Time calibrated morpho-molecular classification of Nassellaria (Radiolaria)

Abstract

Nassellaria are marine radiolarian protists belonging to the Rhizaria lineage. Their skeleton, made of opaline silica, exhibit an excellent fossil record, extremely valuable in micro-paleontological studies for paleo-environmental reconstruction. Yet, to date very little is known about the extant diversity and ecology of Nassellaria in contemporary oceans, and most of it is inferred from their fossil record.Here we present an integrative classification of Nassellaria based on taxonomical marker genes (18S and 28S ribosomal DNA) and morphological characteristics obtained by optical and scanning electron microscopy imaging. Our phylogenetic analyses distinguished 11 main morpho-molecular clades relying essentially on the overall morphology of the skeleton and not on internal structures as previously considered. Using fossil calibrated molecular clock we estimated the origin of Nassellaria among radiolarians primitive forms in the Devonian (ca. 420 Ma), that gave rise to living nassellarian groups in the Triassic (ca. 250 Ma), during the biggest diversification event over their evolutionary history. This morpho-molecular framework provides both a new morphological classification easier to identify under light microscopy and the basis for future molecular ecology surveys. Altogether, it brings a new standpoint to improve our scarce understanding of the ecology and worldwide distribution of extant nassellarians.

Verifiability of genus-level classification under quantification and parsimony theories: a case study of follicucullid radiolarians

“…This raises a question about the distinguishing character(s) of Parafollicucullus at the genus level. This extinct polyphyletic group is also considered in terms of homology, because molecular phylogenetic studies for extant Radiolaria and Phaeodaria except for Spumellaria identify strong homology at the level of superfamilies and suborders (Class Acantharea by Decelle et al [2012], Class Phaeodaria by Nakamura et al [2015], Order Nassellaria by Sandin et al [2019], and Order “living Entactinaria” by Nakamura et al [2020]). The original diagnosis of Parafollicucullus is “bilaterally symmetrical, imperforate siliceous shells of unknown internal structure with apical cone, winged pseudothorax and ring-like pre-pseudoabdominal segment interposed between pseudothorax and pseudoabdomen” (Holdsworth and Jones 1980: p. 285).…”

“…Taxonomic classification at the genus level can involve tensions between splitting and lumping philosophies. When there are living taxa, this kind of controversy can often be resolved by molecular data, but it is impossible to use this approach in extinct fossil groups (Sandin et al 2019).…”