291-5 Reconstructing orthocone cephalopods: did cameral deposits function as counterweights?

Session: Cephalopods Through Time: Insights into Evolution, Ecology, and Environmental Reconstruction

Presenting Author:

Authors:

Abstract:

Throughout the Paleozoic, straight-shelled cephalopods (orthocones) became incredibly successful, occupying a diverse range of environmental settings around the globe. While this morphotype characterizes over a hundred genera with ostensibly similar conch shapes, some clades had different internal characteristics. Many orthoceratoids had enigmatic calcareous structures within their chambered shells, which likely grew during life (cameral deposits). These deposits have been traditionally interpreted as counterweights, offsetting the mass of the soft body and permitting horizontal orientations while swimming. However, these animals must have maintained a delicate balancing act, managing their soft body and cameral deposit proportions to produce a neutrally buoyant condition. Using computer reconstructions, we explore the practical limits of cameral deposit influence on mass distribution, while satisfying neutral buoyancy. We vary body chamber ratio (BCR; body chamber length to total length) and cameral deposit distribution to investigate the practical range of orientation, stability, and maneuverability for orthocones. Cephalopods with 40% BCR are neutrally buoyant with empty chambers, and assume a highly stable, downward-facing orientation. As BCR decreases to 30%, there is some room for cameral deposits, but gains in maneuverability are negligible. Between BCRs of 22-25%, stability decreases to zero, allowing any orientation to be assumed (i.e., highest maneuverability). Reducing stability or altering orientation with theoretical cameral deposit distributions only works within this narrow BCR range. Cameral-deposit-bearing orthocones often express higher body chamber ratios (≥~30%), obscuring a broad generalizable counterweight function for cameral deposits. Alternatively, they could have served as ballasts, strengthened the shell, or offered physiological benefits. Externally shelled cephalopods are unique among swimming animals because they have experimented with differences in stability and maneuverability spanning orders of magnitude. These results add context to how early cephalopods interacted with physical tradeoffs that placed fundamental constraints on their swimming capabilities and life habits. 

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-6378

© Copyright 2025 The Geological Society of America (GSA), all rights reserved.