Xiphograptusand the evolution of virgella‐bearing graptoloids

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Abstract
Abstract: The virgellar spine is one of the most consistent features of the graptolite sicula. It is present in a large number of graptoloid groups, but evolved separately and independently in these as it is seen from the presence of the spine in either ventral (Axonophora) or dorsal (Phyllograptus,Xiphograptus) position. The evolution of the virgellar spine in the Pan‐Bireclinata in the Upper Dapingian to Lower Darriwilian time interval is known to follow four main steps, from a simple rutellum, through a lamelliform rutellum and a lanceolate virgella to the true virgellar spine. For the xiphograptids and inPhyllograptus, the origin and early development is less well documented, but appears to follow a similar path. However, the individual stages are condensed, and a true virgellar spine emerges already in the Floian time interval. A virgellar spine was found inDidymograptellus bifidus, necessitating a revision of the diagnosis of the genusDidymograptellus. A number of species of the virgellate generaXiphograptus,YutagraptusandDidymograptellusare described from isolated material for the first time. The species are useful for the biostratigraphic correlation of endemic mid‐continent North American faunas with the Pacific Type faunal realm.Xiphograptus artussp. nov.,Didymograptellus primussp. nov. andDidymograptellus cowheadensissp. nov. from the Cow Head Group of western Newfoundland are described as new.