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Journal of Paleontology

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The Proximal Development in Cymatograptus (Graptoloidea) from Argentina and Its Relevance for the Early Evolution of the Dichograptacea

¹ CONICET. Departamento de Geología y Paleontología, IANIGLA, Centro Regional de Investigaciones Científicas y Tecnológicas, C.C. 131, 5500 Mendoza, Argentina, <btorogr{at}lab.cricyt.edu.ar>
² Department of Geology, University at Buffalo, SUNY, 772 Natural Science Complex, Buffalo, N.Y. 14260-3050, USA, <jorgm{at}buffalo.edu>

Cymatograptus bidextro sp. nov. from the Floian "Baltograptus deflexus" Biozone of northwest Argentina represents the oldest expansograptid with an artus type proximal development and a prosicular origin of the first theca. The species bears a variably dextral or sinistral proximal development, previously known exclusively from Tremadocian members of the Anisograptidae, but not from the Dichograptidae. The proximal development types of the Dichograptacea are shown to consist of several independently changing characters and cannot be used as phylogenetically useful descriptors. Proximal developments are more variable than expected and several independent origins of an artus type proximal development can be recognized in the Lower to Middle Ordovician.