Isograptus gibberulus (Nicholson) and the isograptids of the Arenig Series (Ordovician) of England and Wales

A revision of collections of Isograptus gibberulus (Nicholson) from the Arenig Series of England and Wales shows that nine separate isograptid species and subspecies have previously been included under that name. They are Isograptus caduceus gibberulus (for which a lectotype is now selected), I. caduceus cf. imitatus (Harris), I. caduceus subsp. indet., I. victoriae divergens (Harris), Pseudisograptus stellus (Hopkinson), P. angel n. sp., P. ? menaiensis n. sp. and Pseudisograptus n. spp. A and B.

The new identifications resolve important taxonomic problems related to the definition of Isograptus. They also allow a reassessment of the I. gibberulus Zone in Britain, and make possible a more accurate correlation of the zone to the graptolite sequences in other countries. The fauna of the zone indicates that it is no older than the Ca 3 Zone (uppermost Castlemainian) of the Lower Ordovician in Australasia.