Having looked at the origin of the Patagonian lakes (Part 1 Here), we can conclude that any "lake monsters" must have had to come from outside the region and the only area where nowadays large mammals can be found in South America, are the subtropical and tropical regions of Northern Argentina, Paraguay, Brazil and Bolivia.
However current aridity in Patagonia and its northern border would impede migration of tropical or temperate region animals into the wetter and forested Andean region of Patagonia. See this arid area in the map below:
The map above shows the "tropical and subtropical" jungles and forests to the north of Argentina, and their southern border (upper red line). It also shows the Andean Forests along the Andes between Argentina and Chile and their border (bottom red line). The area in between is the "Arid Gap" that any creature moving north to south would have to bridge. It comprises not only the dry Patagonian steppe but also an arid shrubland region. See a photograph of the steppe Here.
Besides, Patagonia's cold climate is the opposite to the one prevailing in those creatures' current tropical/temperate habitat.
Nevertheless climatic changes of the post-glacial period, which shifted the Patagonian forest ranges northwards, out of Patagonia and south towards it from the subtropical region, may have allowed the formation of a continuous forest bridge through which animals of these more balmy climes could have moved into the southern Andean forests and thus crossed the current barrier of a dry, waterless stretch of land several hundred kilometers wide.
See a photograph of the Patagonian forest Here.
In Central Chile, at Talinay (30°40 S, 71°35’ W), a relict forest growing in the midst of a currently arid region is proof of this possibility. This forest thrives on the mist blown in from the ocean and its vegetation is very similar to the Valdivian rainforest of Patagonia. It could have originated by the Valdivian forest’s northwards advance during the Ice Ages or, as a survivor of the ample subtropical forests that covered southern South America at the end of the late Cenozoic Era.
The former is more likely and means that the temperate Patagonian forest extended out of Patagonia merging with the tropical forests, habitat of the many subtropical creatures.
Rainfall patterns during the late Cenozoic Era some 30 to 50 thousand years ago confirm the prevalence of humid conditions in what is now the arid horseshoe that separates Patagonian forests from the subtropical jungle in northern Argentina.
These damper conditions could have narrowed this Arid Continental horseshoe maybe even creating a continuous forest coverage that could have allowed exotic mammals to move into Patagonia. Besides additional rain would have provided the necessary water for them to bridge any small arid gaps.
See my post Here on a possible "aquatic route" from the Paraná River basin to the Colorado River in northern Patagonia - maps included.
When the arid conditions were restored after deglaciation, these tropical mammals may have remained isolated in Patagonia.
But, could these creatures, whose usual habitat is the very warm subtropical or tropical jungles in South America and South East Asia survive in the cold Patagonian climate?
And if they did, what kind of creature could they be?
Maybe we can look into this in future posts.
 Villagrán, C. and Hinojosa, L., (2005). Esquema Biogeográfico de Chile. In: Llorente Bousquets, J. and Morrone, J., [Eds.], (2005). Regionalización Biogeográfica en Iberoamérica y tópicos afines. Mexico: Ediciones de la Universidad Nacional Autónoma de México. Chap. 33. pp. 560+
 Secretaría de Ambiente y Desarrollo Sustentable. Convenio sobre diversidad Biológica
Copyright 2009 by Austin Whittall ©