consanguinidad

Greetings,
While reading an informacion matrimonial that contained consanguinidad en tercero con
cuartro grado. Two of the testigos claim that "conocio bien a toda la familia incluso el tronco que lo fue, Francisco Dias". What does "el tronco" mean in this context?
Is Francisco Dias the head of the family or the relative they have in common?
Thanks so much,
Alice BB