No bones about it! What are skeletons?

Skeletons aren’t just bony figures with a skull, ribs and bandy legs that jump out of closets and ask for sweets at Hallowe’en. In fact, many skeletons have no bones at all! Let’s take a far and wide look at skeletons…

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Skeletons fulfil at least four basic functions – they support the body’s structure against gravity or other forces, they help animals move around (acting as levers for muscles and tendons to pull on), they protect the most important parts of organisms from damage, and they act as storage repositories for minerals (such as the hydroxyapatite or calcium salts in bone) or other stiff molecules. But only vertebrates, which make up just 5% of the world’s species, have the bony skeleton that we do, so here’s a guide to what might be haunting the rest of the animal kingdom next October 31st

Endo-skeletons, like ours, are internal scaffolds. Bone is a living, growing tissue unique to vertebrate animals. It allows us to move quickly and precisely, and can support the enormous size of animals like whales or sauropod dinosaurs (read more about bone here). But endoskeletons don’t have to be bone! Sharks and rays (elasmobranchs) and jawless fishes tend to use cartilage, which is more flexible than bone. This allows them greater freedom of movement (check out this hagfish), but is softer and more easily damaged. Bone itself in vertebrates lies along a continuum of tissues from soft squishy cartilage to more calcified cartilage to various forms of true bone. Echinoderms (including starfish, sea urchins and sea cucumbers) use ossicles, discs of calcium carbonate minerals, to keep their shape and protect their insides, but these contribute much less to movement. Even sponges, the most ancient living animals, have an internal network of silica spicules or fibrous spongin which gives them their shape and protects them from the ocean currents.

Exo-skeletons are usually hard, rigid external structures, like body armour. Articulated exo-skeletons are movable, for example the chitinous outsides of insects and other arthropods. The skeleton shapes their limbs and joints, allows complex movement, provides protection and prevents terrestrial animals losing too much water through evaporation. You know when you find dried up spiders behind the microwave? What’s left is the exoskeleton, but because it’s on the outside, it doesn’t look much different from a live spider; the squishy stuff of the live animal is inside. Simple exoskeletons have limited or no movement, and are usually found in slow-moving or stationary animals. Think snail and tortoise shells, for example. For these animals, the skeleton is used primarily for protection and is usually much tougher than the articulated arthropod exo-skeleton – but at the expense of being more cumbersome and less flexible. As usual in evolution, there is no perfect solution for all situations: almost everything is a tradeoff, such as between mobility and protection in this case.

Hydrostatic skeletons are internal, but water-based, and very flexible. That might not sound very useful to us, but many animals including jellyfish and worms use fluids to support them and move around. If you’ve ever played with a water snake, imagine if you squeeze one end. All the water is pushed to the other end and propels the water snake forward. This principle helps such animals to move or swim by constricting muscle around the skeleton (sometimes called a coelom). For obvious reasons, this kind of hydrostatic skeleton is largely found in aquatic animals – once they’re washed up on the beach, jellyfish don’t have much shape to them at all. Hydrostatic skeletons also don’t provide much protection for the internal organs. Instead, defensive systems like the jellyfish’s sting are needed to keep attackers at bay. However, many animals use hydrostatic skeletons for parts of their bodies: for example, our tongues and genital organs are regionalized muscular, hydrostatic skeletons, and the squishy pad of an elephant’s foot can be considered a large hydrostatic skeleton of sorts.

Of course, there are thousands more examples and many animals use combinations of these types for different purposes. Some mammals, like porcupines and pangolins, have pseudo-exoskeletons made from keratin (found in hair and nails; also in the scales of reptiles and other animals) for extra protection, and armadillos and tortoises have bone (called osteoderms) in their external shells. Although echinoderms use ossicles to support their shape, they use thousands of hydrostatic tube feet to move around, and snails have both a protective shell and a hydrostatic foot for locomotion.

Do only animals have skeletons? Not really- we could consider the fibrous woody parts of land plants, the calcified tissues of some algae, the mineralized cores of many microscopic organisms, or even the internal “skeleton” of cells, the cytoskeleton, to be a kind of skeleton of sorts. It’s all a matter of degree, as so much in biology is.

Because they’re so important, biologists can study the way evolution has shaped skeletons to learn more about animals all over the world. Without these crucial systems, we would all just be blobby bags on the floor. Thank you, skeletons!

Want to know more?

Watch: https://www.youtube.com/watch?v=RW46rQKWa-g

https://www.youtube.com/watch?v=NilT05VCwsI

Read: Anatomy and Physiology: an illustrated guide.

Visit: Your nearest natural history museum! As well as bony skeletons, most museums will have a beautiful selection of more unusual examples on display.

Glossary

Arthropods: Animals belonging to a group called Arthropoda, which have an exoskeleton and segmented bodies. This includes insects, spiders, crustaceans and centipedes among others.

Chitinous: Made of chitin, strong modified carbohydrate chains which make up the hard exoskeletons in arthropods.

Coelom: A central body cavity filled with fluid which contains and supports other internal organs.

Echinoderm: A group of invertebrate animals which have pentaradial (5-fold) symmetry and use tube feet, such as sea stars, sea urchins and sea cucumbers.

Elasmobranch: Sharks and rays.

Ossicle: A calcium-based disc which helps echinoderms keep their shape.

Tube feet: Small tentacles filled with water which are found in echinoderms and are used for locomotion and feeding.

Picture credit: Sponge spicules. By Rob W. M. Van Soest, Nicole Boury-Esnault, Jean Vacelet, Martin Dohrmann, Dirk Erpenbeck, Nicole J. De Voogd, Nadiezhda Santodomingo, Bart Vanhoorne, Michelle Kelly, John N. A. Hooper. Reproduced under Creative Commons BY 2.5.

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