Seymouria

The creature that was long proposed as the ancestor of reptiles was the extinct amphibian species Seymouria. However, it then emerged that Seymouria could not be an intermediate form, since reptiles were living on Earth 30 million years before Seymouria first appeared. The oldest Seymouria fossils date back to the Lower Permian stratum of 280 million years ago. Yet Hylonomus, the oldest known reptile species (310 million years old) and Paleothyris (300 million years old) have both been found in Early Pennsylvanian strata, dating back 330 to 315 million years.222

It is of course impossible for the ancestor of reptiles to have lived long after reptiles themselves.

222. http://en.wikipedia.org/wiki/Hylonomus