Lake Dragons (aka monsters): Thought to descend from tanystropheus or plesiosaur. However, tanystropheus is more likely, because plesiosaur probably wouldn't be able to lift its weak neck above the water.
Tanystropheus: Means "long necked one"; A twenty-feet long reptile (including a ten-feet neck); Strong neck; An aquatic or semi-aquatic piscivore (fish eater); Dated from middle Triassic period.
Plesiosaur: Has a broad body with short tail, two sets of flippers; Weak neck; A carnivorous aquatic reptile; Dated to early Jurassic period.
The Water Horse movie shows a plesiosaur with flippers.
Further study:
Plesiosaur Champ
Champ Wiki
Lake Monsters
