Typical Retiolitinae (Graptolithina) tubaria (an alternative term for rhabdosomes) are preserved as a network of lists, formed by bandages of cortical tissue, rarely with preserved fragments of thin membranes, which show evidence of a thin fusellar structure. This study focuses on some remarkable specimens, preserved with nearly complete cortical membranes of the ancora sleeve and metathecae. The studied three-dimensional material belongs to species of Spinograptus and Cometograptus from the upper Wenlock (Silurian) strata in Nunavut, Arctic Canada. The membranes of the ancora sleeve of Spinograptus were studied using infrared microscopy for the first time. Images of the membranes clearly show the arrangement of cortical bandages within the meshes of the networks, formed on the outside of the ancora sleeve in mature colonies. No traces of fusellar increments were observed in the ancora sleeve tissues. In addition, specimens of Cometograptus show traces of membranes in the thecal framework, again made of cortical bandages, with no trace of fusellar tissue connected to it. These new data expand our knowledge of the structure of retiolitine tubaria. During their evolution, the fuselli of the ancora sleeve became completely dominated by the cortical bandages, forming lists and extremely rare thin membranes.
