The biggest (and seemingly sole) evidence for this is found in Mario Party: Island Tour:

. Bowser doesn't specify the exact relation he bears to that Dry Bowser, but it fits nicely with my other Mushroom Civil War Theories to have him as the father of the current King Koopa. It would also explain how Dry Bowser can race/party/and sport alongside the living Bowser, as they are different individuals.

The only addition to his is Dry Bowser, or Morton Koopa Sr, which is my new headcanon.

Here's how I think it might fit. Bowsers father dies only some time before or after the birth of Bowser, as he does not remember his parents in the games. The Koopa prince becomes Koopa King, and the adventures of Mario and co begin. Some where along the way, when Bowser is defeated, he is revived by Bowser Jr as Dry Bowser. Since he was only dead for a short while, soon Bowser regrew his flesh to reassume his living appearance. Inspired by this and having learned of his fathers might as king, he reanimates his remains, but since it has been such a long time since he passed away, the flesh cannot regrow.

The revival of the long dead relative is possible due to the deep connection with magic the Koopas have: Bowser is skilled in the dark arts as seen in Mario Galaxy, Bowser Jr as mentioned revived his old man, and the magikoopas are very powerful soldiers of the Koopa Troop.This enabled Dead Father Koopa to return to life, although while still rocking the skeletal visage.