Impact of ontogeny and spines on the hydrodynamic performance of the Cambrian arthropod Isoxys

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 Isoxys. Pelagic groups evolved from benthic stock, in many cases through neoteny and retention of characteristics from planktic larval stages. However, Isoxys 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 Isoxys, Isoxys auritus and Isoxys minor. 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 Isoxys to predators. Taken together, it is likely that pelagic Isoxys species evolved from benthic stock through the co-option of anti-predatory features.