THE PROXIMAL TYPE DEVELOPMENT IN CYMATOGRAPTUS (GRAPTOLOIDEA) FROM ARGENTINA AND ITS RELEVANCE FOR THE EARLY EVOLUTION OF THE DICHOGRAPTACEA

An early Ordovician graptolite assemblage from Los Colorados section (Purmamarca area, Jujuy Prov.) in the Eastern Cordillera of NW Argentina yielded important well preserved didymograptid graptolites showing characters that enable to better understand early graptoloid evolution.

The graptolites come from the middle-upper levels of the Acoite Formation, a unit with a shallowing upward, shelf succession related to a wave storm-dominated deltaic system. The local biostratigraphic scheme encompasses the Tetragraptus akzharensis (early Arenig), Baltograptus deflexus (upper early Arenig) and Didymograptellus bifidus (middle Arenig) Zones. Graptolites are commonly preserved as flattened films of carbonized periderm, but greenish shales and siltstones, locally interbedded with thin, fine-grained sandstones allowed relief preservation, in which the rhabdosomes were filled by a soft, friable pinkish to whitish mineral. This mineral has been interpreted as an iron oxide, probably goethite, which is the product of oxidation of pyrite. The unusual, optimum preservation of this material permits detailed observation of the proximal development of these specimens.

The specimens show variable dextral and sinistral proximal development of the isograptid type in Cymatograptus sp. The origin of the first theca is apparently in the prosicula, but details are not available, as fusellar structure is not preserved. In all known younger graptoloids the proximal end development is exclusively dextral. A variable development was documented from few graptolites so far (especially Rhabdinopora, Adelograptus tenellus), but three-dimensionally preserved specimens showing these details are few. They include mostly early to mid Tremadoc species, and, thus, this type of development appears to be restricted mainly to the Anisograptidae. Along the early evolution of the Dichograptacea the dextral development type was fixated and retained in younger forms. The new material from Argentina shows that a variable development persisted into early Arenig dichograptids.