The fine structure of the virgular apparatus in Cystograptus vesiculosus



Fragments of a virgular apparatus (nematularium), assigned to Cystograptus vesiculosus (Nicholson 1868), etched out of calcareous intercalations of early Llandoverian age from the South Urals, were investigated with the TEM and the light microscope. The extrarhabdosomal portion of the nematularium is a three-vaned, single-walled structure, built exclusively of fusellar tissue without any cortical covering. Its surface is free of cortical bandages. No traces of an extension of the nema (virgula) proper were found at the junction of all three vanes. The membranous portion of each vane is made of narrow, densely packed strips set at 6–12o to its thickened rim. These microfusellus-like units show highly characteristic undulations of their outer pellicle, providing an additional reinforcement of the structure. Thickened rims are made by the merging of numerous microfuselli which, in their outer part, have a pellicle with a thick outer lamella. A reconstruction of the fine structure of the nematularium is suggested and possible explanations of its morphogenesis and function are discussed. Graptoloidea, nematularium, nema, virgula, ultrastructure, morphogenesis, function, early Llandoverian, southern Urals.