Late Wenlock – early Ludlow was a period of profound change in the graptolite world and two "crises" are evident. The most important changeover occurred within the late Wenlock Homerian Stage, during which time Cyrtograptus became extinct, monograptids, while severely reduced, introduced new thecal and and sicular types, and plectograptines underwent a striking diversification. The second, lesser, change coincided with the Wenlock – Ludlow boundary, at which time monograptids reemerged as the dominant taxon group, and plectograptines declined and, shortly thereafter, became extinct.Two species, "Pristiograptus" praedeubeli (Jaeger) and Lobograptus? cornuatus.n.sp., recovered in isolated and uncompressed form, are described and illustrated. The presence of the former species permits the subdivision of the upper Homerian of the Canadian Arctic into two biozones; the lower "Pristiograptus" praedeubeli Biozone and the upper "Pristiograptus" ludensis – "Pristiograptus" sherrardae Biozone. Both species exemplify the novel thecal and sicular morphologies that appeared during the late Homerian and continued into the Ludlow.

You do not currently have access to this article.