What sort of disorder is abnormally fused skull bones?

Craniosynostosis. The skull is made of many bone plates which are not fully fused at birth, allowing for adequate brain development. Fibrous tissue between the plates (cranial sutures) calcifies as brain growth slows during the first five or so years. Premature fusion of the plates can cause a mis-sharpen head or inhibit brain growth. Not all fused sutures are worrisome. The only treatment, if required, is surgery.