Exoskeleton
Examples of exoskeletons in animals include the cuticle skeletons shared by arthropods (insects, chelicerates, myriapods and crustaceans) and tardigrades, as well as the skeletal cups formed by hardened secretion of stony corals, the test/tunic of sea squirts and sea urchins, and the prominent mollusc shell shared by snails, clams, tusk shells, chitons and nautilus.
Exoskeletons contain rigid and resistant components that fulfil a set of functional roles in addition to structural support in many animals, including protection, respiration, excretion, sensation, feeding and courtship display, and as an osmotic barrier against desiccation in terrestrial organisms.
[4] Arthropod exoskeletons contain chitin; the addition of calcium carbonate makes them harder and stronger, at the price of increased weight.
The armour of reptiles like turtles and dinosaurs like Ankylosaurs is constructed of bone; crocodiles have bony scutes and horny scales.
[citation needed] In contrast, moulting reptiles shed only the outer layer of skin and often exhibit indeterminate growth.
For instance, the strong layer can resist compaction, allowing a mould of the organism to be formed underneath the skeleton, which may later decay.
The evolution of a mineralised exoskeleton is considered a possible driving force of the Cambrian explosion of animal life, resulting in a diversification of predatory and defensive tactics.
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.
[7] Some Precambrian (Ediacaran) organisms produced tough but non-mineralized outer shells,[11] while others, such as Cloudina, had a calcified exoskeleton,[14] 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 shells appeared at the same time.
[7] However, the relative abundance of calcite- and aragonite-using lineages does not reflect subsequent seawater chemistry – the magnesium/calcium ratio of the oceans appears to have a negligible impact on organisms' success, which is instead controlled mainly by how well they recover from mass extinctions.
[17] A recently discovered[18] modern gastropod Chrysomallon squamiferum that lives near deep-sea hydrothermal vents illustrates the influence of both ancient and modern local chemical environments: its shell is made of aragonite, which is found in some of the earliest fossil molluscs; but it also has armour plates on the sides of its foot, and these are mineralised with the iron sulfides pyrite and greigite, which had never previously been found in any metazoan but whose ingredients are emitted in large quantities by the vents.
