Graptoloidea Lapworth, 1873

Kozłowska, Anna & Bates, Denis E. B., 2008, Kirkigraptus, a new retiolitid graptolite from Poland, Acta Palaeontologica Polonica 53 (1), pp. 105-112 : 107

Order Graptoloidea Lapworth, 1873 Family Retiolitidae Lapworth, 1873

Emended diagnosis.—Diplograptid rhabdosome composed of cortical bandages forming lists with the sicular and fusellar walls not usually preserved. Ancora present, a structure of four lists formed by forking at the end of the virgella; from the ancora there develops the ancora umbrella and its prolongations composing the ancora sleeve which encloses the thecae. The thecal framework comprises the nema, virga, virgella, transverse rods, lateral apertural rods, thecal lips, and connecting rods. Some structures of the rhabdosome may be reduced.

Discussion.—The last diagnosis of the retiolitids was published by Obut and Zaslavskaya (1983), who also included the Ordovician Archietiolitidae. Subsequently many investigations of retiolitids have been made mostly using the scanning electron microscope (e.g., Bates and Kirk 1984, 1992, 1997; Bates 1990; Lenz 1994, 1995; Lenz and Kozłowska−Dawidziuk 2001, 2004; Kozłowska−Dawidziuk 2002, 2004; Lenz and Kozłowska 2006). The new data allow us to understand better and define the retiolitid structures as well as the evolutionary tendencies of the group. The terminology of retiolitids is explained in Bates et al. (2005).

One of the main tendencies in retiolitid evolution is reduction of the rhabdosome elements; thus as a result there are many forms with rhabdosome structures reduced, e.g., instead of transverse rods only sockets occur in most Plectograptinae ; and no ancora sleeve is developed in Rotaretiolites exutus Bates and Kirk, 1992 . In the new form described herein, Kirkigraptus inexpectans sp. nov., there is no ancora. This is the first record of such an absence.