A transitional fossil shows the evolutionary development from one species to another. For example, if organism 1 existed 70 million years ago, and organism 2 shows up in the fossil record 5 million years later, then theoretically there should be intermediate species in this 5 million year gap, which shows gradual progression from one species to another. The lack of these "transitional" fossils is proof to young earth creationists that evolution is false.

Evolutionists have shown that indeed there are transitional fossils, and there are plenty of examples of them. For instance, see this article.1 Here is the key point...even if young earth creationists accept these examples of transitional fossils, they will still claim that there are no transitional fossils! These fossils will be called either unique species, or they will come up with some reason (disease, birth defect, etc) that accounts for the apparent transition feature.

Naturally, they will say, "Where are the transitional fossils between these transitional fossils?" If we had a clear fossil record, showing progression every 10,000 years for millions of years, they will not believe it, and will want the "transitional" fossils for the missing 10,000 year period. No amount of evidence will convict them that their belief is wrong.

The same thing could be said of progressive creationists as well. Progressive creationists believe in an old earth, but that God created each species a unique creation, and not evolved from an earlier species. I happen to be one of these myself. However, we must be careful not so say our view is the only one that is valid. Dr. Hugh Ross of the old earth ministry Reasons to Believe, has put forth many arguments against evolution. However, when you consider the possibility that within Theistic Evolution, you have God guiding the evolutionary process, then all bets are off. Yes, evolution by itself could not have happened...as Dr. Ross explains, 13.7 billion years is not nearly enough time, statistically speaking, for evolution to occur. However, with God's supernatural intervention and guidance, it could have easily happened.

I'm not saying that evolution is right, but what I am saying is that with God, all things are possible, including evolution. We should not be so quick, as progressive creationists, to condemn evolution.

Conclusion

The fact that young earth creationists will not be convinced, no matter how much evidence is presented, makes this a weak argument. The argument is not based on science, but on assumptions based on a young earth interpretation of creation.