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Palaeontological significance

Palaeontological significance

Borings in exoskeletons can provide evidence of animal behaviour. In this case, boringsponges attacked this hard clamshell after the death of the clam

Exoskeletons, as hard parts of organisms, are greatly useful in assisting preservation of organisms, whose soft parts usually rot before they can be fossilised. Mineralised exoskeletons can be preserved "as is", as shell fragments, for example. The possession of an exoskeleton also permits a couple of other routes to fossilisation. For instance, the tough layer can resist compaction, allowing a mould of the organism to be formed underneath the skeleton, which may later decay.^[3] Alternatively, exceptional preservation may result in chitin being mineralised, as in the Burgess shale,^[4] or transformed to the resistant polymerkeratin, which can resist decay and be recovered.

However our dependence on fossilised skeletons also significantly limits our understanding of evolution. Only the parts of organisms that were already mineralised 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.^[3] 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 fossilised.^[5]

Mineralised skeletons first appear in the fossil record shortly before the base of the Cambrian period, 550 million years ago. The evolution of a mineralised 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,^[3] while others, such asCloudina, had a calcified exoskeleton.^[6] Some Cloudina shells even show evidence of predation, in the form of borings.^[6]

Evolution

Further information: Small shelly fossils

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. We do know that in a very short course of time just before the Cambrianperiod 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-mineralised outer shells,^[3] while others, such as Cloudina, had a calcifiedexoskeleton,^[6] but mineralised 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 the same time.^[8] Most other shell forming organisms appear during the Cambrian period, with the Bryozoans being the only calclfying 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 andpolysaccharides required for the shell's composite structure, not in the precipitiation of the mineral components.^[1] 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.^[7]

Ocean chemistry may also control which mineral shells are constructed of. Calcium carbonate has two forms, the stable calcite, and themetastable aragonite, which is stable within a reasonable range of chemical environments but rapidly becomes unstable outsite 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.

With the exception of the molluscs, whose shells often comprise both forms, most lineages use just one form of the mineral. The form used appears to reflect the seawater chemistry - thus which form was more easily precipitated - at the time that the lineage first evolved a calcified skeleton, and does not change thereafter.^[2] 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.^[9] A recently-discovered modern gastropod 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 armor 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.^[1]

Artificial "exoskeletons"

Humans have long used armour as an artificial exoskeleton for protection, especially in combat. Exoskeletal machines (also called powered exoskeletons) are also starting to be used for medical and industrial purposes, while powered human exoskeletons are a feature of science fiction writing, but are currently moving into prototype stage. Orthoses are a limited, medical form of exoskeleton.

An orthosis (plural orthoses) is a device which attaches to a limb, or the torso, to support the function or correct the shape of that limb or the spine. Orthotics is the field dealing with orthoses, their use, and their manufacture. An orthotist is a person who designs and fits orthoses. Aprosthesis (plural prostheses) is a device that substitutes for a missing part of a limb. If the prosthesis is a hollow shell and self-carrying, it is exoskeletal. If internal tubes are used in the device and the cover (cosmesis) to create the outside shape is made of a soft, non-carrying material, it is endoskeletal. Prosthetics is the field that deals with prostheses, use, and their manufacture. A prosthetist is a person who designs and fits prostheses.

Parenthetically, the exoskeleton has been used as an architectural model. See the lighthouse at St. Martin Island.

Perhaps the first animals to use a naturally-occurring "artificial exoskeleton" were the hermit crabs, the majority of which are obliged constantly to "wear" an empty gastropod shell, in order to protect their soft abdomens.