Morphological variation in Coryphidium from the Arenigian Series (Lower Ordovician) of northwestern England

Abstract

Three morphotypes of Coryphidium have been identified in the Skiddaw Group of Arenigian (Early Ordovician) age in northwestern England. Lower Arenigian rocks, probably of the Tetragraptus phyllograptoides graptolite Biozone, contain Coryphidium aff. C. elegans; middle Arenigian rocks, spanning the Didymograptus deflexus, D. nitidus and the lower part of the Isograptus gibberulus graptolite biozones, have yielded Coryphidium bohemicum?; and upper Arenigian rocks, from the upper part of the I. gibberulus and the Didymograptus hirundo graptolite biozones, contain Coryphidium aff. C. bohemicum (referred to as C. bohemicum by other authors). Coryphidium can therefore be used as a guide fossil in the Arenigian of northwestern England. The three morphotypes are distinguished by vesicle shape, process shape and size, and process distribution. Vesicle shape changes from generally straight-sided in C. aff. C. elegans and C. bohemicum? to predominantly concave-sided in C. aff. C. bohemicum. The processes of C. aff. C. elegans and C. bohemicum? are comparatively long and slender whereas those of C. aff. C. bohemicum are relatively short and stout. The processes appear to be distributed randomly on C. aff. C. elegans whereas those of C. bohemicum? and C. aff. C. bohemicum tend to be concentrated at the corners of the vesicle.