In the Book of Mormon, there is a prophet named Abinadi (like Moses, Jeremiah, etc.) who castigates a corrupt priest class for their worship of false gods and other vices.
He quotes the 10 commandments to these men in Mosiah 12:36:
Thou shalt not make unto thee any graven image, or any likeness of any thing in heaven above, or things which are in the earth beneath.
A little later in Mosiah 13:12:
And now, ye remember that I said unto you: Thou shalt not make unto thee any graven image, or any likeness of things which are in heaven above, or which are in the earth beneath, or which are in the water under the earth.
This is echoed in Exodus 20:4:
Thou shalt not make unto thee any graven image, or any likeness of any thing that is in heaven above, or that is in the earth beneath, or that is in the water under the earth:
Wow! Sounds to me like he's denouncing the worship of reptilians or other cave-dwellers. Of course, reptilians aren't the only creatures who dwell underground: chimeras, little people, etc. But I can't think of many others who can dwell "in water under the earth."
Reptilian creatures are known to live underwater in many cases, e.g., crocodiles, alligators, snakes, etc. At least, they spend great portions of their time underwater.
Giant reptiles who live underwater are also mentioned in The Coming Race (chapter 18). The Vril-ya called them "Kreks." They were extremely aggressive and dangerous creatures with a vicious appetite. Some of them exhibited above-average intelligence for what was supposed to be a dumb animal. Perhaps they have two-legged cousins with intelligence which dwarfs that of man...
Perhaps the people in ancient America were building idols to please reptilian creatures? The same sort of thing happened in ancient Mesopotamia and Asia! Maybe they even saw the real deal from time to time. Food for thought.