The balancing act ofNipponites mirabilis(Nostoceratidae, Ammonoidea): managing hydrostatics during a complex ontogenetic trajectory

Abstract

AbstractNipponitesis a heteromorph ammonoid with a complex and unique morphology that obscures its mode of life and ethology. The seemingly aberrant shell of this Late Cretaceous nostoceratid seems deleterious. However, hydrostatic simulations suggest that this morphology confers several advantages for exploiting a quasi-planktic mode of life. Virtual, 3D models ofNipponites mirabiliswere used to compute various hydrostatic properties through 14 ontogenetic stages. At each stage,Nipponiteshad the capacity for neutral buoyancy and was not restricted to the seafloor. Throughout ontogeny, horizontally facing to upwardly facing soft body orientations were preferred. These orientations were aided by the obliquity of the shell’s ribs, which were parallel to former positions of the aperture during life. Static orientations were somewhat fixed, inferred by stability values that are slightly higher than extantNautilus. The initial open-whorled, planispiral phase is well suited to horizontal backwards movement with little rocking.Nipponitesthen deviates from this coiling pattern with a series of alternating U-shaped bends in the shell. This modification allows for proficient rotation about the vertical axis, while possibly maintaining the option for horizontal backwards movement by redirecting its hyponome. These particular hydrostatic properties likely result in a tradeoff between hydrodynamic streamlining, suggesting thatNipponitesassumed a low energy lifestyle of slowly pirouetting in search for planktic prey. Each computed hydrostatic property influences the others in some way, suggesting thatNipponitesmaintained a delicate hydrostatic balancing act throughout its ontogeny in order to facilitate this mode of life.