The story I heard was that one day in 716 B.C., Romulus disappeared in a cloud of smoke, and his soldiers figured that the god Mars had taken him away. If there is any truth to this, he could have been struck by lightning or a meteor. But either way, I don’t think there would have been much left of him, maybe nothing at all, to put in the sarcophagus.
He was taken by aliens to what would be the planet Romulus...
Maybe that's what got cremation started in Rome...