Small Shelly Fauna - Evolution of Skeletons and Biomineralization

Evolution of Skeletons and Biomineralization

Biomineralization is the production of mineralized parts by organisms. Hypotheses to explain the evolution of biomineralization include physiological adaptation to changing chemistry of the oceans, defense against predators and the opportunity to grow larger. The functions of biomineralization in SSFs vary: some SSFs are not yet understood; some are components of armor; and some are skeletons. A skeleton is any fairly rigid structure of an animal, irrespective of whether it has joints and irrespective of whether it is biomineralized. Although some SSFs may not be skeletons, SSFs are biomineralized by definition, being shelly. Skeletons provide a wide range of possible advantages, including : protection, support, attachment to a surface, a platform or set of levers for muscles to act on, traction when moving on a surface, food handling, provision of filtration chambers and storage of essential substances.

It has often been suggested that biomineralization evolved as a response to an increase in the concentration of calcium in the seas, which happened around the Ediacaran-Cambrian boundary, and that biomineralization's main benefit was to store harmlessly minerals that might have disrupted organisms' internal processes. For example Mikhail A. Fedonkin suggested that an increase in the length of food chains may have contributed, as animals higher up the food chain accumulate greater amounts of waste products and toxins relative to their size, and biomineralization may have been a way of isolating excess carbonates or silicates consumed with prey. However, biomineralizing a skeleton is a fairly expensive way to dispose safely of excess minerals, as the main construction cost is the organic matrix, mostly proteins and polysaccharides, with which minerals are combined to form composite materials. The idea that biomineralization was a response to changes in ocean chemistry is also undermined by the fact that small shelly fossils made of calcite, aragonite, calcium phosphate and silica appeared virtually simultaneously in a range of environments.

Organisms started burrowing to avoid predation at around the same time. Jerzy Dzik suggested that biomineralization of skeletons was a defense against predators, marking the start of an evolutionary arms race. He cited as another example of hardened defenses from this time the fact that the earliest protective "skeletons" included glued-together collections of inorganic objects — for example the Early Cambrian worm Onuphionella built a tube covered with mica flakes. Such a strategy required both anatomical adaptations that allowed organisms to collect and glue objects and also moderately sophisticated nervous systems to co-ordinate this behavior.

On the other hand Bernard Cohen argued that biomineralized skeletons arose for "engineering" reasons rather than as defenses. There are many other defensive strategies available to prey animals including mobility and acute senses, chemical defenses, and concealment. Mineral-organic composites are both stronger and cheaper to build than all-organic skeletons, and these two advantages would have made it possible for animals to grow larger and, in some cases, more muscular — in animals beyond a certain size, the larger muscles and their greater leverage produce forces all-organic skeletons are not rigid enough to withstand. The development of modern brachiopods includes a progression from all-organic to mineral-organic composite shells, which may be a clue to their evolutionary development. The evolution of rigid biomineralized exoskeletons may then have started an arms race in which predators developed drills or chemical weapons capable of penetrating shells, some prey animals developed heavier, tougher shells, etc.

Fedonkin suggested another explanation for the appearance of biomineralization around the start of the Cambrian: the Ediacara biota evolved and flourished in cold waters, which slowed their metabolisms and left them with insufficient spare energy for biomineralization; but there are signs of global warming around the start of the Cambrian, which would have made biomineralization easier. A similar pattern is visible in living marine animals, since biomineralized skeletons are rarer and more fragile in polar waters than in the tropics.