Dermal Skeleton

a system of bony formations that develops in the connective-tissue layer of skin, or derma, of vertebrates.

The dermal skeleton performs protective and supportive functions. It is most highly developed in fish and loricate Agnatha. The original type of dermal skeleton was the placoid scale (dermal denticles), which covered the head and body of the animal evenly (fossil telodonts and Euselachii). The dermal denticles could fuse into more or less large plates, forming a shell. In the evolution of fish, the dermal skeleton differentiated into a cephalic section, consisting of large ossifications forming a dermatocranium, and a truncal section, consisting of rows of scales (cosmoid and ganoid). Vertebrate phylogeny has shown a gradual sinking of the elements of the dermal skeleton beneath the skin, bringing the dermatocranial ossifications into closer contact with the cranium and gradually reducing the shell of truncal scales (well developed in many fossil vertebrates). In extant terrestrial vertebrates the rudiments of the primitive dermal skeleton are preserved in the form of bony scales in apodal amphibians and ventral ribs in tuataras and crocodiles. In certain vertebrate forms, such as turtles and armadillos, a heavy dermal skeleton developed secondarily. The skeletal formation in certain invertebrates is sometimes called a dermal skeleton, but such formations differ from the dermal skeleton of vertebrates in content (for example, calcareous plates and the spines of echinoderms).