Exoskeleton

An exoskeleton is a hard outer shell that supports and protects an animal's body. Unlike humans, who have bones inside their bodies, many animals carry their skeletons on the outside. This hard covering helps keep their shape and shields important parts inside from getting hurt.

Many insects, like beetles and butterflies, have exoskeletons made from a material called a cuticle. Crustaceans, such as crabs and lobsters, also have exoskeletons that protect them. Even tiny creatures called tardigrades, often known as water bears, have exoskeletons to help them survive tough conditions.

Some animals, like turtles, have both an inner skeleton and an outer shell for extra protection. Other sea creatures, including snails and clams, also have hard outer shells that act like exoskeletons. These outer skeletons are very important for keeping these animals safe and helping them move and grow.

Role

Exoskeletons are hard outer coverings that help animals in many ways. They give support, protect inside parts, and help with breathing and feeling. They also help animals move, find food, and show off to find mates. Exoskeletons keep animals safe from harm and pests, and they give places for muscles to connect.

Arthropod exoskeletons are made from a material called chitin. When mixed with calcium carbonate, they become harder and stronger but also heavier. Special parts of these exoskeletons, called apodemes, help muscles attach and can stretch to store energy for jumping, like in locusts. Some animals, like molluscs, use calcium carbonates for their shells, while others, like diatoms, use silica. One special mollusc even uses iron sulfides for protection. Some tiny organisms build their outer layers by sticking sand and shell pieces to themselves. Unlike some people think, echinoderms do not have exoskeletons—their outer layer is always inside living tissue.

Exoskeletons have developed many times in different animal groups. Eighteen groups evolved hard, calcified exoskeletons. Other animals, like armadillos and pangolins, have tough outer coatings made from bone or hair. Reptiles like turtles and dinosaurs like Ankylosaurs had bony armor, and crocodiles have bony and horny scales.

Growth

Main article: Ecdysis

Because an exoskeleton is hard and cannot grow, animals with them must shed it when they get bigger. This process is called moulting or ecdysis. When it's time to shed, the animal makes a new, soft exoskeleton under the old one. After shedding the old shell, the new one is soft at first and can still grow a little before it hardens. During this time, the animal usually hides to stay safe until its new shell is ready. If an animal cannot shed its old exoskeleton when it becomes too small, it may not survive or be able to grow up properly. Some insect medicines work by stopping this shedding process.

Paleontological significance

Exoskeletons are very important for scientists because they help preserve ancient animals. Since soft parts of animals usually break down before they can become fossils, hard exoskeletons can often be preserved as shell pieces. This hardening helps protect the shape of the animal and can even create a mold of it.

However, because we mostly find fossils of hard parts, we miss a lot of information about soft-bodied animals that did not leave behind fossils. This means we know less about two-thirds of animal groups. The first mineralized exoskeletons appeared just before the start of the Cambrian period, about 550 million years ago. The development of these hard shells may have helped cause a big increase in animal life and new ways animals interacted with each other. Some older animals also had tough outer shells, and we can even see signs that they were eaten by others.

Evolution

Further information: Small shelly fauna

Long ago, many animals developed hard outer shells to protect themselves. These shells were made from minerals like silica and calcium compounds. Just before a big change in Earth's history called the Cambrian period, many different types of shells appeared quickly. Most of these shells were made from a mineral called calcium carbonate, which stayed the same even when conditions changed.

Some very old animals had tough outer shells that weren't made from minerals. Others, like Cloudina, had shells made from minerals. These mineral shells didn't become common until the start of the Cambrian period, when many small shell-making animals appeared suddenly. This might look quick because the conditions needed to preserve these shells also appeared at the same time.

The type of mineral in shells depends on the ocean's chemistry. Calcium carbonate can be in two forms: calcite and aragonite. Aragonite is stable when there is more magnesium in the water, but becomes less stable when magnesium levels drop. Most animals use just one form of this mineral, depending on the ocean's conditions when their shells first developed.

A special modern sea creature, Chrysomallon squamiferum, shows how both old and new ocean conditions affect shells. It lives near deep-sea vents and has a shell made of aragonite, like some of the earliest fossil molluscs. It also has armor plates made from iron sulfides, which had never been found in animals before but are common near these vents.