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.