A post-glacial bryozoan fauna from the Upper Ordovician (Hirnantian) of the Argentine Precordillera

CARRERA, M.G.; HALPERN K.

Alcalá de Henares

Simposio; International Symposium on the Ordovician System; 2011

Instituto Geológico y Minero de España

The Late Ordovician extinction was the second largest loss of diversity in the history of life. Almost 48% of marine genera disappeared during this event. Nevertheless, the communities rapidly recovered due to low structural changes in terms of ecological organization. Detailed taxonomic studies are frequent in the literature on this subject. On the contrary, studies on paleoecological dynamics of this event are scarce or treated broadly. However, these preliminary attempts are of enormous value and allow continuing research. An accepted paleoclimatic interpretation of the Latest Ordovician, based on different sources of data, points out that the polar region of Gondwana was covered by an ice sheet for a short time, and it is consistent with the presence of glacial deposits in Africa and in South America (Sutcliffe et al., 2001 and references therein). Since the pioneer studies of Benedetto (1986, 1990) and Sánchez et al. (1991) very few taxonomic or paleoecological studies on the Hirnantia fauna in the glacial deposits of the Argentine Precordillera have been carried out. Only some taxonomic studies of brachiopods or paleogeographic implications of their distribution were made. Bryozoans have been listed as very scarce components of this fauna. New fossil collections in the post -glacial deposits in the Don Braulio section, Villicum range (Fig. 1), allow us to recognize numerous bryozoan specimens. The brachiopod Hirnantia and the bivalve Modiolopsis were considered as the dominant components of the fossil communities previously described (Sánchez et al., 1991). These dominance values should be reevaluated in the light of the amount of bryozoans found in the new collected material. In this contribution, we report the occurrence of two bryozoan genera in the Upper Ordovician (Hirnantian) Don Braulio Formation, Argentinean Precordillera. They are associated with the typical Hirnantia Fauna (Benedetto, 1986), representing the first community that flourished after the Late Ordovician Glaciation.