How & why was Charlemagne directly descended from a Roman aristocrat named Flavius Syragius?

Update: said he was a aristocrat that ruled over region around modern day Lyon...which back during Roman empire was named what?

and then he begat Saint Arnulf of Metz, who was a bishop?
Update 2: Wikipedia says
Arnulf gave distinguished service at the Austrasian court under Theudebert II. He distinguished himself both as a military commander and in the civil administration. Arnulf is the male-line grandfather of Pepin of Herstal, great-grandfather of Charles Martel and 3rd great-grandfather of Charlemagne.
1 answer 1