VARIATION IN SEPTAL SPACING AND LIVING CHAMBER SIZE AMONG ORTHOCONIC NAUTILOIDS (CEPHALOPODA)

The phragmocone and the living chamber are two components that constitute the shell of a nautiloid cephalopod. Because each septum was sequentially the last existing septum, the septa furnish a convenient record of the phragmocone's past growth. The aperture, however, grew more continuously, making the apertural boundary of past living chambers hard to recognize. Moreover, complete living chambers are rare in the fossil record. Especially in larger nautiloid fossils, the living chamber typically endures only in part: sometimes its aperture is broken or poorly defined; other times it is entirely crushed.

The logarithmic growth of the extant nautilus is well known, but orthoconic nautiloids, in the broad sense, provide models that are simpler in geometry but distinct from that of coiled cephalopods. Mathematical functions that relate living chamber size to phragmocone size can only aid the reconstruction and restoration of fragmentary fossils. Determining the living chamber's volume helps constrain estimates of a cephalopod's mass and buoyancy, information helpful in understanding its ecology. A dearth of such formulas has hampered historical reconstructions, which by default extrapolated measurements from cephalopods only distantly related. Determining the relative proportions of the phragmocone and living chamber is also necessary to establish the total relative size of the conch in nautiloids, where estimates have ranged up to 9 m.

The goal of this study is to gather linear measurements of living chambers and phragmoconic chambers from a variety of mostly orthoconic nautiloid taxa. The data come from Paleozoic nautiloids from North America, either collected for this study or already residing in US museums. Plotting these data can uncover patterns of growth, whether within a single individual, within species, or across taxa. For example, some orthocerids grew isometrically, producing a linear plot. Some Actinoceras, by contrast, present a more sigmoidal growth curve. The crowding of the last few septa coincides with a constriction of the aperture, which may challenge the hypothesis that septal approximation adjusts for an allometric expansion of the living chamber.