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      A method to the madness: Ontogenetic changes in the hydrostatic properties of Didymoceras (Nostoceratidae: Ammonoidea)

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          Abstract

          The seemingly aberrant coiling of heteromorphic ammonoids suggests that they underwent more significant changes in hydrostatic properties throughout ontogeny than their planispiral counterparts. Such changes may have been responses to different selective pressures at different life stages. The hydrostatic properties of three species of Didymoceras ( D. stevensoni, D. nebrascense, and D. cheyennense) were investigated by creating virtual 3D models at several stages during growth. These models were used to compute the conditions for neutral buoyancy, hydrostatic stability, orientation during life, and thrust angles (efficiency of directional movement). These properties suggest that Didymoceras and similar heteromorphs lived low-energy lifestyles with the ability to hover above the seafloor. The resultant static orientations yielded a downward-facing aperture in the hatchling and a horizontally facing aperture throughout most of the juvenile stage, before terminating in an upward direction at maturity. Relatively high hydrostatic stabilities would not have permitted the orientation of Didymoceras to be considerably modified with active locomotion. During the helical phase, Didymoceras would have been poorly suited for horizontal movement, yet equipped to pirouette about the vertical axis. Two stages throughout growth, however, would have enhanced lateral mobility: a juvenile stage just after the formation of the first bend in the shell and the terminal stage after completion of the U-shaped hook. These two more mobile phases in ontogeny may have improved juvenile dispersal potential and mate acquisition during adulthood, respectively. In general, life orientation and hydrostatic stability change more wildly for these aberrantly coiled ammonoids than their planispiral counterparts.

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          Most cited references 42

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          Calculation and simulation of ammonoid hydrostatics

          The buoyancy, stability, and orientation of a shelled cephalopod in water are the predictable products of shell geometry, body chamber length, and such physical parameters as shell, tissue, and water densities. Given such physical characteristics as shell geometry, shell, tissue, and water densities, and shell thickness, the hydrostatic characteristics of planispiral shelled cephalopods, including orientation, centers of mass and buoyancy, stability, and neutrally buoyant body chamber length, can be calculated and simulated using microcomputer-based techniques. Individual variables such as geometry, body chamber length, and shell thickness are linked in a calculable manner to orientation, neutral buoyancy, and stability. LivingNautilusprovides a means of testing the model and for making hydrostatic comparisons between ammonoids and nautiloids. The close agreement between calculated versus observed body chamber lengths in five species of Mississippian ammonoids shows that neutral buoyancy, and (with one exception)Nautilus-like orientations, were at least feasible for these species.
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            Ammonoid Life and Habitat

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              Pelagic palaeoecology: the importance of recent constraints on ammonoid palaeobiology and life history

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                Author and article information

                Contributors
                (View ORCID Profile)
                Journal
                Paleobiology
                Paleobiology
                Cambridge University Press (CUP)
                0094-8373
                1938-5331
                May 2020
                April 15 2020
                May 2020
                : 46
                : 2
                : 237-258
                Article
                10.1017/pab.2020.14
                © 2020

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