Psammosteid microremains from the Middle Devonian (Givetian) of Estonia

The Estonian locality of Essi in the Baltic Middle Devonian (Givetian) Abava Substage, called by Gross the "Ubergangs-Stufe", has yielded numerous well preserved microremains of Psammosteus sp. They show a variety of structure and growth types: single elements that somewhat resemble thelodont and certain chondrichthyan scales on the one hand, and complex elements that are similar to tesserae of primitive heterostracans or to platelets ("tesserae") and scales of psammosteids on the other. Among the latter, specimens of both synchronomorial and cyclomorial growth types occur. These elements reveal the differences between Psammosteus and the other psammosteids and shed new light on the growth and relationships ofheterostracans in general. The derivation of psammosteids from pteraspids and the secondary nature of psammosteid "tesserae" are equivocal issues