Plesiosaur fossils are found all over Patagonia, from its northern reaches to its southernmost areas. They have been dead for roughly 65 million years so they can't[*] be related to the "creatures" reported in several Patagonian lakes (i.e. Nahuelito, and the "Plesiosaur Lake creature").
[*]The long dead Plesiosaurs were sea monsters. The Patagonian lakes are freshwater lakes. These lakes were carved quite recently by the glaciers during the last Ice Ages.
For plesiosaurs to live in them we would have to explain how they survived in the oceans for millions of years and then, suddenly -the glaciers melted about 10,000 years ago- opted to live in cold land-locked lakes.
Why did they swim up the long Negro and Limay rivers from the Atlantic to set up their home in a mountain lake?
The mesozoic reptile theory is too flimsy and should be dismissed.
Today I came across an incredible piece of evidently "fake evidence" regarding Nahuelito. It is an article Other Strange Sea Creatures,which in good faith includes the following image assuming it to be true:
The text that includes the photograph refers to different "Sea Monsters" and includes Nahuelito, it repeats the usual brief comments on the 1922 Plesiosaur expedition and Garrett's 1910 "sighting".
But the most surprising part of the article is the following, which refers to the above image:
To the left is a depiction of an ancient carving of Nahuelito showing a horned reptilian form (from Eggleton & Suckling, The Book of Sea Monsters, 1998, p. 90.)
In other words we must believe that the local Tehuelche (Poya) or Mapuche people carved that amazingly oriental dragon head in rock!
There are no known comparable or similar carvings by these people. And this one seems to be Chinese, but not native American.
Below is a great book on Mapuche art, compare the style of wood carvings (totems) which they called rehue shown on the cover with the "stone reptile" shown above. Absolutely no similarity. Here is another example, notice how coarse it is in comparison to the dragon.
You can read more about the Mapuche's sculptures Here.
How come I have never seen it in all the years I have spent in Bariloche? It would be a top tourist attraction. I must somehow find the location of the ledge, cliff or cave where this sculpture is and organize tours (I may make a fortune).
I have not been able to check the original source, but please do it for me and let me know if the image really appears there, and what is the text that goes with it.
 Salgado, Leonardo, Parras, Ana y Gasparini, Zulma. Un plesiosaurio de cuello corto (Plesiosauroidea, Polycotylidae) del Cretácico Superior del norte de Patagonia. Ameghiniana. [online]. abr./jun. 2007, vol.44, no.2, p.349-358. ISSN 0002-7014.
 Otero, Rodrigo A; Suarez, Mario E; LE ROUX, Jacobus P. First record of Elasmosaurid Plesiosaurs (Sauropterygia: Plesiosauria) in upper levels of the Dorotea Formation, Late Cretaceous (Maastrichtian), Puerto Natales, Chilean Patagonia. AndGeo, Santiago, v. 36, n. 2, jul. 2009. doi: 10.4067/S0718-71062009000200008.
 Suckling, Nigel and Eggleton, Bob. (1998). The book of sea monsters. Overlook Press.
 Molina, Joaquin. (2007) Mapuche: arte de los pueblos del sur.Fundación Nicolás García Uriburu.
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