The Triassic Los Menucos Basin is located in northern Patagonia, in the central region of the Río Negro province (figure 1). Its infilling consists of 150 m of nonmarine volcaniclastic rocks deposited by ephemeral river systems under explosive volcanic activity and developed over a local low gradient volcanic relief (Llambías, 1999; Kokogian et al ., 2001; Labudía and Bjerg, 2001). The volcanism in the region during the Late Triassic is related to trans-arc extensional phenomena (Franzese and Spalletti, 2001; Macdonald et al ., 2003) that produced the development of volcanic arcs on the active western margin of the continent and an important outpouring of silicic extensional volcanism (Macdonald et al ., 2003).
The Triassic sequence (Los Menucos Group) is well exposed near the town of Los Menucos, approximately 40? 53' S - 63? 11' W; it unconformably overlies sedimentary and igneous paleozoic rocks and it is covered by Cretaceous sedimentary rocks and Tertiary volcanics (Labudía and Bjerg, 2001). The Triassic succession is lithostratigraphically divided into two units (figure 2), the Vera Formation and the overlying Sierra Colorada Formation (Stipanicic et al ., 1968; Labudía et al ., 1995; Labudía and Bjerg, 2001). The Vera Formation consists of tuffaceous sandstones and mudstones interfingered with volcanic rocks, whereas the Sierra Colorada Formation is exclusively rhyolitic (Stipanicic et al ., 1968; Pesce, 1974; Rosenman, 1979; Labudía and Bjerg, 2001).
As mentioned above, the footprints are recorded in the Vera unit from several levels of fine-grained tuffaceous pale yellowish brown and light brown sandstones interbedded with brown grayish mudstones bearing a rich " Dicroidium -type Flora" (Casamiquela, 1964; Stipanicic et al ., 1968; Stipanicic and Methol, 1972; Pesce, 1974; Artabe, 1985, 1986; Labudía et al ., 1995; Labudía and Bjerg, 2001). The sandstones containing the footprints are quarried for slabs; all studied material came from two quarries, the "Cantera Vieja" (= "La Vieja" sensu Leonardi and Henrique de Oliveira, 1990; = "Cantera Tscherig" sensu Leonardi and Henrique de Oliveira, 1990; Leonardi, 1994) and the "Cantera Nueva" (= "La
Cantera Nueva" sensu Leonardi and Henrique de Oliveira, 1990; = "La Nueva" sensu Leonardi and Henrique de Oliveira, 1990; Leonardi, 1994). These quarries are located in the proximity of the Cerro Las Lajas, west of the town of Los Menucos, and they are still under excavation.
Recent radiometric analyses in volcanics from the Sierra Colorada Formation (222 ± 2 Ma, Rapela et al ., 1996) place this sequence in the middle Carnian ( sensu Gradstein et al ., 1995), thus giving a minimum Carnian age to the Vera beds and to the track-bearing levels. This is in agreement with the Late Triassic age previously assigned to the Vera unit based on its stratigraphic relationships, radiometric data and floral content (Stipanicic et al ., 1968; Stipanicic and Methol, 1972; Labudía et al ., 1995).