Impact of ontogeny and spines on the hydrodynamic performance of the Cambrian arthropod <i>Isoxys</i>

A metazoan-dominated biological pump was established early in the Phanerozoic, a time that saw the evolution of the first pelagic euarthropod zooplankton such as some species of the Cambrian bivalved euarthropod <i>Isoxys</i>. Pelagic groups evolved from benthic stock, in many cases through neoteny and retention of characteristics from planktic larval stages. However, <i>Isoxys</i> brooded eggs and did not have a planktic larval stage, precluding this route into the pelagic realm. Computational fluid dynamics was used to quantify hydrodynamic performance through the ontogeny of two hyperbenthic species of <i>Isoxys, Isoxys auritus</i> and <i>Isoxys minor</i>. Coefficients were used to quantify forces for different carapace shapes over a range of biologically relevant sizes and swimming speeds. Streamlining and lift generation were greater for later growth stages, a consequence of carapace asymmetry and elongated anterior and posterior spines. Simulations performed with anterior spines artificially removed demonstrate the importance of this feature for lift generation, with a minimal impact on drag. Elongated spines and faster swimming can also be considered anti-predatory, and the reduction of drag would have reduced the detectability of <i>Isoxys</i> to predators. Taken together, it is likely that pelagic <i>Isoxys</i> species evolved from benthic stock through the co-option of anti-predatory features.