Nearly all of our knowledge about the Triassic tetrapod faunas in southernmost South America comes from two Late Triassic successions in Patagonia: the El Tranquilo Group in Santa Cruz province and the Los Menucos Group. Whereas the El Tranquilo has yielded exclusively dinosaur remains (prosauropods and ornithischians), the situation represented by the Los Menucos ichnites is completely different.
Until now, all well known Late Triassic Gondwanan tetrapod faunas have been archosaur-dominated (crurotarsal archosaurs and dinosaurs), such as those recorded in the Ischigualasto and Los Colorados formations in central-western Argentina ( e.g. Bonaparte, 1971,1997; Arcucci et al ., 2004), the upper part of the Santa María Supersequence in southern Brazil (Schultz at al ., 2000; Zerfass et al ., 2003), and the Elliot-Clarens succession in southern South Africa ( e.g. , Kitching and Raath, 1984; Olsen and Galton, 1984; Lucas, 1998; Lucas and Hancox, 2001).
The present analysis of Los Menucos tracks, which relied on a relatively large number of specimens, supports the existence of a terrestrial tetrapod fauna characterized by rather small animals. The dominant component of the fauna was largely the non-mammalian therapsids, probably eutheriodonts, both in terms of taxonomic diversity and relative abundance. Nevertheless, the presence of mammals in the assemblage cannot be ruled out. Therefore, the Los Menucos assemblage was unique, relative to other coeval Gondwanan tetrapod faunas, in which archosaurs (basal archosaurs and dinosaurs) are not the dominant component. This unusual situation might be either taphonomically biased toward small, predominantly terrestrial tetrapods or the record of an endemic fauna actually dominated by therapsids. The likelihood of a preservational bias as an explanation is difficult to assert because no taphonomic studies of the collected material are available.
The second interpretation is that the assemblage really represents an endemic fauna. The deposition of the Los Menucos succession was controlled by volcanic processes and deposited by ephemeral river systems under explosive volcanic activity, conditions that probably continuously changed the configuration of the landscape (see Lockley, 1990). This scenario might contributed to the development of an endemic fauna which reflects the instability of the landscape in which the animals lived.