Abstract
The internal architecture of chambered ammonoid conchs profoundly increased in complexity through geologic time, but the adaptive value of these structures is disputed. Specifically, these cephalopods developed fractal-like folds along the edges of their internal divider walls (septa). Traditionally, functional explanations for septal complexity have largely focused on biomechanical stress resistance. However, the impact of these structures on buoyancy manipulation deserves fresh scrutiny. We propose increased septal complexity conveyed comparable shifts in fluid retention capacity within each chamber. We test this interpretation by measuring the liquid retained by septa, and within entire chambers, in several 3D-printed cephalopod shell archetypes, treated with (and without) biomimetic hydrophilic coatings. Results show that surface tension regulates water retention capacity in the chambers, which positively scales with septal complexity and membrane capillarity, and negatively scales with size. A greater capacity for liquid retention in ammonoids may have improved buoyancy regulation, or compensated for mass changes during life. Increased liquid retention in our experiments demonstrate an increase in areas of greater surface tension potential, supporting improved chamber refilling. These findings support interpretations that ammonoids with complex sutures may have had more active buoyancy regulation compared to other groups of ectocochleate cephalopods. Overall, the relationship between septal complexity and liquid retention capacity through surface tension presents a robust yet simple functional explanation for the mechanisms driving this global biotic pattern.
Highlights
Are simple, straight or sinuous in nautiloids
Each of these major shifts occurred between global mass extinctions: most species with goniatitic sutures went extinct during the end-Permian extinction, species with ceratitic sutures becoming dominant and going extinct at the end-Triassic mass extinction, species with ammonitic sutures flourishing in the Jurassic and Cretaceous (Fig. 1F)
It should be noted that other suture types are less common, but present within clades that persist through these mass extinctions (e.g., Prolecanitida with goniatitic sutures in the Late Paleozoic to Triassic; Phylloceratida with ammonitic sutures originating in the Triassic and persisting until the Late Cretaceous)
Summary
Are simple, straight or sinuous in nautiloids. This group includes the only extant ectocochleates—the nautilids. ~ 420 Ma a go[31], ammonoids produced several derived clades with increasingly complex suture expressions: agoniatitic and goniatitic (simple sinuous, zig-zags; Fig. 1C), ceratitic (sinuous with accessory elements on the lobes; Fig. 1D), and ammonitic (several levels of accessory elements due to the fractal-like frilling of the septal margins; Fig. 1B,E). Each of these major shifts occurred between global mass extinctions: most species with goniatitic sutures went extinct during the end-Permian extinction, species with ceratitic sutures becoming dominant and going extinct at the end-Triassic mass extinction, species with ammonitic sutures flourishing in the Jurassic and Cretaceous (Fig. 1F). We use 3D printed models of virtually reconstructed septa to investigate the outstanding hypotheses regarding cameral liquid retention capacity and surface tension
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