Jurassic ammonite aptychi: functions and evolutionary implications
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Jurassic ammonite aptychi: functions and evolutionary implications Horacio Parent1 • Gerd E. G. Westermann
Received: 20 January 2015 / Accepted: 31 August 2015 Ó Akademie der Naturwissenschaften Schweiz (SCNAT) 2015
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 wellformed 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.
Deceased: Gerd E. G. Westermann. & Horacio Parent [email protected] 1
Laboratorio de Paleontologı´a, IFG-FCEIA, Universidad Nacional de Rosario, Pellegrini 250, 2000 Rosario, Argentina
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. Keywords Ammonitina Aptychus multifunctionality Protection Feeding Locomotion Aalenian diversification
Introduction Aptychi (sensu stricto) are calcitic, bivalved plates commonly found singly or in pairs, isolated or associated with ammonites where they usually occur in the body-chamber. Aptychi are universally accepted as integral parts of the ammonite organism and there is wide consensus that aptychi were part of the buccal mass (e.g. Dzik 1981; Dagys et al. 1989; Lehmann and Kulicki 1990; Nixon 1996). They are paired outer calcareous plates of the lower jaw, typically wing-shaped, showing a range of morphotypes. Aptychus morphologies commonly differ distinctly among Ammonitina families and occur in both sexual dimorphs (see Arkell 1957: L437–L440; Lehmann 1981; Dagys et al. 1989; Morton and Nixon 1987; Dzik 1981; Schweigert 2009; Parent et al. 2011, 2014 and references therein). The types of Jurassic aptychi
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