Chapter 13: Neogene vertebrates from Argentine Patagonia: their relationship with the most significant climatic changes

TONNI, E. P., CARLINI, A. A..

Late Cenozoic of Patagonia and Tierra del Fuego

ELSEVIER B.V.

Lugar: ISSN 1571-0866; Año: 2008; p. 269 - 282

Pascual and Odreman Rivas (1973) made an excellent update of the mammal-bearing units of the Patagonian Cenozoic and their relationships with diastrophic processes. In what these authors called ‘‘Patagonic area’’, they distinguished two regions with mammal-bearing sediments, belonging to two different basins limited southward by the Patagonian massifs: (1) the region between the Macizo de Somun Cura or North Patagonian Massif and the Macizo del Deseado (Deseadan Massif), and (2) south of the Deseadan Massif. Both regions correspond mainly to the San Jorge Gulf Basin and the Austral Basin, respectively (Fig. 1). In the San Jorge Gulf Basin, sediments with faunas pertaining to the Paleogene prevail, while in the Austral Basin the beginning of the Neogene is best represented with the outstanding exposures of the Santa Cruz Formation, already known since the end of the twentieth century through the fieldwork of Carlos Ameghino and the paleontological descriptions of his brother Florentino (see Ameghino, 1889, 1906). According to Pascual et al. (2002), the oldest Cenozoic mammals recorded in southern Patagonia (Santa Cruz Province) belong to the Paleogene, the Casamayoran (Late Eocene) and Deseadan (Late Oligocene) stages. In turn, the Neogene is represented by the Santacrucian (the latest Early Miocene) stage, followed by younger units up to the Mayoan stage of the ‘‘Estratos del Guenguel’’ (late Middle Miocene) in the northwest of the province (Dal Molin and Franchi, 1996). The Santacrucian stage is undoubtedly the richest of these Neogene units in mammal remains within the Argentine territory, and probably the entire South American continent (Pascual et al., 2002), whereas in Chubut Province there are also older sediments assigned to the Peligran and Riochican (Paleocene) stages. The continental vertebrate-bearing units are interfingered with marine sedimentary rocks rich in paleontological content, both invertebrate and vertebrate remains. Among the latter, abundant and diverse remains of condrichthyan fish with significant biostratigraphic value (Arratia and Cione, 1996) and spheniscid birds (Simpson, 1972; Cione and Tonni, 1981; Acosta et al., 2004) are recorded. For the Late Paleogene and the Neogene, several marine units have been recognized. The ‘‘Patagoniano’’, ‘‘Patagoniense’’ or ‘‘Patagonia Formation’’ (including the San Julia´n, Monte Leo´n and Gaiman formations, see Zinsmeister, 1981; Cione and Cozzuol, 1990; Cione, 2002) ranges from the Late Eocene to the Early Miocene; in northeast Patagonia, it is represented by the Puerto Madryn Formation which is Middle–Late Miocene in age (Arratia and Cione, 1996; Cione et al., 1996). In the Patagonian region, no mammals that may be clearly correlated with those characterizing the uppermost Miocene–Pliocene units of the extra-Patagonian area (Montehermosan, Chapadmalalan and Marplatan stages; Cione and Tonni, 1999, 2005) have been recorded. The single exception are the few reports of the Puerto Madryn Formation (Dozo et al., 1999, 2002) or those of the Cerro Azul Formation at the northern boundary of the studied area (Montalvo, 2000, 2001, 2003; Urrutia and Scillato-Yane´, 2003; Montalvo and Verzi, 2004; among others). Those of the Early Pleistocene (Ensenadan stage) are not present either. The remains of the Late Pleistocene are relatively frequent, but almost restricted to those representing approximately the last 15,000 yrs, many of which are directly or indirectly associated with archeological sites. Tonni et al. (1982: 149) pointed out that during the Pleistocene ‘‘. . . gran parte del territorio patago´nico estuvo habitado por megamamı´feros de las mismas especies o muy cercanamente emparentadas a las que habitaron el a´rea pampeana’’ [‘‘. . . a large portion of the Patagonian territory was inhabited by megamammals of the same species or very close related to those inhabiting the Pampean region’’]. The successive faunas recognized since the Neogene in Patagonia will be described, as well as their contribution to the chronology of the host sediments and their relationships to climatic and environmental changes. Given that the vertebrate record in Patagonia is very scarce since the Late Miocene, it was necessary to base the analysis on the northern extra-Patagonian faunas (Pampasian), which are highly diversified and better known, to interpret the changes of the Patagonian region more precisely.