THE SIGNIFICANCE OF THE GRAPTOLOID AMPHIGRAPTUS DIVERGENS FROM THE (MID-RAWTHEYAN) TYPE SECTION OF THE ASHGILL SERIES, ORDOVICIAN

Abstract

Amphigraptus divergens Lapworth is known to be restricted, in Britain, to the linearis Biozone, and its occurrence in the type Ashgill Series reinforces Rickards' prior independent attribution of that part of the type Ashgill to the linearis Biozone. The thecal structure revealed in this paper allies A. divergens to the dicellograptids rather than the leptograptids, hence it is placed in the family Dicranograptidae. The evolutionary origin of Amphigraptus is almost certainly rooted in Dicellograptus.