Osteoplax

An osteoplax (plural: osteoplaxes or osteoplax) is a dermal bone embedded within the skin of certain animals. These structures serve various protective and structural functions. While the term literally translates to "bone plate," it is important to distinguish osteoplaxes from osteoderms. The key difference lies in the presence of a cartilaginous precursor in osteoderms, which is absent in osteoplaxes. Osteoplaxes are formed through direct ossification within the dermis.

Osteoplaxes are notably found in turtles, particularly in the carapace (upper shell) and plastron (lower shell). In these animals, they contribute significantly to the overall rigidity and protection afforded by the shell. The number, shape, and arrangement of osteoplaxes can vary considerably among different turtle species.

The function of osteoplaxes is primarily skeletal, offering structural support and a protective barrier against physical trauma. Their presence reduces the flexibility of the skin in the area where they are located. In turtles, the osteoplaxes fuse with the ribs and vertebrae during development, forming a rigid protective shell.