Paleontologists are the folks who dig in the earth to find fossils of creatures long extinct. They also classify the bones according to families, genus-es & species. Based on their findings, they tell us that something like 99% of all species which ever existed have gone extinct.
I don't know the number of extinct species they've identified, but it's at least hundreds, probably thousands.
And EVERY ONE OF THEM could, in some sense, be called a "missing link." Of course, now that it's found and identified, it's no longer "missing" is it? ;-)
But it has create two "missing links" on either side of it. "missing links" are increasing twice as fast as they are discovered. Take that, Darwin!