Lower Palaeozoic Trilobita : the Cheiruridae ; a preliminary account of the Llandoverian trilobite fauna of the type area

Abstract

The classification of the trilobite family Cheiruridae has been revised. The work has included visiting the institutions having the major palaeontological collections in this country, and such material has been borrowed for study and photography in Newcastle. The British species of Cheirurids described before about 1950 have been redescribed and figured. The nomenclature of the family at the specific, generic and sub familial levels has been clarified. The subfamilies considered to belong to the Cheiruridae are as follows; Cheirurinae, Cyrtometopinae, Pilekiinae, Sphaerexochinae, Deiphoninae, Artinae and Acanthoparyphinae. The subfamily Hammatocneminae is excluded from the family and probably requires the erection of a new monotypic family. The genus Onycopyge is also excluded and considered to form a new monotypic subfamily of the Encrinuridae. With reference to all available world literature on the family, as complete as possible a list of Cheirurid species has been built up and a phylogeny of the genera is proposed. The distribution of these genera throughout the time range of the family (Upper Cambrian to Middle Devonian) is dealt with; the Upper Cambrian and Tremadocian genera (Pilekiinae) being widespread, the Ordovician genera being provincial in distribution in common with the pattern shown by many other trilobite families at that time, and the Silurian and Devonian Cheirurids having worldwide distribution. A very short preliminary account of the Llandoverian trilobites of the type area is included at the end of the thesis. The preliminary determinations of the trilobites, which are uncommon in these rocks, indicate about 12 genera to be present in the collections made by the author and in that made by Prof. O. T. Jones which is housed at the Sedgwick Museum, Cambridge University.