Phyllogenetic relationships of Ordovician graptolites from Argentina

VENTO, B.A.; TORO, B.A.; MALETZ J.

Córdoba

Jornada; Reunión de Comunicaciones de la Asociación Paleontológica Argentina; 2013

Asociación Paleontológica Argentina

Modern systematic classifications of planktonic graptolites have emphasized the necessity to apply statistical methodologies to quantify characteristics of the proximal end of isolated specimens from rocks, to establish their phylogenetic relationships. In traditional studies, the genera and families of graptolites were proposed on the basis of a limited number of morphological characteristics of the tubarium, some of which have been shown to have a strong homoplastic character, and therefore are of questionable value for understanding the phylogeny of the group. More recently, a greater focus on the characteristics of the proximal end of the colonies was preferred to establish ancestral relationships. The main purpose of this work was to test several phylogenetic hypotheses that will help to explain the evolutionary relationships of dichograptid graptolites from Early-Middle Ordovician of northwestern Argentina. In this analysis the use of characters of phylogenetic importance of the proximal end was preferred, and characters related to the distal part of the colony were used to a lesser extent. The cladistic analysis clarified the ancestral phylogenetic relationships by recognizing monophyletic groups. The consensus trees for the selected taxa showed two major clades, corresponding to the families Tetragraptidae and Didymograptidae of the traditional systematic classification, or Tetragraptinae and Dichograptinae in recent proposals. The didymograptid cladogram shows the genus Baltograptus as a monophyletic clade, as well as suggests that the genera Expansograptus and Cymatograptus share a common ancestor. The contribution of future phylogenetic studies to the knowledge of the evolutionary history of the graptolites is highlighted on this work.