Three-Dimensional Bookstein Shape Coordinates and Functional Morphology of Passive Suspension Feeding in Composita (Brachiopoda, Athyridida)
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Abstract
The effects of shell morphology on passive fluid circulation are examined in Late Mississippian through Late Pennsylvanian Composita (Brachiopoda, Athyridida) from Texas, Kentucky, and Iowa. The first three-dimensional derivation of Bookstein shape coordinates is given in association with an in-depth discussion of the advantages and potential drawbacks of the technique for geometric shape analysis applications. Three-dimensional morphometric analysis of Composita shows a single shape distribution corresponding to progressive differentiation of the commissure into vertically displaced parasulcate, lateral, and sulcate gapes with increasing shell size, indicating an ontogenetic shape change trend. Gaping models of three specimens (including a simulated lophophore in the largest specimen) representative of different ontogenetic stages were used to observe the effects of morphology on passive flow circulation in a recirculating flume tank. All models showed medially-inhalant and laterally-exhalant passive circulation when oriented with the sulcus facing upstream, with more vigorous flow associated with increasing shell size.