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Facts about Exoskeletons for Kids

An exoskeleton is the external skeleton that supports and protects an animal’s body, in contrast to the internal skeleton (endoskeleton) of, for example, a human. The shells of the various groups of shelled mollusks, including those of snails, clams, tusk shells, chitons and nautilus, are also exoskeletons.

  • Mineralized exoskeletons first appeared in the fossil record about 550 million years ago, and their evolution is considered by some to have played a role in the subsequent Cambrian explosion of animals.
  • Exoskeletons contain rigid and resistant components that fulfil a set of functional roles including protection, excretion, sensing, support, feeding and acting as a barrier against desiccation in terrestrial organisms.
  • These structures are composed of chitin, and are approximately 6 times as strong and twice as stiff as vertebrate tendons.
  • Chitin forms the exoskeleton in arthropods including insects, arachnids such as spiders, crustaceans such as crabs and lobsters (see arthropod exoskeleton), and in some fungi and bacteria.
  • Calcium carbonates constitute the shells of molluscs, brachiopods, and some tube-building polychaete worms.
  • Exoskeletons have evolved independently many times; 18 lineages evolved calcified exoskeletons alone.
  • Exoskeletons, as hard parts of organisms, are greatly useful in assisting preservation of organisms, whose soft parts usually rot before they can be fossilized.
  • For instance, the tough layer can resist compaction, allowing a mold of the organism to be formed underneath the skeleton, which may later decay.
  • We do know 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.
  • 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”.
  • 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.
  • Skeletonisation 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 skeletonisation was likewise a response to increased pressure from predators.