Radiolaria from the Lower Silurian of the Cape Phillips Formation, Cornwallis Island, Nunavut, Canada

Well-preserved Radiolaria were recovered from a nearly continuous succession of Llandovery to lower Wenlock strata from the Cape Phillips Formation, Nunavut, Canada. Age control is well-constrained by graptolite biostratigraphy. Only one previous radiolarian study had a comparable degree of stratigraphic continuity in the Llandovery. The present study thus provides a baseline of data on radiolarian taxonomy and stratigraphic distribution in the Llandovery for future comparative studies.; The Secuicollactidae include Secuicollacta (nine species described), Diparvapila (four species), and Rotasphaera (three species). The Palaeoscenidiidae include Goodbodium (2 species), Insolitignum (three species), Palaeodecaradium (three species), Palaeoephippium (three species) Palaeopyramidium (one species), and Protoentactinia (one species). The Haplotaeniatumidae include Haplotaeniatum (seven species described) and Orbiculopylorum (three species described). The Inaniguttidae are not assigned to genera. Five taxa are described. The genus Labyrinthosphaera of uncertain family is represented by three species.; The radiolarian fauna changed at three stratigraphic levels. At the Rhuddanian-Aeronian boundary abundance shifts from Haplotaeniatum to Secuicollacta, and Orbiculopylorum appears. In a poorly constrained interval (upper turriculatus to crispus zones) Orbiculopylorum and some species of Haplotaeniatum disappear, and the Inaniguttidae first appear in the formation. Also poorly constrained, across the Llandovery-Wenlock boundary species abundances are rearranged, and Ceratoikiscum and Helenifore appear. The Rhuddanian-Aeronian change may be associated with a global shift in graptoloid dominance. The upper turriculatus to crispus zones change is dubiously associated with the Stimulograptus utilis graptolid event. The Llandovery-Wenlock change apparently coincides with one or both of the Cyrtograptus lapworthi and C. murchisoni graptoloid events and the Ireviken oceanic event.; Changes in the radiolarian fauna allow the distinction of four biostratigraphically successive zones. The Haplotaeniatum Zone extends from basal Llandovery to topmost cyphus graptolite Zone. Boundaries of the next three zones are only approximate due to nonrecovery of radiolaria. The Orbiculopylorum Zone ranges from the top of the Haplotaeniatum Zone to presumably the crispus Zone. The Inaniguttid Zone presumably extends from the crispus Zone to the Llandovery-Wenlock boundary. The Ceratoikiscum Zone follows the Inaniguttid Zone; the top of the interval is not established.