Graptolites from the Dictyonema Shales of Quebec

Abstract

Summary The fossiliferous Dictyonema Shales of Matane are represented in older collections by a rich graptolite fauna which is here fully described on the basis of material sent to Lapworth at the end of last century, and the Richardson collection and other material from McGill University, Montreal. Included in this fauna is Dictyonema canadense (figured but not described by Lapworth) and four new species of Dictyonema , together with six species and varieties of Anisograptus, Triograptus and Staurograptus, and a species of Didymograptus . From other less fossiliferous localities spread along the south shore of the St. Lawrence River come some of the above, with another new species of Dictyonema , a Bryograptus and a monotypic new genus Radiograptus , intermediate between Anisograptus and Dictyonema . The stratigraphical relations of these various localities are unknown, but the whole constitutes a fauna of Tremadoc age which is assigned either to the highest part of the Dictyonema Shales (of Scandinavia) or the lower part of the Upper Tremadoc. Structurally, all the species of Anisograptus , the Bryograptus and the Staurograptus are shown to possess stolothecae, and bithecae can be identified in most of them; the occurrence of this dendroid branch structure is also believed to characterize Triograptus and Radiograptus . Accordingly, all these genera, together with Clonograptus and Adelograpius , are separated from the Dichograptidae where they were previously placed, and united into a new family, the Anisograptidae, which is transferred to the Dendroidea. No phylogeny can be indicated with any assurance, but the whole group seems to constitute a plexus intermediate between the Dendroidea and the Graptoloidea.