<P> Mineralized skeletons first appear in the fossil record shortly before the base of the Cambrian period, 550 million years ago . The evolution of a mineralized exoskeleton is seen by some as a possible driving force of the Cambrian explosion of animal life, resulting in a diversification of predatory and defensive tactics . However, some Precambrian (Ediacaran) organisms produced tough outer shells while others, such as Cloudina, had a calcified exoskeleton . Some Cloudina shells even show evidence of predation, in the form of borings . </P> <P> On the whole, the fossil record only contains mineralised exoskeletons, since these are by far the most durable . Since most lineages with exoskeletons are thought to have started out with a non-mineralised exoskeleton which they later mineralised, this makes it difficult to comment on the very early evolution of each lineage's exoskeleton . It is known, however, that in a very short course of time, just before the Cambrian period, exoskeletons made of various materials--silica, calcium phosphate, calcite, aragonite, and even glued - together mineral flakes--sprang up in a range of different environments . Most lineages adopted the form of calcium carbonate which was stable in the ocean at the time they first mineralised, and did not change from this mineral morph - even when it became the less favorable . </P> <P> Some Precambrian (Ediacaran) organisms produced tough but non-mineralized outer shells, while others, such as Cloudina, had a calcified exoskeleton, but mineralized skeletons did not become common until the beginning of the Cambrian period, with the rise of the "small shelly fauna". Just after the base of the Cambrian, these miniature fossils become diverse and abundant--this abruptness may be an illusion, since the chemical conditions which preserved the small shellies appeared at the same time . Most other shell - forming organisms appear during the Cambrian period, with the Bryozoans being the only calcifying phylum to appear later, in the Ordovician . The sudden appearance of shells has been linked to a change in ocean chemistry which made the calcium compounds of which the shells are constructed stable enough to be precipitated into a shell . However this is unlikely to be a sufficient cause, as the main construction cost of shells is in creating the proteins and polysaccharides required for the shell's composite structure, not in the precipitation of the mineral components . Skeletonization also appeared at almost exactly the same time that animals started burrowing to avoid predation, and one of the earliest exoskeletons was made of glued - together mineral flakes, suggesting that skeletonization was likewise a response to increased pressure from predators . </P> <P> Ocean chemistry may also control which mineral shells are constructed of . Calcium carbonate has two forms, the stable calcite, and the metastable aragonite, which is stable within a reasonable range of chemical environments but rapidly becomes unstable outside this range . When the oceans contain a relatively high proportion of magnesium compared to calcium, aragonite is more stable, but as the magnesium concentration drops, it becomes less stable, hence harder to incorporate into an exoskeleton, as it will tend to dissolve . </P>

An exoskeleton helps which of the following organisms to live