Interstipe webbing in the Silurian graptolite Cyrtograptus murchisoni and its palaeobiological significance.

Abstract

Although it has long been recognized that the Graptoloidea constituted a diverse group of planktic organisms, the precise hydrodynamics of the various colony morphotypes has been a source of debate. Recent discoveries of specimens of Cyrtograptus murchisoni with a complex suite of webs or vanes between the central coiled stipe and the cladial branches have shown that the hydrodynamic modifications of at least this taxon were considerably more complex than previously thought. These webs are composed of very thin peridermal tissue and stretch between the first or second order cladial branches and the main stipe, the webs overlapping to give a screw-type morphology to the rhabdosome. The form of the webbing also has implications for the mode of life and mobility of individual zooids within the colony, as the main areas of web construction are in regions in which the zooids were enclosed within restricted thecal apertures