What ate the ancient ancestor of a sea turtle?

Answer 1

I guess you mean what ate the Archelon... Although there is also and older relative called the Protostega gigaswhich is much older and is also smaller. Although something of that size would rarely have many predators it would be much the same as it is today with larger predators that were designed to bite, tear, and rip pieces of their prey for swollowing being the only viable predators.

Archelon's main suspected predators were the large sharks and the mosasaur of that era. Then again this is all speculative since no one was around to witness this and we have not found conclusive evidence in fossils, at least not in any of the reports I have read to date. Remember that his "shell" was flattened ribs and unlike the turtles today it was not solid but had thick leather plates between these flattened ribs.

Much the same story for the Protostega with large predatory animals of that era being the main enemy. It should be noted that shark teeth are embedded in Protostega bones housed at a museum in Chicago.