Earliest Arenig radiolarians from the Cow Head Group, western Newfoundland

Radiolarians have been recovered from lowermost Arenigian rocks of the Cow Head Group in Gros Morne National Park, Newfoundland. Two new faunal assemblages include eight families, 13 genera, and 25 species. Among these, the family Neoaspiculumidae, the genus Neoaspiculum, and 11 species, Beothuka aitchisoni, B. grosmornensis, B. stougei, B.? stellata, Neoaspiculum cancellium, N. headense, N. transformum, N. laxum, N.? amplum, Proventocitum cylindricum, and P. piriforme, are new. This study elucidates the reason for the significant difference in species composition between pre-Arenigian and Arenigian radiolarian faunas. This difference does not appear to be due to mass extinction, but rather to phylogenetic evolution: most radiolarian families that flourished during pre-Arenig time were gradually but significantly reduced by the earliest Arenig. Characteristic features of the earliest Arenig assemblages that distinguish them from their Tremadocian predecessors are 1) the development of a distinctive skeletal constructional element referred to as "bandage" structure that evolved through modification of a mesh of thin bars and 2) the appearance of multiple concentric shells including a proloculus (or a microsphere), which developed from a primitive three-dimensionally interwoven meshwork or spongy shell wall. These forms, represented by beothukids, antygoporids, and inaniguttids, are rare but they appear to originate in the earliest Arenig. Transitional characteristics among those forms are developed in the Neoaspiculumidae, which flourished during the earliest Arenig. The precise biostratigraphic age of one of the assemblages was determined with the recovery of the index conodont Prioniodus adami Stouge and Bagnoli, 1988 from strata belonging to the Tetragraptus approximatus Graptolite Zone, which is at the base of the Arenigian.