AN ENGIMATIC AQUATIC MESOFOSSIL FROM THE ALBIAN(-?CENOMANIAN) OF MARYLAND, USA

During standard processing for mesofossils (flowers, megaspores, etc.) from Potomac Group (Cretaceous) sediments in Maryland, four specimens—two complete and two partial—of a large, oblate morphotype bearing multiple processes were recovered. The specimens are 700-890 µm in overall diameter. Each specimen is composed of two convex valves joined at their edges and bearing a very low relief reticulate sculpture over both surfaces. Each valve is constructed of a solid wall forming a dome ~530 µm in diameter, to which a wing 80-120 µm wide is attached. The wing of the designated upper valve extends up and over the dome leaving only ~25% of the dome’s diameter exposed at the apex. The wing of the designated lower valve is much thinner and nearly all of the dome remains exposed. The most complete specimen bears 14, possibly 15, processes positioned sub-equatorially on the designated upper valve. Each process is 100-170 µm long and terminates in an ‘anchor’ with two recurved tips. The reticulate sculpture on the dome is formed from a slight thickening of the wall, while the reticulum on the wings directly overlie a network of walls that partition the volume of the wing into compartments. The compartments are connected by 2-4 µm circular pores positioned in the middle of walls, with only a few off axis. The wall forming the dome is solid, without porosity, and without any layers detectable by either TEM or FIB-SEM. Neither the dome nor the wings are divided into discernible plates and no dehiscence aperture are visible; the contents appear to have been released at the equator by separation of the valves. The overall morphology and wall structure do not conform to those of terrestrial plants nor to those of dinoflagellates. The morphology closely approximates Pterospermella but is more than an order of magnitude larger. The morphotype here most closely resembles some Proterozoic acritarchs but differs in aspects of wall structure.