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 the result of a reduction of the basal layer of lamellar bone. Despite the reduction of the basal layer, the microanatomy of the scales is still comparable with that of Late Silurian and Early Devonian forms. The gap in the fossil record of Middle Devonian osteostracans may be the result of the disintegration of the dermal armor into isolated tesserae.