During the Upper or Later Cretaceous (roughly spanning the period from 100 to 65 Million years ago) most of what is now Patagonia was covered by a shallow sea, part of the Atlantic Ocean. Southern South America was a group of islands, which comprised the current Somuncurá plateau and the Puerto Deseado massif (see map below).
At this time, the Andean volcanic ring began forming along the western edge of this sea closing off its connection with the South Pacific Ocean.
In this sea, lived plesiosaurs such as the short-necked plesiosaur (Plesiosauroidea, Polycotylidae), though most of the regional fossils belong to Elasmosauridae plesiosaurs, which were long-necked.
The difference is that short-necked Plesiosaurs had 13 vertebrae in their necks while the Elasmosaurs with their long necks had up to 76 vertebrae. However some believe that short-necked plesiosaurs should be classified as Pliosaurs.
These had a short neck with a long head and strong jaws (they were carnivorous) and could grow up to 15 m (45 ft.) long. They can be seen at the Olsacher Museum in Zapala, Neuquén province, Argentina, which holds the remains of several pliosaurs found in northern Patagonia.
Extant pliosaurs and plesiosaurs
If the alleged sightings of lake monsters are true, it is highly unlikely that they are one of these reptiles (pliosaurs or plesiosaurs). The reasons are quite simple:
- Time frame: they would have had to survive over 65 Million years. Outliving all other reptiles of their time.
- Adaptation: they would have had to mutate, adapting to a freshwater environment because they were sea creatures. This is not a simple feat and requires biochemical changes. Though, there is some evidence of freshwater plesiosaurs.
- Size: they were giants, and as such required a lot of food. Being carnivores they would quickly deplete the fish from Patagonian lakes. And for them to have survived for so long, a reasonable sized population in the same geographic region would be required. This would add strains to the food chain.
Patagonian lake mosnters are not reptiles that survived the massive extinctions at the end of the Cretaceous.
The Plesiosaur Directory
 Salgado Leonardo, Parras Ana, Gasparini Zulma. Un plesiosaurio de cuello corto (Plesiosauroidea, Polycotylidae) del Cretácico Superior del norte de Patagonia. Ameghiniana [revista en la Internet]. 2007 Jun [citado 2010 Sep 21] ; 44(2): 349-358.
 Fernández, Marta, (2002). En los Mares de la Araucania Ciencia Hoy, vol. 12,Oct.-Nov. 2002 No.71 pp.22-29.
 Aguirre-Urreta M. Beatriz, Casadío Silvio, Cichowolski Marcela, Lazo Darío G., Rodríguez Débora L.. (2008)Afinidades paleobiogeográficas de los invertebrados cretácicos de la Cuenca Neuquina. Ameghiniana [revista en la Internet]. 2008 Sep [citado 2010 Sep 21] ; 45(3): 591-611.
Patagonian Monsters - Cryptozoology, Myths & legends in Patagonia2010 International Year of Biodiversity Copyright 2009-2010 by Austin Whittall ©