<P> Since exoskeletons are rigid, they present some limits to growth . Organisms with open shells can grow by adding new material to the aperture of their shell, as is the case in snails, bivalves and other molluscans . A true exoskeleton, like that found in arthropods, must be shed (moulted) when it is outgrown . A new exoskeleton is produced beneath the old one . As the old one is shed, the new skeleton is soft and pliable . The animal will pump itself up to expand the new shell to maximal size, then let it harden . When the shell has set, the empty space inside the new skeleton can be filled up as the animal eats . Failure to shed the exoskeleton once outgrown can result in the animal being suffocated within its own shell, and will stop subadults from reaching maturity, thus preventing them from reproducing . This is the mechanism behind some insect pesticides, such as Azadirachtin . </P> <P> Exoskeletons, as hard parts of organisms, are greatly useful in assisting preservation of organisms, whose soft parts usually rot before they can be fossilized . Mineralized exoskeletons can be preserved "as is", as shell fragments, for example . The possession of an exoskeleton also permits a couple of other routes to fossilization . For instance, the tough layer can resist compaction, allowing a mold of the organism to be formed underneath the skeleton, which may later decay . Alternatively, exceptional preservation may result in chitin being mineralized, as in the Burgess Shale, or transformed to the resistant polymer keratin, which can resist decay and be recovered . </P> <P> However, our dependence on fossilized skeletons also significantly limits our understanding of evolution . Only the parts of organisms that were already mineralized are usually preserved, such as the shells of molluscs . It helps that exoskeletons often contain "muscle scars", marks where muscles have been attached to the exoskeleton, which may allow the reconstruction of much of an organism's internal parts from its exoskeleton alone . The most significant limitation is that, although there are 30 - plus phyla of living animals, two - thirds of these phyla have never been found as fossils, because most animal species are soft - bodied and decay before they can become fossilized . </P> <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>

What does the animal do as soon as it has a new skeleton