Pelagic or benthic? Mode of life of the remopleuridid trilobite Hypodicranotus striatulus

The mode at life of the remopleuridid trilobite Hypodicranotus striatulus Walcott (1875) was examined hydrodynamically with a special focus on the relationship between the autecological performances of swimming and feeding. To understand the effect of swimming height from the sea bottom on the hydrodynamic performance of the exoskeleton, we performed computational fluid dynamics simulations on four models at differing distances from the sea bottom. The results indicated that Hypodicranotus could launch itself from the sea bottom with a relatively strong hydrodynamic lift force from slow walking or swinuning speeds. However, the lift force decreased as the swimming height increased at slow swimming speeds. Hence. Hypodicranotus would have had to increase its swimming speed to greater than 0.2 m/s and to obtain the most stable lift force at a swimming height equal to half of its own body height. Its exoskeletal morphology, with a forked hypostome, enabled it to launch itself at a slow velocity and swim at a modest distance, i.e., close to its own height, from the sea bottom. Feeding from the median vortex flows along the food groove between the two prongs of the hypostome may have been the best strategy near the sea bottom, where a large amount of food matter would have been available. Because arthropod musculature consists of striated muscles, which exhibit inferior endurance, Hypodicranotus most likely adapted to the near-bottom environment, where it could rest at times on the sea bottom as part of a nektobenthic mode of life.