CONICET | Buscador de Institutos y Recursos Humanos

Abstract. Recent taxonomic reviews of well preserved graptolites from the Eastern Cordillera enabled to obtain new insight on didymograptid rhabdosome structure and evolution. Earlier forms of the genus Baltograptus have an isograptid type proximal development, as does the biostratigraphically important Baltograptus sp. nov., which ranges through a considerable interval of the Floian "B. deflexus" Biozone, and it is replaced in the Didymograptellus bifidus Biozone by the true Baltograptus deflexus with an artus type proximal development. Cymatograptus sp. nov., which was recently recognized in the upper part of the "B. deflexus" Biozone, bears an artus type proximal development, variably developed undulating dorsal side of the stipes, and approximately equal number of dextral and sinistral specimens. It represents the oldest known didymograptid species with artus type development. A variable dextral and sinistral development is known from early to mid Tremadoc anisograptids (especially Rhabdinopora, Adelograptus tenellus). During the early evolution of the Dichograptacea the dextral development type was fixed and retained in younger forms, but Cymatograptus sp. nov. shows that a comparably high intraspecific variation persisted into the early Arenig. An artus type proximal development evolved several times during the Early and Middle Ordovician. It can be recognized in Cymatograptus, Baltograptus, Didymograptus, Xiphograptus, Kinnegraptus and a number of sigmagraptines and may be present in other dichograptids as well. The extreme plasticity of the rhabdosome development in early dichograptids may be the reason for the strong effects of homoplasy and parallel evolution in planktic graptolites and led to the problems of understanding early Ordovician graptolite evolution.