Paul Schrader -- a legendary Hollywood screenwriter and director -- spoke with Variety to promote his new flick "Oh, Canada," and revealed he had a fearsome run-in with Marty's dog which pulled a move worthy of a "Goodfellas" gangster.

The way Schrader tells it ... he went over to Scorsese's crib in December for dinner -- and, when he went to pet a little Scottish terrier, it went full Cujo and bit him, ripping off and eating part of his thumb!

It sounds like the little dog -- who's more Martin's daughter's than his -- ended up with a decent-sized portion of Schrader. Thankfully, Paul says MS has a live-in nurse, so he didn't need to rush to the hospital.

Seems like everything's fine between the 2 old pals BTW -- Schrader plays off the attack as no big deal. The pals first worked together on "Taxi Driver" and also collaborated on hit films like "Raging Bull" and "The Last Temptation of Christ."

Schrader's also carved out quite the career for himself outside of his Scorsese ties ... directing Richard Gere -- who also stars in his new film -- in the classic, "American Gigolo," and more recently helming the Oscar Isaac movie "The Card Counter."