Strictly speaking, pachyostosis is a non-pathological condition in vertebrate animals in which the bones experience a thickening, generally caused by extra layers of lamellar bone. It often occurs together with bone densification (osteosclerosis), reducing inner cavities. This joint occurrence is called pachyosteosclerosis. However, especially in the older literature, “pachyostosis” is often used loosely, referring to all osseous specializations characterized by an increase in bone compactness and/or volume. It occurs in both terrestrial and, especially, aquatic or semi-aquatic vertebrates.
Most giant deer showed pronounced pachyostosis of the mandible and skull. It has been suggested that this served to store minerals for antler growth. Many Pachycephalosauria and most members of the Dinocephalia clade of therapsids had thickened skull bones, probably used in head-butting contests.
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