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Abstract
Ancient fenestrate bryozoans (Fenestellidae, Polyporidae) exhibit a range of fenestrule widths that can be investigated to gain insights about constraints on meshwork morphology. Theoretical morphospace analyses of branch width and spacing reveal that there is a minimum fenestrule width common to both groups. We propose that the minimum fenestrule width observed in fenestrate colonies was constrained by hydraulic resistance (the hydraulic-resistance-constraint hypothesis). Modelling that considers both the viscous and inertial components of fluid-flow resistance through hypothetical fenestrate meshworks corroborates this hypothesis for the smaller fenestrate meshworks of the biserial fenestellids and for polyporids with branch widths less than about 0.6 mm. However, the geometry of the polyporid meshworks in species with branches wider than 0.6 mm does not appear to have been determined by a hydraulic-resistance constraint. Instead, maximum branch densities in the larger-branched polyserial polyporid colonies appear to have been constrained by a limit on the number of zooid rows that these colonies could develop on their branches (the zooid-row-limit hypothesis). The observed decrease in the minimum fenestrule width in these colonies is the indirect geometric result of the inability of the larger polyserial polyporid colonies to develop meshworks with high branch densities.
Acknowledgements
We acknowledge with gratitude the generous editorial help that Wolf-Ernst Reif gave us in preparing our previous study for publication in the Neues Jahrbuch für Geologie und Paläontologie (Starcher and McGhee Citation2002), and McGhee acknowledges with gratitude the many occasions when Wolf-Ernst Reif assisted him with ever-generous and good-humoured help during his stays of research at the University of Tübingen. McGhee remembers Reif as a brilliant individual, but one who also was unfailingly kind and concerned with helping others even when struggling with the difficulties that his own ill health constantly presented him. We further thank S. Hageman for constructive comments that helped us substantially improve this present paper. This research was supported in part by grants from the National Science Foundation (USA) and from the Petroleum Research Fund, administered by the American Chemical Society.