Fig. 7.

Galeaspida (early Devonian). (A) Schematic illustration of the posterior part of the head-shield of Bannhuanaspis. The shield is thought to have formed by the fusion of multiple mineralized units. Each unit is composed of lamellar, acellular bone with many large, perpendicular fiber-like bundles (Sharpey's fibers). This lamellar bone is sometimes ornamented with small tubercles. The dermoskeleton is underlain by mineralized cartilage associated with the neurocranium; there is no evidence of a perichondrium. (B) SEM section of a polybranchiaspid scale-like element demonstrating the unique organization of the matrix. Note the perpendicular Sharpey's fibers. Scale bar: B = 20 µm.