Jurassic ammonite aptychi: functions and evolutionary implications

Abstract

Nine proposals of aptychus (sensu stricto) function have been published (in historical order): operculum, micromorphic males, lower mandible, protection of gonades, ballast for lowering of aperture, flushing of benthic prey, filtering microfauna, pump for jet propulsion, and active stabilizer against rocking produced by the pulsating jet during forward foraging and backward swimming. Some ammonites bear thick, laevaptychus- and lamellaptychus-type aptychi (aspidoceratids and haploceratoids) that may have improved lowering of the aperture as part of a mobile cephalic complex, enabling many of these functions. Aptychi were multifunctional, most commonly combining feeding (jaw, flushing, filtering) with protection (operculum), and/or with propulsion (ballast, pump, diving and stabilizing plane). Multifunctionality would have been a strong constraint in ontogeny and evolution as shown by the limited diversity of aptychi with respect to the wide variety of shell morphologies known in the Mesozoic Ammonitina. Calcification of aptychi in the Jurassic Ammonitina is known from the Early Toarcian Hildoceras which is also the first ammonite with males bearing well-formed lateral peristomatic projections or lappets. Calcification allowed aptychi to be involved in functions, which would have improved, in different degrees and combinations, feeding, propulsion and protection. It is herein suggested that multifunctional calcareous aptychi allowed the gradual development of a wide variety of new life-styles. These new life-styles would have led to the origin and early evolution of haploceratids and stephanoceratids producing the wide diversification of the Ammonitina observed from the Early Aalenian.