No, they weren't. I originally wasn't going to pick on you about this point, but the more research I did I realized it was kind of important. Here's a quick little link about it.
http://www.ucmp.berkeley.edu/vertebrates/tetrapods/amphibfr.html
and a more detailed link about amphibian/reptile evolution.
http://www.myherp.com/articles/other/evolution.htm
Frogs as we know them were not around when the amphibian-reptile transition occured. They have survived, in one form or another, for millions of years since then.
Your position is characteristic of a VERY common misconception about the way evolution works. New species do not completely replace old species unless there is direct competition between them. Amphibians occupied a specific niche in the ancient ecosystem. They could move about on land and thus avoid competition from fish and other water-based organisms. Reptiles had traits that made them even less dependent on water and thus could occupy a NEW niche in the ecosystem. They DID NOT replace amphibians. They didn't have to. Reptiles could entirely avoid places where amphibians ruled. That's why the traits that define reptiles were such an advantage: scales, shelled eggs, better skeletons. Amphibians were perfectly capable of competing with speicies of fish and occupied a new enviornment from them. Reptiles occupied still another enviornment. The Nautulis(sp?) has existed relatively unchanged since it occupies a perfectly suitable little enviornmental niche all its own and nothing better has come along that directly competes with it enough to force into extinction. Reptiles did not "replace" amphibians.
Back to your point about frogs not changing. Individual speices of frogs, and there are oodles, are widely varied and have changed drastically since the first frog-like ancestor species evolved. The basic "frog format" however, is well-adapted to the conditions these species tend to occupy. Hence it is superior to other speices that might have tried to compete with it and has not died out.
There are no contradictions in the theory of evolution. What you are discussing here is a very common misconception about how evolution works. You're right! The position you're arguing against here doesn't make sense and is, in fact, not the way the system works.
While very interesting, it is irrelevant to the point being made. Frogs have been around for some 200 million years (at least) and the ones I quoted about are virtually unchanged. There are many other creatures which have been virtually unchanged for hundreds of millions of years - according to evolutionists and all legitimate science. The question posed many posts ago - which remains unanswered is - when did frogs and other species know when to stop evolving? If mutation happens all the time and the environment changes all the time as evolutionists claim their theory accounts for new species, then how is it possible for any species to remain unchanged for hundreds of millions of years. Clearly, evolution is false.