The initial period of rule by emperors had some short-lived
dynasties during its beginning and towards the end. The
Julio-Claudian dynasty (5 emperors) ended with a rebellion during
which Nero had is assistant kill him as he did not manage to commit
suicide and a civil war. The Flavian dynasty (3 emperors) ended
with the murder of Domitian and the Nerva-Antonine (8 empeorors)
dynasty ended with the murder of Commodus and also involved a civil
war. The Severan(5 emperors) dynasty ended with the overthrow of
Alexander Severus.
There was a 70-year period without dynasties. The emperors were
generals appointed by their troops. The Constantinian dynasty ended
because of murders of family members by Constantine's sons, wars
between them and the death in battle of Julian. The Valentinian
dynasty was replaced by the Theodosian dynasty via marriage and
this last dynasty ended with murder and usurpation.