Abstract

The Arisaig Group in northern mainland Nova Scotia has the most complete sequence of Silurian, fine grained, siliciclastic, near-shore marine sediments in Western Avalonia. In this setting, the Arisaig Group received a nearly continuous supply of autochthonous marine and allochthonous nonmarine palynomorphs. One hundred and seventy-seven samples, ranging in age from latest Ordovician?–early Llandovery to Přídolí (Beechhill Cove to Stonehouse formations) yield the most diverse spore assemblage yet known from the Silurian of North America. This paper describes the spores and cryptospores and reexamines age relationships of the Arisaig Group, placing both within context of early land plant evolution.

Thirty-six species of spores and cryptospores were found in the Arisaig Group. Six new species are Ambitisporites marginatus sp. nov., A. capitaneus sp. nov., Scylaspora asperverruca sp. nov., Chelinospora textilis sp. nov., Hispanaediscus scabiosus sp. nov., and Hispanaediscus imbricatus sp. nov. A new genus, Vermiverruspora is created and the original description of ChelinosporaAllen 1965 is resurrected. Six species of Chelinospora are reassigned to Vermiverruspora gen. nov. (C. paravermiculata Loboziak et al. 1988, C. vermiculata Chaloner & Streel 1968, C. ?vermiculataMcGregor & Camfield 1976, C. retorrida Turnau 1986, C. rumneyiBurgess & Richardson 1995, C. vermolutaBurgess & Richardson 1995) and one species transferred to Stellatispora (pattersoniiWellman 1993). Four additional sporomorphs believed to be new, are not formally named because better preserved specimens are needed.

The Arisaig Group (West Avalonia) is similar to sporomorph-rich type sections in the Anglo-Welsh Basin (East Avalonia). Four assemblage zones (avitus–dilutus, brevicosta–verrucatus, libycus–poecilomorphus, and tripapillatus–spicula), and four assemblage sub-zones (A. brevicosta, E. protophanus, C. obscura, and S. inframurinata var. inframurinata) have been recognized. Sporomorphs indicate that the contacts between the Lower and Upper Members of the McAdam Brook Formation and Green Member of the Moydart Formation may be one to two graptolite biozones older than previously reported.

The Arisaig palynomorphs support the theory that a major radiation of vascular land plants occurred during the latter half of the Silurian Period. Llandovery and early Wenlock deposits at Arisaig contain only a few species of smooth walled spores and cryptospores. Sixteen trilete spore and fifteen cryptospore species appear between the Homerian and the late Přídolí. The tempo and composition of this diversification reflect evolutionary events observed elsewhere in the world.

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