Modelling flow patterns in conical dendroid graptolites

Abstract

Life-sized wire models of several different morphologies of conical dendroid graptolites were tested in a flume to observe the flow patterns within the rhabdosome. Testing over a range of unidirectional current speeds from 3 to 15 cm/s showed that upward-directed currents are produced within the cone, and a significant amount of the water that enters the cone through the upstream wall exits the top opening. These upward currents were most evident at speeds of 3–10 cm/s. Shorter cones, wide cones, less porous mesh, and downstream tilting each increased the amount of water exiting the top of the cone, especially at higher speeds. These findings are consistent with a mode of feeding in which conical forms received afferent currents into the sides of the cones, where food particles were captured, and waste efferent currents went out the top opening. This does not support the hypothesis that downward-directed, ciliary feeding currents produced the force necessary to propel the earliest planktic graptolites into the water column in the evolutionary transition from a benthic to a planktic mode of life. The suggestion that differing rhabdosomal morphologies may represent adaptations to differing paleocurrent regimes is supported. Dendroid rhabdosomal morphology may prove to be a useful paleoenvironmental tool, particularly as an indicator of current strengths, especially in fine-grained sediments where other current-indicative structures may be lacking.Graptolite, dendroid, flow, feeding, paleoecology.