INTERSTIPE WEBBING IN THE SILURIAN GRAPTOLITE CYRTOGRAPTUS-MURCHISONI

Although it has long been recognized that the Graptoloidea constituted a diverse group of planktic organisms, the precise hydrodynamics of t he various colony morphotypes has been a source of debate. Recent disc overies of specimens of Cyrtograptus murchisoni with a complex suite o f webs or vanes between the central coiled stipe and the cladial branc hes have shown that the hydrodynamic modifications of at least this ta xon were considerably more complex than previously thought. These webs are composed of very thin peridermal tissue and stretch between the f irst or second order cladial branches and the main stipe, the webs ove rlapping to give a screw-like morphology to the rhabdosome. The form o f the webbing also has implications for the mode of life and mobility of individual zooids within the colony, as the main areas of web const ruction are in regions in which the zooids were enclosed within restri cted thecal apertures.