Marcusodictyon Bassler from the Lower Ordovician of Estonia: not the earliest bryozoan but a phosphatic problematicum

Marcusodictyon priscum (Bassler 1911) from the Tremadocian Ungulite Sandstone (A2) of Jegelecht Falls, Estonia, is redescribed following SEM study of type and other specimens. The fossil consists of a polygonal network of low ridges known to exist only on the exterior surfaces of valves of the inarticulate brachiopod Obolus. Electron microprobe analysis demonstrates that, like its host substratum, Marcusodictyon is composed of calcium phosphate. This, together with the detailed morphology and dimensions of the ridge system, is used to negate Bassler's assignment of the genus to the ctenostome Bryozoa. Instead Marcusodictyon resembles the Silurian problematicum Gochtia Eisenack. The inferred growth pattern of Marcusodictyon suggests that it may have been a symbiont of a living brachiopod.