OK--not dinosaurs, but clearly early dinosaurs and other archosaurs during the Triassic had nasty encounters with giant temnospondyl amphibians.
Whether temnospondyls are amphibians is a question I'm working on!
Abstract Capitosaurs were among the largest amphibians that have ever lived. Their members displayed an amphibious lifestyle.
There were amphibious and even outright terrestrial temnospondyls, but the capitosaurs seem to have been about as "amphibious" as a catfish.
BTW, in case anyone is wondering, the "tabular horns" aren't anything that would have been visible in the living animal; they're just the dorsocaudolateral corners of the skull roof drawn out into triangular processes that may have served as attachment sites for muscles or ligaments that held the head steady during swimming.