TESTING THE HYDRODYNAMIC BEHAVIOUR OF THE FIRST SWIMMING APEX PREDATORS – COMPARISONS BETWEEN CAMBRIAN AND ORDOVICIAN RADIODONTS

The precise drivers of the Cambrian Explosion remain largely unresolved, but there is overall

agreement that the origin and evolution of the first apex predators was instrumental in the

evolutionary arms race among the earliest complex animals. The radiodonts, large bodied stem-

group arthropods with diverse autecologies ranging from sediment sifters to filter feeders, likely

played a key role in the function of these early marine biosphere. The functional morphology of

radiodonts has received substantial attention, particularly that of their frontal appendages, which

are the only arthropodized parts of their bodies and whose anatomical features reflect the main

ecological adaptations in these organisms. However, there is a major gap in knowledge with

regards to how the sclerotized dorsal carapaces of these animals would have worked in terms

of their hydrodynamic behaviour, which in turn affects their locomotion and trophic mode. In this

study, we tested the hydrodynamic properties of several radiodont species from the Cambrian

and Ordovician through a series of numerical simulations using Computational Fluid Dynamics

(CFD) to compare the effect of head shield morphology on water flow regimes. We compare the

disparate dorsal carapace morphologes of five hurdiid species (Hurdia victoria, Hurdia

triangulata, Pahvantia hastata, Cambroraster falcatus and Aegirocassis benmoulae) to explore

adatpations to different modes of life and feeding strategies, as well as overall body size. We

find evidence for considerable variation across the resulting flow regimes in these species. Our

results indicate that Cambroraster falcatus does not have a favorably hydrodynamic profile,

which concurs with its suggested eudemersal mode of life. Other hurdiid species show greater

effectiveness in terms of streamlining that reduces the effects of turbulence around the

carapace, suggesting adaptations for efficient swimming in the water column.