OXYGEN-CONTROLLED EARLY ORDOVICIAN TRILOBITE ASSEMBLAGES:THE THYSANOPYGE FAUNA FROM NORTHWESTERN ARGENTINA

WAISFELD, B.G Y VACCARI N.E.

Advances in trilobite research

Instituto Geológico y Minero de España

Lugar: Madrid; Año: 2008; p. 421 - 425

The well known Thysanopyge fauna, formerly defined as a group of endemic asaphids that flourished during the Floian in western Argentina (Harrington and Leanza, 1957), is re-evaluated upon new insights into its composition, spatio-temporal distribution, as well as ecological and environmental aspects. Thysanopyge is a large aspahid, reaching 40 cm sagittal length, able to cope with a relatively wide range of environmental conditions, evidenced by its occurrence in a variety of lithologies, including green, grey, and black silts and shales. Although the Thysanopyge fauna was previously characterized by the presence of the alleged ubiquitous Thysanopyge argentina Kayser, six different species has been recently recognized by Waisfeld and Vaccari (submitted). Apart from T. argentina, T. frenguellii Harrington, T. taurinus (Harrington), T. clavijoi Harrington and Leanza, Thysanopyge  n.sp. 1, and Thysanopyge n.sp. 2 are now established. The Thysanopyge fauna is widespread in the Argentine Cordillera Oriental, the southern extreme of the Central Andean Basin, along present day 250 km, from nearby Salta city to Santa Victoria, in the border with Bolivia (Fig. 1). Its age is now bracketed between the late Tremadocian (A. murrayi Zone) to late Floian (upper part of B. deflexus Zone), involving about 11 m.y. The Thysanopyge fauna is recorded in different units (San Bernardo, Parcha, and Acoite formations), involving largely fine-grained successions, of overall 1500 m thickness. These successions display no significant lithofacies changes, except for shale/ silt variation in grain size and colour, and only locally, interbedded sandstones and shell beds account for high energy events mostly related to tempestite episodes. According to recent interpretations the Cordillera Oriental represents the forebulge (or peripheral bulge) depozone of an extended Ordovician foreland basin systems in northwest Argentina. In the Cordillera Oriental platform deposits developed in a low gradient ramp-like setting, under the influence of large-scale prograding deltaic systems from the east, and of an active volcanic arc complex from the west (Bahlburg and Furlong, 1996; Astini, 2003).   The onset of the Thysanopyge Fauna is associated with a significant faunal turnover from the previous “Notopeltis Fauna”, linked to a basin-wide flooding that took place since middle-late Tremadocian generating widespread subtidal environments throughout the basin (cf. Astini, 2003). Floian records occur in successions of dark, laminated silts and shales, that account for a relatively protracted interval of deficient oxygen content, related to environmental restriction in prodelta settings during aggradation stage of a highstand system tract (Astini, 2003; Waisfeld y Astini, 2003). In the uppermost Floian shallowing related to  the progradation stage of the highstand system tract brought about a change in oceanographic conditions  that led to the extinction of the Thysanopoyge Fauna and its replacement by the Famatinolithus Fauna (Waisfeld et al, 2003, and references therein)  Thysanopyge conforms an array of assemblages that are here differentiated by their trilobite composition, richness, and co-occurrence of other benthic groups. The particular geodynamic and environmental setting featured particular ecological and biogeographical signatures which are briefly discussed.