Locomotory and morphological evolution of the earliest Silurian graptolite (Demirastrites) selected by hydrodynamics

Interpretation of the locomotion for biostratigraphic important graptolite taxa is rare and rendered problematic due to their lack of close modern analogues and soft tissues. In this study, based on well-preserved specimens of the early Silurian low-helical spiral Demirastrites Eisel, we reconstructed three-dimensional (3D) Demirastrites models and simulated their locomotion by using computational fluid dynamics. Hydrodynamic properties (outer-wall pressure fields and velocity fields) were obtained and used to test the prevailing hypothesis that the Silurian low helical spiral graptolite Demirastrites could rotate in seawater. The Demirastrites models kept rotating at different velocities in the simulation field, which helped to counteract the impact of the water current and achieve stability. During rotation, higher velocity fields could be observed near the thecal apertures, which meant better access to more nutrient particles in the sea water. Our simulation thus confirmed the rotating locomotory pattern of the Silurian low conical graptolite Demirastrites for the purpose of better feeding efficiency and turbarium stability. Moreover, we analysed how the evolution of structural innovations, such as the density and width of thecae and the curvature angle of the rhabdosome within the recovered geological lineages of Demirastrites, were influenced and selected by hydrodynamics. The results showed that Demirastrites lineages evolved towards increased stability and higher rotation velocity. Our study highlights the importance of hydrodynamic constraints serving as hidden abiotic factors shaping the evolution of planktonic graptolites.