Double-helical macrostructure aids the passive movement of extinctive graptolites (Dicellograptus) revealed by CFD simulation

Abstract Understanding how marine organisms move in the water column is critical for comprehending their palaeoecology and palaeobiogeography. However, interpreting the locomotion of extinct invertebrates can be problematic and difficult because of the lack of close modern analogues or preserved soft tissues. In this study, we chose a biostratigraphically important Ordovician graptolite taxon, Dicellograptus, and reconstructed three-dimensional (3D) models of it. By simulating their rotation patterns via computational fluid dynamics (CFD), we tested three prevailing locomotory hypotheses (“V”-shaped structure, double-helix structure or independently spiralling twin turbaria structure) for Dicellograptus. The simulated hydrodynamic properties (outer-wall pressure fields and velocity fields) suggest that a double-helical rotating locomotory pattern was the most likely for the Ordovician graptolite Dicellograptus because it would have conveyed better feeding efficiency and turbarium stability. Moreover, we analysed whether the evolution from the lineages Jiangxigraptus to Dicellograptus was influenced and selected for by hydrodynamics. The results revealed that the modification of the proximal pattern with a broader first pair of thecae in Dicellograptus than in Jiangxigraptus resulted in reduced rotational velocity and increased stability. This study highlights the close relationship between traditional paleontological analysis and modern computational methods and provides a more comprehensive understanding of the functional morphology of these ancient marine plankton.