Adding a new dimension to investigations of early radiolarian evolution

Abstract

Knowledge of the detailed architecture of the earliest radiolarian microfossils is key to resolving the evolution and systematics of this important group of marine protozoans. Non-destructive methods for observing the complexity within the internal structures of their siliceous skeletons have long eluded paleontologists. By developing methodologies that overcome some limitations of existing micro-computed tomography (micro-CT) we demonstrate a technique with potential to provide new insight into their evolution. Using 3D micro-CT data to generate models for six well-preserved siliceous radiolarian skeletons from the Middle Cambrian Inca Formation in far north Queensland, Australia and the Middle Ordovician Piccadilly Formation, in western Newfoundland, Canada, we can reconstruct phylogenetic relationships amongst some of the earliest radiolarians. Better knowledge of early radiolarian morphologies clarifies the vital function of internal structures and hierarchical diagnosis across a range of taxonomic affiliations.

Figure 1

Workflow for detailed radiolarian taxonomic study with the micro-CT. (a) General overview of the micro-CT scanning and reconstruction. (b) Scanning electron micrographs of the multiple specimen mount, where alignment of specimens is along the central rotational axis. (c) micro-CT models of processed data to show the digital analysis of the internal feature. Planar extraction of the apical view and basal view of the inner sphere. MB = median bar, rb = radial beam, TG = transversal gate.

Figure 2

Micro-CT models of various radiolarian groups illustrating the initial spicule (red) in relation to their heteropolar skeleton. Each segment is shown at the same scale unless indicated. (a) Segmented spicules of Archeoentactinia incaensis Won from the Middle Cambrian Inca Formation, far northwest Australia. Green sphere digitally inserted to test the sphericity of the central capsule. (b) Ectopically positioned spicule of Sphaeroentactinia sp. aff. S. integrata (Maletz and Bruton) from the Middle Ordovician Piccadilly Formation, western Newfoundland. Fusing points of the meshwork and initial spicule illustrated. (c) Internal features of Spongentactinia armillata (Nazarov) from the Piccadilly Formation. Six-rayed internal spicule with a median bar (red) is digitally segmented from the meshwork (grey).

Figure 3

A compilation of line drawings and micro-CT models of various radiolarians from the Middle Ordovician Piccadilly Formation, western Newfoundland with the innermost sphere highlighted (red). Each segment is shown at the same scale. (a) micro-CT models of Haplotaeniatum sp. cf. H. vertigospongum (Jones and Noble) capturing the complex spiral growth of the skeleton. (b) Triplococcus acanthicus Danelian and Popov demonstrating a complete morphological observation of the relationship between spheres and spines without compromising the integrity of the specimen. (c) Undescribed new species of the genus Westernbrookia demonstrating a primitive tetrahedral initial spicule similar to that of Triassic.

Figure 4

Entactinarian phylogeny as inferred from internal morphological studies. In our new understanding, spumellarians are a Suborder within the Order Entactinaria. The nature of internal elements needs to be treated as a family and genus level feature.

Figure 5

Initial examination of the Middle Ordovician samples from the Piccadilly Quarry, western Newfoundland, Canada was undertaken with the SEM to assess the preservation of the internal features in the broken specimens. Scale bar: 100 μm.