Article: The Amoeboceras zonation of the Boreal Upper Oxfordian
The ammonite family Perisphinctidae, on which the standard zonation of the Middle and Upper Oxfordian has hitherto been based, is poorly represented in the Boreal Province. A new scheme is proposed based wholly on the family Cardioceratidae. It has been worked out from extensive collections from a complete section at Staffin, Isle of Skye, with supporting material from East Greenland, the Scottish mainland, Yorkshire, East Anglia, and Dorset. Published evidence indicates that it should be applicable to the whole of the Boreal Province including Canada, Alaska, and the northern U.S.S.R. The Boreal and Sub-Mediterranean standard zonal successions diverge at the base of the Middle Oxfordian. In the Boreal scheme, the Middle Oxfordian now consists of a Densiplicatum Zone, divided into Vertebrale and Maltonense Subzones, and a Tenuiserratum Zone with abundant small Cardioceras (Miticardio-ceras), divided into Tenuiserratum and Blakei Subzones. At the base of the succeeding Glosense Zone Cardioceras evolved into Amoeboceras and this makes a convenient change on which to base the Boreal Middle-Upper Oxfordian boundary. Four principal faunas of Amoeboceras delimit successively the Glosense, Serratum, Regulare, and Rosen-krantzi Zones, each, except the Regulare Zone, divisible into two Subzones. To this end it was necessary to review the taxonomy of existing species of Oxfordian Amoeboceras. Nineteen species are described, including the type-species, A. alternans (v. Buch), and two new species, A. newbridgense and A. koldeweyense. A new species of early Ringsteadia, R. caledonica, is described. We also describe the Oxfordian section at Staffin, which is taken to be the type-section for the Zones in the new scheme.