The balancing act of Nipponites mirabilis (Nostoceratidae, Ammonoidea): Managing hydrostatics throughout a complex ontogeny

Abstract

Nipponites is 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 of Nipponites mirabilis were used to compute various hydrostatic properties through 14 ontogenetic stages. At each stage, Nipponites had the capacity for neutral buoyancy and was not restricted to the seafloor. Throughout ontogeny, horizontally facing to upwardly facing soft body orientations were preferred at rest. These orientations were aided by the obliquity of the shell's ribs, which denote former positions of the aperture that were tilted from the growth direction of the shell. Static orientations were somewhat fixed, inferred by stability values that are slightly higher than extant Nautilus. The initial open-whorled, planispiral phase is well suited to horizontal backwards movement with little rocking. Nipponites then deviated from this bilaterally symmetric 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 that Nipponites assumed a low energy lifestyle of slowly pirouetting in search for planktic prey. Each computed hydrostatic property influences the others in some way, suggesting that Nipponites maintained a delicate hydrostatic balancing act throughout its ontogeny in order to facilitate this mode of life.

Buoyancy control in ammonoid cephalopods refined by complex internal shell architecture

“…Live Nautilus vary from up to 32% overall fluid retention in juveniles, to about 12% or less in adults 19 , 21 . Estimates for ammonoids range from nearly empty to 40% 56 – 60 . Alternatively, positive buoyancy could have been offset by simply increasing the relative size of the soft body instead of retaining more liquid.…”

“…Other interpretations suggest that the marginal crenulations of septa could have retained liquid by surface tension, possibly improving buoyancy adjustment 24 , 25 , 28 , 47 , 54 , 55 . The retention of chamber liquid is supported by empirical assessments of ammonoid buoyancy, demonstrating that some amount of liquid is required for a near neutrally buoyant condition 56 – 60 . Extant Nautilus are slightly negatively buoyant, which is easier for the living animal to manage 21 .…”

“…The Damesites camera was extracted from the CT (computed tomography) scan from Inoue and Kondo 63 . The Menuites camera was constructed using the methods of Peterman et al 59 , 60 . A specimen of Menuites oralensis was prepared to expose the entire suture pattern (including the internal portion).…”

Recent advances in heteromorph ammonoid palaeobiology

“…Novel methods will allow us to study intraspecific variation quantitatively in orthoconic, gyroconic, and vermiform shapes (Okamoto, 1988 ab , c , 1996; Tsujino, Naruse & Maeda, 2003; De Baets et al ., 2013 a ; Urdy, 2015; Ward et al ., 2015; Hoffmann et al ., 2019). Even taxa that seemingly grow chaotically like Nipponites have been demonstrated to have a regular growth pattern consistent with a free‐living, pelagic life habit (Okamoto, 1988 c ; Peterman, Mikami & Inoue, 2020 c ). In many cases, more traditionally defined species have been synonymised, leading to more comprehensive analyses of drivers of the mode and degree of intraspecific variation in different taxa (Ward et al ., 2015; De Baets et al ., 2015 b ; Hoffmann et al ., 2019).…”

Vertical escape tactics and movement potential of orthoconic cephalopods

“…The transverse cross-section of shrimpfish (Fish & Holzman, 2019) is also similar to many baculitid ammonoids (see Larson et al, 1997), which infers drag would be reduced in the dorsal direction. Although this horizontal mode of locomotion in orthocones would not be very efficient due to the higher hydrodynamic drag relative to vertical movement, and their low source of jet thrust (i.e., the thrust angle; Okamoto, 1996;Peterman, Mikami & Inoue, 2020). The vertical orientation of shrimpfish is thought to be associated with camouflage (Fish & Holzman, 2019); a function unlikely for comparatively larger orthocones, and those in habitats lacking structure in which to hide.…”