DIRECTIONAL EVOLUTION IN THE HISTIODELLA LINEAGE

G. L. ALBANESI; N. FELTES

FOURTH INTERNATIONAL CONODONT SYMPOSIUM. ICOS IV ?PROGRESS ON CONODONT INVESTIGATION?

INSTITUTO GEOLÓGICO Y MINERO DE ESPAÑA

Año: 2017; p. 283 - 286

Several species of the Ordovician genus Histiodella have been used as index fossils (e.g., Sweet, 1984; Maletz, 2009; Stouge, 2012). This taxon is distributed through low to mid-high latitude latitudinal regions and characterizes deep platform or platform margin to slope depositional environments (Stouge, 2012). Histiodella is particularly abundant in the Las Aguaditas creek stratigraphic section of the Central Precordillera, Argentina, where it presents a continuous record through the upper part of the San Juan Formation and the lower member of the Las Aguaditas Formation, Darriwilian in age (fig. 1). The upper part of the San Juan Formation presents a high diversity and abundance of conodont elements (Feltes et al., 2016), including those of Histiodella. In the lower and middle parts of the lower member of the Las Aguaditas Formation the conodont diversity decreases sharply because of the flooding event that submerged the carbonate platform. The sea level change caused unfavourable conditions for the development of the conodont fauna in this geological setting. In the upper part of the lower member, the diversity recovers and Histiodella species reappears. In the study section, five species of Histiodella were identified following Ethington and Clark (1981), McHargue (1982), Stouge (1984) and Bauer (2010); i.e., H. sinuosa, H. serrata, H. holodentata, H. cf. holodentata, and H. kristinae, which share a certain ancestor-descendant relation according to the phylogenetic hypotheses of McHargue (1982) (from H. sinuosa to H. serrata), and Stouge (1984) (from H. holodentata to H. kristinae). Elements that occupies the Pa position in the apparatus of Histiodella are predominant in the collection; these are the most useful elements in the taxonomy to distinguish between different species of the genus. The aim of this work is to analyze the evolutionary transformation experienced by this taxon across the Yangtzeplacognathus crassus to the Eoplacognathus pseudoplanus zones.