Segmental growth in planulate ammonites: inferences on costal function

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Abstract
In planulate Ammonitina, the directions of costae and their parts coincide with the growth lines, both being accreted in parallel and subtriangular increments, i.e. segments. This implies that the growing margin was consistently corrugated (plicated) parallel to the edge of the peristome. The adventral separation of the ribs caused by the coiling increased with whorl involution and was compensated for by costal division into primaries and secondaries. We propose that costation of the shallower-water, nektobenthic planulates reduced the stiffness and increased effective thickness of their growing peristome. Ribs functioned (primarly or secondarily) as a protection for the immature, fragile aperture against predators and impacts.