Hexangulaconulariids (Cambrian stages 1‒2) are an extinct group of medusozoan polyps having a biradially symmetrical, fan-shaped periderm that is distinct from those of medusozoan polyps showing three-, four-, five-, or six-fold radial symmetry. Hexangulaconulariids exhibit substantial variation in gross morphology, including variation in the number of faces on each of the two major sides of the periderm. An intermediate taxon of hexangulaconulariids with ten faces (five on each major side) was expected. Here we describe a new hexangulaconulariid, Decimoconularia isofaciem n. gen. n. sp. from Bed 5 of the Yanjiahe Formation (Cambrian Stage 2) in the Three Gorges area of Hubei Province, China. The new taxon differs mainly from other hexangulaconulariids (Arthrochites, Hexaconularia, and Septuconularia) in possessing a total of 10 faces. The two lateral margins are each marked by a ridge in about the apertural half of the periderm and by a collinear furrow in about the apical half, while the five faces on each major side are bounded by a furrow in about the apertural half and by a collinear ridge in about the apical half. Among hexangulaconulariids, Decimoconularia and Septuconularia may be more closely related to each other than either genus is to Arthrochites or Hexaconularia.

The specimens were picked under a binocular microscope from insoluble residues produced by digestion of rock samples with 10% acetic acid. The fossils were coated with gold and then imaged using a FEI Quanta 400 FEG scanning electron microscope (SEM) at an accelerating voltage of 15 eV (secondary electron mode). The holotype specimen (CUBar172-3) was imaged using the Zeiss Xradia 520 at the State Key Laboratory of Continental Dynamics, Northwest University, Xi’an, China, at a beam current/voltage of 80 µA/40 kV with a voxel size of 2.23 µm. The micro-CT data were processed using VG Studio 2.2 Max for 3D volume rendering and movies.