The Body Chamber Length Variations and Muscle and Mantle Attachments in Ammonoids

Abstract

AbstractThe varying body chamber lengths and the different attachment of muscles and mantle to conch wall belong to the major adaptations to their diverse modes of locomotion. Therefore, these traits are indirect indicators of different life styles. The sparse record of ammonoid body chamber lengths and attachment marks has impeded the understanding of this aspect of ammonoid paleobiology. The examination of body chamber length revealed that the decrease of the ammonitella body chamber lengths shows is the long-term trend characterizing the evolutionary development of the Goniatitida–Prolecanitida–Ceratitida–Phylloceratida branch of the Ammonoidea. The analysis of the body chamber lengths and the attachment marks leads to conclusion that a precondition for the jet-powered swimming of ammonoids is less than one whorl body chamber length and the position of the attachment marks in sites from where the cephalic retractor and funnel retractor muscles would be able to extend straight across to the head and to the funnel. This is the case of goniatitids and ammonitids possessing moved forward large ventrolateral muscle marks; jet-powered swimming is highly probable for them. None of the universal small dorsal, umbilical and ventral marks may be left in the attachment sites of the cephalic retractor and funnel retractor muscles. In the hook-shaped terminal body chambers of heteromorph ammonoids, like Audouliceras, the long tongue-like umbilical marks perhaps indicate the moved forward strong umbilical muscles adapted for regular change of the mantle cavity volume for sucking and filtering seawater. This suggests that such ammonoids fed on fine plankton or suspended organic rich substance. Their irregular coiled spiral shells, best suited to floating and perhaps vertical (diurnal) migrations, support the view above. The fossilized mantle so far described in the ceratitid ammonoid Austrotrachyceras has a laminated structure fibrous seen in internally shelled Jurassic belemnotheutis and Loligosaepia.KeywordsAmmonoidsBodychamberlengthMantle and muscle attachment marksJet-powered swimmingMantle fossilizationAttachment markspreservation