Micro-computed tomography is increasingly important for the taxonomic analysis of early Palaeozoic radiolarians as it provides the opportunity to conduct detailed observations of the most internal parts of radiolarian skeletons in a non-destructive way. Our observations on 3D models of the species Protoentactinia gracilispinosa from lower Tremadocian strata of Newfoundland (Canada) indicate that during the first two stages of growth the skeleton develops symmetrically with respect to the median bar. However, subsequently, progressive reduction of the number of rays placed proximally, combined with full growth of rays distally, results in the formation of an external subglobular shape during stages 6 to 8. Although our results confirm the previously assumed growth pattern of the skeleton, they reveal for the first time the number and pattern of branched spicule iterations. More importantly, we establish that some of the stages of skeleton growth are in a preferential direction, highlighting a clear heteropolarity to skeleton development, although the end product is a subspherical spicular test.

Polycystine radiolaria, Entactinaria, skeletogenesis, micro-computed tomography