A likely candidate for Patagonian lake creatures. All of the cryptozoolgy articles that I have read about gigantic cryptid lake Creatures always suggest that they are Plesiosaurs. I have come across another more likely candidate (though, we would still have to explain how it managed to survive the massive dinosaur extinction 65 million years ago).
The creature by the way lived in Patagonia in the late Jurassic Period some 135 million years ago, it was a strange and entirely new species of crocodile, the Dakosaurus andiniensis . The name Dakosaurus means ‘tearing lizard’. It is the first discovery of a croc looking dinosaur.
It had a bullet-shaped skull with a high and flat head (with T-Rex like snout) . Its teeth were large and had a serrated-edge, all of which is very different from the other crocs (which have long narrow muzzles and thin teeth).
Unlike the crocodiles we know today, the Dakosaurus lived entirely in the water, and was well adapted to its habitat: it had fins instead of legs and a fish-like tail. Its size and shape earned it the nickname “Godzilla.”
Its remains were discovered in 1996 at Pampa Tril in Nequén province by Argentine scientists Sergio and Rafael Cocca.
“Godzilla” measured roughly 4 meters long (13 ft.) and its interlocking 10 cm long (4 in.) teeth show that it was a predator that fed on other big sea reptiles.
It was, according to Diego Pol (one of the team who worked on the remains) “an abundant and evolutionary successful group that occupied many ecological niches currently occupied by other species such as mammals…” this would indicate a relatively adaptable and smart creature that could have survived beyond the Dinosaur Age.
In the late Jurassic, the Andes had not yet risen out of the ocean, and Nequén was a tropical region. Pampa Tril was a warm deep bay along on the coast of the ancient Pacific Ocean. 
So we have the right creature in the right place, all we need to do is get the right time frame.
 Iglesias, M. Descubren en Neuquén un fósil de cocodrilo con aspecto de dinosaurio. 11.11.2005. Clarin. Buenos Aires.
 Gasparini Z., et al. An Unusual Marine Crocodyliform from the Jurassic-Cretaceous Boundary of Patagonia. Science 6 January 2006: 70-73. DOI: 10.1126/science.1120803
Patagonian Monsters - Cryptozoology, Myths & legends in Patagonia2010 International Year of Biodiversity Copyright 2009-2010 by Austin Whittall ©