What Is an Osteoderm? Bone Grown Inside the Skin

An osteoderm is a plate of bone that develops within the dermis, the deeper layer of skin, rather than growing out from the skeleton underneath. It forms through a process distinct from the one that produces limbs and vertebrae, and it is not connected to them, which means such plates can appear, enlarge, multiply or disappear over evolutionary time without any corresponding change to the skeleton. Each plate is generally covered in life by a horny sheath, so what is seen on a living crocodile or a lizard is keratin over bone, and the fossil preserves only the bone. Internally they range from solid to spongy with a rich blood supply, and the internal structure varies between groups in ways that relate to what the plate was doing.

The distribution across animals is wide and scattered, which is the clue to their nature:

• •Crocodilians, where the plates form rows along the back and are among the most familiar examples
• •Many lizards, where small plates give the skin a beaded texture
• •Armadillos, where the plates form the bands and shields that define the animal
• •Armoured dinosaurs, where they range from small studs to metre-long plates and spikes
• •Sauropods in some groups, where scattered plates were found unexpectedly and changed reconstructions
• •Numerous extinct groups including early amphibians and various reptile lineages, which is a distribution best explained by the capacity being ancient and repeatedly expressed

Defence is the obvious answer and it is one of several. Protection against predators is well supported for the heavily armoured groups, where the plates cover vulnerable regions and are thick enough to matter. Mineral storage is a documented function in crocodilians, where the bone acts as a calcium reserve drawn on during egg laying and released to buffer acidity during long dives. Temperature regulation has been proposed for the large plates of some dinosaurs and for crocodilians basking, since the blood supply allows heat exchange. Structural support has been suggested where plates are embedded in a rigid shield. Display is plausible for conspicuous arrangements. As with most large conspicuous features the sensible position allows several functions at once, with the balance differing between groups and changing over an animal's life.

Because these plates develop in skin they follow the rules of skin rather than of the skeleton, and the consequences are visible in living animals. They appear progressively as an animal grows rather than being present from hatching, with young crocodilians developing their rows over months and armadillos hardening their bands after birth. They can be added throughout life, so an old individual carries more and larger plates than a young one, which makes them a rough indicator of age in some species. They remodel internally in response to load and to the demands placed on them for mineral. They can be damaged and repaired, and healed injuries are visible in both living animals and fossils. And in some lineages they are lost entirely over evolutionary time without any other change, which is what would be expected of a feature whose development is independent of the skeleton.

For palaeontologists these plates are both informative and awkward. Because they sit in skin rather than attaching to bone, they scatter after death and are usually found isolated, which means an excavation recovers a pile of plates with no record of where each sat on the animal. Reconstructing the arrangement therefore depends on the rare specimens found articulated, and several famous restorations were revised when such a specimen appeared, most notably the arrangement of the plates along the back of stegosaurs, which was argued over for a century. Isolated plates are also difficult to identify, since similar shapes occur in unrelated animals, and names have been given to material that later turned out to belong to something already known. The general lesson is that a structure not anchored to the skeleton is easy to find and hard to place.

Bone forming within the skin rather than growing from the skeleton, covered in life by a horny sheath, which is why such plates appear and disappear across unrelated groups without skeletal change. Defence, calcium storage, heat exchange and display all have support in different animals. Because they are not anchored, they scatter after death, which is why reconstructing their arrangement took so long.