Modern medusae are primarily macroscopic, free-swimming marine organisms with a tetraradial symmetry. But the early Cambrian medusozoans, such as biradial Hexaconularia sichuanensis and tetraradial Quadrapyrgites quadratacris from the basal Cambrian Kuanchuanpu Formation in South China, were millimeter-scale and sedentary forms. Although well-documented as common components in a shelf environment, little is known about their in situ life conditions since they are easily transported by tidal flow before burial. Specifically, the interactions between their exoskeleton shapes and the hydrodynamic conditions are poorly known. Here we use fluid-structure interaction computational fluid dynamics to analyze the functional performance of the two taxa using 3D mesh models constructed from scanned fossils. The simulation experiments reveal that: (a) due to their millimeter-scale size, most sedentary small shelly fossils (SSFs), including those of medusozoans lived within the viscous sublayer range of tidal flow; (b) the biradial symmetry of Hexaconularia was more structurally stable than the tetraradial symmetry of Quadrapyrgites; (c) different orientations of Hexaconularia, relative to the water flow, resulted in different force distributions on the surface. This research provides quantitative data of the hydrodynamic conditions of SSFs in shelf environments and critical insights into the biotic replacement of the early Cambrian benthic medusozoan community.