Phylogenetic affinity between <i>Archeoentactinia</i> and the Entactinaria: evidence from the middle Cambrian Inca Formation, Georgina Basin, Australia, and Lower Ordovician Cow Head Group, Newfoundland

Abstract To investigate early radiolarian evolution and phylogeny, exceptionally well-preserved materials from the middle Cambrian and Lower Ordovician were examined using laboratory-based and synchrotron X-Ray micro-computed tomography (MCT). From the middle Cambrian (Wuliuan–Drumian) Inca Formation of the Georgina Basin, three families, including one new family, the Fungomaculidae n. fam., have been recovered. Analysis of MCT three-dimensional models of radiolarians reveals significant intraspecific variations in Archeoentactinia incaensis Won in Won and Below, 1999 and A. hexactinia Won in Won and Below, 1999 with six morphotypes recognized. Two new species, A. heptactinia n. sp. and A. pentactinia n. sp, are established. The first median-bar-centered initial spicule was identified in a specimen of A. incaensis morphotype 4. Specimens of Varispiculum ectospiculatum Won and Iams, 2015 from the Lower Ordovician (upper Floian) Cow Head Group, Newfoundland, are digitized with the aid of the Australian Synchrotron MCT. The exclusively spicular composition of V. ectospiculatum is recognized. Morphological comparison suggests possible phylogenetic affinity between A. incaensis and V. ectospiculatum. At least one Entactinarian lineage appears to have evolved from the point-centered spicular Echidninidae. Additional detail revealed by closer examination of Archeoentactinia hexactinia Won in Won and Below, 1999 morphotype 2 includes the possibility that rays of the initial spicule are hollow. This previously unrecognized feature is significant in the broader context of the mode of skeletal growth and early radiolarian evolution. UUID: http://zoobank.org/7edbe7fa-1238-4914-b57b-f455037e1b7e