Checa, Antonio&Westermann, Gerd E. G. 1989 01 15: Segmental growth in planulate ammonites: inferences on costal function.Lethaia, Vol. 22, pp. 95–100. Oslo. ISSN 0024–1164.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 shallow‐water, nekto‐benthic plandates reduced the stiffness and increased effective thickness of their growing peristome. Ribs functioned (primarily or secondarily) as a protection for the immature, fragile aperture against predators and impacts.□Ammonite shell, Jurassic, morphology, growth lines, ornamentatio