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Relation of shell strength to evolution in the Ammonoidea
Hewitt, R.A. (1993). Relation of shell strength to evolution in the Ammonoidea, in: House, M.R. (Ed.) The Ammonoidea: environment, ecology, and evolutionary change. pp. 35-56
In: House, M.R. (Ed.) (1993). The Ammonoidea: Environment, ecology, and evolutionary change. The Systematics Association Special Volume, 47. Clarendon Press: Oxford. ISBN 0-19-857765-6. 353 pp.
In: The Systematics Association Special Volume
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  • Hewitt, R.A.
Abstract
    The last septum sealing the unflooded part of the phragmocone was probably the weakest part of both ammonoid and nautiloid shells during loading by depth-correlated water pressure. The concave-out axial curvature of the fluted septum between the saddles of an ammonoid septal suture, produced a higher critical buckling pressure than cylindrical flutes of equivalent weight, and caused a local radial displacement of the suture which produced a spring-like support for the phragmocone wall. The resultant sutural complexity was poorly correlated with bathymetry, unlike the ratio between the skeletal thickness and minimum radius of curvature of various toroidal segments within the phragmocone. The latter indicate that relatively few ammonoids migrated below the depth limit of 240 m required for efficient and sustained pumping of water from the phragmocone by simple osmosis. The water pressure and other growth limitations maintained a conservative globular embryonic shell morphology, supported against circumferential pressure by the fluted proseptum. It was probably hatched in an epipelagic environment which contrasted with the shallow benthic habitat of most fossil nautiloids and the potentially mesopelagic dispersal stage of the hatched ammonoids. The relatively active adult ammonoids presumably improved their circulatory system as an adaptation for life on the margins of deoxygenated benthic water masses which were likely to support lower-density populations of the less active and bottom-feeding nautiloids. The shallow-water ammonoids were more prone to extinction than the similarly shallow-water, Jurassic-Tertiary nautiloids, and the deeper-water ammonoids. New morphological depth criteria are developed for future studies of nautiloid and ammonoid habitat depth limits. But the distinction between epipelagic and epibenthic habitats must be largely made from geological data.
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