Skeletal growth pattern of Protoentactinia gracilispinosa Kozur, Mostler & Repetski, 1996, a late Cambrian - Early Ordovician radiolarian species, revealed by X-ray tomography

Micro-computed tomography is increasingly important for the taxonomic analysis of early Palaeozoic radiolarians. It provides an 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 basal Tremadocian strata of western Newfoundland (Canada) indicate that, during the first two stages of growth, the skeleton develops symmetrically with respect to the bar-centred entactinarian spicule. 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 branching 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 branching iterations. More importantly, we establish that some stages of skeletal growth occur in a preferred direction, highlighting a clear heteropolarity of skeleton development, even though the end product is a subspherical spicular test.