Mode of life and soft body shape of heteromorph ammonites
ABSTRACT Ebel, K. 1992 04 15: Mode of life and soft body shape of heteromorph ammonites. Lethaia, Vol. 25, pp. 179–193. Oslo. ISSN 0024–1164.Using the idea of a benthic mode of life for ammonites, based on a gastropod-like shell position, it is possible to reconstruct the development of all heteromorph ammonites by regarding single growth stages and the presumable acting forces. The reconstruction of shell formation, particularly the final shell position of heteromorphs with a hook, indicates that the soft body of the ammonite animal was considerably larger than comparison with the present-day Nautilus would suggest.
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ABSTRACT: Previous analyses of ammonite functional morphology have assumed that the animal filled the entire body chamber, and that movement of the animal (such as withdrawal of the head and arms into the shell) had little effect on orientation. An alternative anatomy is proposed here: the ammonite animal was small, mobile, and capable of moving away from the aperture when threatened. The effect of this anatomy on the hydrostatics of aspinoconic, ancycloconic, and hamiticonic heteromorph ammonites is analysed. This analysis suggests that movement of the body would significantly alter the distribution of mass and hence orientation.
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ABSTRACT: A review of fossil evidence supports a pelagic mode of life (in the water column) of ammonoids, but they may have spent their life close to the seabottom (demersal), planktonically, or nektonically depending upon the ontogenetic stage and taxon. There are good indications for a planktonic mode of life of ammonoid hatchlings, but a broad range of reproductive strategies might have existed (egglaying, fecundity). Isotope and biogeographical studies indicate that some forms migrated or swam for considerable distances, whereas others may have been primarily transported by oceanic currents during early and/or late ontogeny. Diverse ammonoid habitats are also supported by evidence from predator–prey relationships derived from characteristic injuries and exceptional fossil finds, which indicate chiefly predatory or scavenging lifestyles. Sublethal injuries preserved in some ammonoid shells, as well as rare stomach and coprolite contents, provide evidence of predation by other cephalopods, arthropods and various jawed vertebrates. Various lines of evidence suggest that different groups of ammonoids had quite different ecologies, but shell shape alone can only give upper constraints on ammonoid capabilities, a matter that needs to be considered when interpreting their diversity and evolutionary history.Journal of Zoology 04/2014; 292(4):229-241. DOI:10.1111/jzo.12118 · 1.95 Impact Factor
Article: Ammonites as Cartesian Divers[Show abstract] [Hide abstract]
ABSTRACT: Many constructional and taphonomic incongruencies dissolve if ammonites are modeled as active Cartesian divers, whose last septum remained a rubber-like, uncalcified membrane until the next chamber cycle began. If the prochoanitic invagination of the septal mantle served as a gas gland able to produce gas at or above ambient pressure, the chamber behind could be operated like a swim bladder by sutural muscles spanning across the septal mantle. Other septal functions probably served as evolutionary stepping stones on the way to this unique design.Palaios 12/1995; 10(6):493. DOI:10.2307/3515089 · 1.90 Impact Factor