate' between both of these groups. These are common constituents of Lower

Cambrian The Cambrian ( ) is the first geological period of the Paleozoic Era, and the Phanerozoic Eon. The Cambrian lasted 51.95 million years from the end of the preceding Ediacaran period 538.8 Ma (million years ago) to the beginning of the Ordov ...

small shelly fossil The small shelly fauna, small shelly fossils (SSF), or early skeletal fossils (ESF) are biomineralization, mineralized fossils, many only a few millimetres long, with a nearly continuous record from the latest stages of the Ediacaran to the end o ...

assemblages.

References

Cambrian invertebrates {{paleo-mollusc-stub