Article: Fluid dynamics of the graptolite rhabdosome recorded by laser doppler anemometry

A precise laser based technique has been used to measure changes in fluid velocity over a range of graptolite models mounted in a wind tunnel. Results from this laser Doppler anemometer (LDA) show the flow to be altered significantly by spines on the sicula and by the morphology of the thecae. A single virgellar spine retards flow along the 'naked' (ventral) side of the sicula and directs it instead over the thecae. More complicated sicular spine arrays in Ordovician biserial graptolites produce trailing vortices and turbulence. These results are important for three reasons. First, they demonstrate that this tool offers a means of quantitatively and non-intrusively assessing the hydrodynamic function of aspects of graptolite morphology and has the potential to enable us to understand the specific oceanic conditions for which graptolites evolved. Second, they show that, with flow controlled by sicular and thecal morphology, the zooids were unlikely to have fed within the stagnant zones of the thecal apertures; it is more likely that they fed at some distance from these apertures, either with lophophores extended into the sea or having themselves crawled along spines. The stagnant or quiet zones provided a resting position. However, it remains to be tested if food particles have a tendency to accumulate in these stagnant zones. Third, as graptolite models are stable in fluids only when flow is from sicula to nema, it seems likely that graptolites with relatively simple metathecae arrayed themselves in this fashion relative to motion in the oceans.