Many years ago, Discover magazine had an article which discussed Dale Russell's speculation of Troodons evolving into sentient tool-users if they weren't wiped out by the mass extinction (supposedly Alvarez's asteroid strike) that cost us all the other dinosaurs (or those that didn't become birds).
You'll recall an episode of Voyager with a similar theme, this time postulating that hadrosaurs had actually developed warp-drive and left the planet a long time ago, seeding the far side of the galaxy with dinosaur descendents.
It's fun to speculate about such things. The fact of the matter is that we really haven't been around for that long, and we've been tool-users for less time than there have been hominids, so maybe there's plenty of room for intelligent amphibians in our past, dinosaurs, tool-using mammoths, who knows? The only problem with such fantasy creatures is ... there's no sign of them. No fossilized specimens with big enough brains, no spark plugs embedded in geodes (no, the Coso Geode isn't as old as it looks), no sign of a previous technological culture with the exception of earlier versions of h. sapiens.
That's not proof, of course. Fossils aren't easy to make in the first place, and we're finding new species of extinct animals in the fossil record every year. Furthermore, if a species evolved tool and fire-using intelligence, but never got beyond wood and bone implements, I doubt there'd be much trace of their artifacts after millions of years.
Right now, it's the stuff of fiction. I think it's unlikely to have happened, but that's not the same as impossible.
