Originally Posted by Lynchburg
You are partly right... there are no joint locks in Muay Thai, they come from Muay Boran. This is the father of Muay Thai that came from Krabi Krabong
The closest you will get to a joint lock in Muay Thai is a shoulder roll when a clinch goes on.
It is a good point you make though
There's a couple more you can borrow from Muay Boran and use in the ring - a crafty grappling coach will show. Cambodian's are good for this stuff and learnt some good ones attacking the shoulder, elbows and spine. The elbow lock off the shoulder roll is done to weaken the elbow and thus the punch is one.