Graptolites     Radiolaria     Stratigraphy

Radiolarian Taxonomy & Evolution

Radiolarians are a group of siliceous microfossils with a high biostratigraphic potential, as they show a fast rate of evolution through time. Even though few faunas are described adequately from the early Paleozoic, these show this potential by their distinct compositions. As most radiolarian faunas are etched from silicified rocks, fine details of the tests are often not preserved and a massive bias towards thick shelled and robust forms has to be assumed. The discovery of well-preserved radiolarian faunas in limestone is a rare opportunity to investigate details not found in most faunas and to gain insight into the actual diversity of the faunas, as well as information for a better phylogenetic understanding of this group of skeleton-bearing marine protists.

In recent years more information became available from a number of regions where Cambrian and Ordovician radiolarians have been found in some quantity, enabling a rare glimpse into their early evolution and diversification. As the radiolarian biostratigraphy is based on isolated occurrences from few and widely scattered regions, the ranges of these faunas are not well established. Many of the isolated occurrences are poorly dated by other fossil groups and the biostratigraphic framework was pieced together from various biogeographically unrelated regions.

Figure 1. General trends in radiolarian evolution from the Cambrian to the Silurian. A. Svalbardospiculum arenigium. B. Secuicolacta sp., Dalarna, Llandovery. C. Sphaeroentactinia hexaspinosa. D. Echidnina sp. E. Parechidnina sp.? F. Spumellarian microsphere. G. Haplotaeniatum sp.? (Maletz, unpublished).

In many species the radiolarian skeleton based on an initial spicule, with associated additional elements forming a more or less regular spherical body. The development of this skeleton can be used to differentiate major groups of radiolarians.

Figure 2. A. Spherical spumellarian with point-centered spicule and shell. B, C. Spherical spumallarian, spicule preserved as remnants on inner shell, shown complete in (B), beams connect to outer – incomplete – shell. D. Spicule association in Echidninids. E. Spherical spumellarian with bar-centered spicule and two shells. F. Spherical spumellarian with bar-centered spciule in eccentric position. Shell produced from branched apophyses. G. Spiculumid with point-centered spicule, distal branching forms a vaguely spherical shell. H. Palaeospiculum sp. (based on Maletz & Bruton 2007).

Early diverse radiolarian assemblages are reported from the Middle Cambrian of Australia, the late Cambrian to early Ordovician of the Cow Head Group of western Newfoundland and from Hunan, China. These show a variety of spicular forms identified as Archeoentactiniidae, Echidninidae, Palaeospiculumidae, Protoentactiniidae and Aspiculumidae.


Figure 3. Early Ordovician radiolarians from western Newfoundland and Spitsbergen. A, B. Echidnina sp., Pendeograptus fruticosus Biozone, Spitsbergen. C. Protoentactinia bifurcata, Spitsbergen. D. Svalbardospiculum arenigium, Spitsbergen. E, F. Spicular forms indet. Spitsbergen (Maletz, various sources).

In the Arenig, radiolarians with one or more distinct spherical shells appear as the dominant faunal elements in the fossil record. These forms are often difficult to distinguish from each other depending on the preservation. Thus, they are poorly known so far. New and diverse faunas were recently discovered in western Newfoundland and Spitsbergen, showing high potential for detailed biostratigraphic use of early Ordovician radiolarians.

Figure 4. Various Castlemainian (mid-Arenig) spumellarian radiolarians from Spitsbergen, transmitted light photos. Specimens prepared by Holdsworth (Collection at Geological Museum, Oslo, Norway, PMO)  A, F. Nyfrieslandia sphaeroidea. B. Antygopora sp. C. Antygopora microspina. D, E. Antygopora bella (Maletz, unpublished).

It is often difficult however, to recognize and identify radiolarians in transmitted light. Comparisons with SEM photographs are difficult as the SEM shows the surface ornamentation only, while transmitted light also shows internal skeletal structures – internal spheres and spicules. 

Figure 5. Different preservational aspects in Arenig radiolarians. SEM photographs. Specimen A is a perfectly preserved, undescribed spherical parechidninid with numerous slender outer spines. Specimens B and C represent Antygopora ordovicica, preserved as a broken  pyritic specimen, showing internal and external spheres (B) and (C) an inner sphere with partial pyritization (lower right corner of photo). The material comes from the Arenig of the Cow Head Group of western Newfoundland (Maletz, unpublished).

Figure 6. Beothuka terranova Aitchison, Flood & Malpas. The left and middle specimens come from the Isograptus victorie lunatus Biozone of western Newfoundland, while the specimen to the right is from the Didymograptellus bifidus Biozone of Spitsbergen. The specimen from Spitsbergen is completely pyritized and thus, the original skeletal structure is strongly modified. The Newfoundland specimens are preserved as silica with little modification from the original structure, even though the original opal-A is recrystallized to quartz.

In the late Middle Ordovician (Darriwilian) the radiolarian faunas changed considerably and new faunal elements appear. In general, the species that are much larger and have spicular forms are extremely rare. Spherical species of the Inaniguttidae with a variable number of spheres connected by spines and beams are associated with labyrinthine species.

Figure 7. Radiolarians and sponge spicule from the Middle Ordovician Table Head Group, western Newfoundland, upper part of Holmograptus lentus to Nicholsonograptus fasciculatus Biozone at the West Bay Centre Quarry, Port-au-Port Peninsula. A. ?Inanibigutta sp. B. Spongy, bipolar radiolarian, one of the main outer spines broken. C. Sponge spicule.

In the Llandovery, the radiolarian faunas are dominated by the Secuicollactidae, represented by small spherical radiolarians with a variable number of primary units or spicules. They are similar in their development to the late Cambrian to early Ordovician Echidninidae. The species are associated with various labyrinthine radiolarians of the genera Haplotaeniatum, Gyrosphaera and Labyrinthosphaera and various spicular radiolarians.

Figure 8. Radiolarians from the Llandovery (early Silurian) of Solberga, Dalarna, Sweden. A. Diparvapila sp. B. Gyrosphaera raneatela. C. Labyrinthosphaera macdonaldi. The specimens are associated with graptolites indicating the Streptograptus sartorius Biozone.