Osteostracan tesserae from the Baltic Middle Devonian: morphology and microanatomy

Isolated tesserae of Middle Devonian osteostracans are described in detail for the first time. We suggest that the formation of isolated tesserae is t he result of a reduction of the basal layer of lamellar hone. Despite the r eduction of the basal layer, the microanatomy of the scales is still compar able with that of Late Silurian and Early Devonian forms. The gap in the fo ssil record of Middle Devonian osteostracans may be the result of the disin tegration of the dermal armor into isolated tesserae.