FLUID-DYNAMICS OF THE GRAPTOLITE RHABDOSOME RECORDED BY LASER-DOPPLERANEMOMETRY

A precise laser based technique has been used to measure changes in fl uid velocity over a range of graptolite models mounted in a wind tunne l. Results from this laser Doppler anemometer (LDPL) 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 graptoli tes produce trailing vortices and turbulence. These results are import ant for three reasons. First, they demonstrate that this tool offers a means of quantitatively and non-intrusively assessing the hydrodynami c function of aspects of graptolite morphology and has the potential t o enable us to understand the specific oceanic conditions for which gr aptolites evolved. Second, they show that, with flow controlled by sic ular and thecal morphology, the zooids were unlikely to have fed withi n the stagnant zones of the thecal apertures; it is more likely that t hey 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 remai ns to be tested if food particles have a tendency to accumulate in the se stagnant zones. Third, as graptolite models are stable in fluids on ly when flow is from sicula to nema, it seems likely that graptolites with relatively simple metathecae arrayed themselves in this fashion r elative to motion in the oceans.