Basically what the title says: what happened to the Merovingian dynasty after it was deposed by the Pepin the Short? We know that the last king Childeric III and his son Thierry were forcefully tonsured, with the former dying in the mid-late 750s and the later either dying young or living as a monk, but nothing else. Were they the last of the dynasty or were there other (verifiable) branches that survived?
Honestly, I don't know. Never heard of them after.
But I guess nobody likes competition, even for the position of a puppet king (who might be used by somebody).
So, I presume, if the relatives of the Merovingians did not disappear before the deposition, they disappeared after.
Having in mind that killing relatives was kind of a family trait among the Merovingians since
Clovis I, there were not too many of them left, I think.