Swimming capability of the remopleuridid trilobite Hypodicranotus striatus: Hydrodynamic functions of the exoskeleton and the long, forked hypostome

The sophisticated hydrodynamic performance achieved by the exoskeleton and the long, forked hypostome of the remopleuridid trilobite Hypodicranotus striatus was demonstrated using image-based modelling and computational fluid dynamics simulation techniques. To understand the function of the long, forked hypostome, we examined two types of exoskeletal models, one with and one without the hypostome. We simulated the flow structures around the exoskeletal models under several ambient flow velocities to evaluate the shapes of the streamlines, the values of the drag and lift forces and the relevant coefficients acting on the models. The simulation results showed that the long, forked hypostome prevents the formation of a ventral vortex; thus, it stabilises the flow structure under all of the ambient velocities tested. Moreover, the hypostome functions to create positive lift, with stable lift coefficients observed under a wide range of velocities, and to reduce the drag coefficient as velocity increases. These results imply that the hypostome can reduce viscous drag with a modest lift force, which is an essential requirement for actively swimming animals. We conclude that the long, forked hypostome evolved to provide an active and stable swimming system, and we therefore hypothesise that Hypodicranotus exoskeletal morphology resulted from the adaptation to be a high-performance swimmer.

► Swimming capability of trilobite Hypodicranotus was verified through CFD simulations.
► Form of exoskeleton is well streamlined with less flow detachment around the model.
► Effaced exoskeleton reduced drag force under conditions of higher flow velocity.
► Long-prolonged labium structure (hypostome) obtained positive modest lift force.
► Hypodicranotus evolved to swim faster with hydrodynamic benefits in morphology.