FIGURE 1. Depiction of general terms used within this study. Orthocones refer to straight shelled cephalopods, whereas cyrtocones are slightly curved. Long and short varieties of these shells are referred to by the prefixes longi- and brevi-, respectively. Exogastric and endogastric shells have a convex and concave venter, respectively (ventral direction towards the right for both cyrtocones). Each figure is modified from Teichert et al. (1964).

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FIGURE 2. 1, Isometric 3D model of Nautilus pompilius with soft body retracted (flush with aperture). 2, Isometric 3D model of Nautilus pompilius with soft body extended (Nautilus head model provided by Peter Minister). 3, 3D model of Nautilus pompilius created from a CT-scanned specimen (from Hoffmann et al., 2014; Lemanis et al., 2015, 2016). Retracted (4) and extended soft body (5) hydrostatic models that have evenly distributed cameral gas and liquid in phragmocone. 6, CT-scanned Nautilus pompilius hydrostatic model. Green = soft body, red = cameral gas, blue = cameral liquid, grey = shell. The tip of the upright pyramid coincides with the center of gravity while the tip of the inverted pyramid coincides with the center of buoyancy. Φgas = percent of phragmocone occupied by cameral gas at neutral buoyancy. St = stability index.

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FIGURE 3. Hydrostatic models of the ellesmerocerid Plectronoceras. 1, A possible minimum estimate of body chamber length (40% body chamber length to total length) estimated from Furnish and Glenister (1964, p. K146, fig. 81, 1b). 2, A body chamber ratio (35% body chamber length to total length) nearly resulting in neutral buoyancy. 3, Whorl section. Green = soft body, red = cameral gas, grey = shell. The tip of the upright pyramid coincides with the center of gravity while the tip of the inverted pyramid coincides with the center of buoyancy. Φ = percent of phragmocone that would need to be occupied by cameral gas to achieve neutral buoyancy. St = stability index.

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FIGURE 4. Hydrostatic model of a mesodomic orthocerid (body chamber to total length = 33%). 1, Φgas distributed adapically. 2, Φgas distributed adorally. 3, Φgas evenly distributed within each camera. 4, Whorl section. Green = soft body, red = cameral gas, blue = cameral liquid, grey = shell. The tip of the upright pyramid coincides with the center of gravity while the tip of the inverted pyramid coincides with the center of buoyancy. Φgas = percent of phragmocone occupied by cameral gas at neutral buoyancy. St = stability index.

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FIGURE 5. Hydrostatic model of an orthocerid (body chamber to total length = 44%). 1, Φgas distributed adapically. 2, Φgas distributed adorally. 3, Φgas evenly distributed within each camera. 4, Whorl section. Green = soft body, red = cameral gas, blue = cameral liquid, grey = shell. The tip of the upright pyramid coincides with the center of gravity while the tip of the inverted pyramid coincides with the center of buoyancy. Φgas = percent of phragmocone occupied by cameral gas at neutral buoyancy. St = stability index.

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FIGURE 6. Hydrostatic model of a mesodomic orthocerid with cameral deposits (body chamber to total length = 33%). 1, Φgas distributed evenly among all camerae. 2, Φcd distributed adapically. 3, Φgas distributed ventrally (rotates model by 1.66o upwards in the dorsal direction). 4, Whorl section of 1 and 2. 5, Whorl section of 3. Green = soft body, red = cameral gas, purple = cameral deposits, grey = shell. The tip of the upright pyramid coincides with the center of gravity while the tip of the inverted pyramid coincides with the center of buoyancy. Φcd = percent of phragmocone occupied by cameral deposits at neutral buoyancy. θ = angle of aperture inclination from vertical. St = stability index.

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FIGURE 7. 1, Orthocerid-like hydrostatic model with large siphuncular modification similar to an endocerid. 2, Hydrostatic model of the endocerid, Endoceras. 3, Whorl section of 1. 4, Whorl section of 2. Green = soft body, red = cameral gas, purple = cameral deposits, grey = shell. The tip of the upright pyramid coincides with the center of gravity while the tip of the inverted pyramid coincides with the center of buoyancy. Φcd = percent of siphuncle occupied by endosiphuncular deposits at neutral buoyancy. St = stability index.

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FIGURE 8. 1, Hydrostatic model of the oncocerid, Oonoceras. 2, Hydrostatic model of the oncocerid, Euryrizocerina. 3, Whorl section of 1. 4, Whorl section of 2. Green = soft body, red = cameral gas, blue = cameral liquid, grey = shell. The tip of the upright pyramid coincides with the center of gravity while the tip of the inverted pyramid coincides with the center of buoyancy. Φgas = percent of phragmocone occupied by cameral gas at neutral buoyancy. St = stability index.

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FIGURE 9. Hydrostatic models of the discosorid, Phragmoceras. 1, Adapical Φgas. 2, Adoral Φgas. 3, Evenly distributed Φgas. 4, X-ray view of the Phragmoceras model. 5, Transverse view showing adult apertural constriction (dorsal opening and ventral hyponomic sinus). Green = soft body, red = cameral gas, blue = cameral liquid, grey = shell. The tip of the upright pyramid coincides with the center of gravity while the tip of the inverted pyramid coincides with the center of buoyancy. Φgas = percent of phragmocone occupied by cameral gas at neutral buoyancy. St = stability index.

figure9