THE ORGANIC MATRIX MYSTERY: TOO MANY PHYLA, TOO LITTLE TIME

The most widespread form of carbonate biomineralization, characteristic of foraminiferans, poriferans (including archaeocyathids), coelenterates, bryozoans, articulate brachiopods, mollusks, arthropods (cirripeds and trilobites), and possibly other groups, involves an organic matrix, seemingly physically identical among all these groups. Despite some degradation, recognizable and characteristic remnants of matrix have been found in fossils as old as Early Cambrian (trilobites and archaeocyathids), and in some abundance among various phyla in the Ordovician. Present-day relatives from these phyla still utilize this matrix for biomineralization.

The mystery lies in the evolution of the organic matrix, or at least in the evolution of the genes that control it. If it evolved specifically to control biomineralization, how does it happen to be present among phyla which diverged millions of years before the first matrix-controlled biomineralization is known? And alternatively, if the genes that direct matrix formation evolved for some other purpose, and just happen to function well in biomineralization, how did so many phyla maintain the genes' integrity throughout those long years? One possibility, although calling for an elastic imagination, would be the Lynn Margulis concept of a parasitic prokaryote, evolving the mechanism for this type of biomineralization in the Early Cambrian, and then 'infecting' members of many phyla with this ability. Any other ideas?