Theory: Some Oviparous can Regenerate their Eggshell

Some oviparous have the ability to regenerate their eggshell.

Arguments

 * Argument 1: Adult Yoshis have been shown turning into eggs. In Super Mario World 2: Yoshi's Island and Yoshi's Island DS, when Baby Mario gets a Star, Yoshi turns into an egg, and in some games like Mario Power Tennis and Mario Strikers Charged, Yoshi´s special ability is to be encased in an egg (a giant one in Strikers case).


 * Argument 2: In Super Mario RPG: Legend of the Seven Stars, Mario and company meet Birdo while she is still inside her egg. Mario and company battle her, they break the egg, then fight Birdo who gives up and, after this happens, Birdo is again inside her egg, even if the egg was broken in the battle.


 * Argument 3: In Super Mario Galaxy 2, Mario battles Peewee Piranha. First, Mario cracks its egg, then Peewee shows his legs and head, while the rest of his body is still inside the eggshell. When Mario completely breaks the eggshell to attack Peewee´s vulnerable point, Peewee reforms the eggshell.


 * Argument 4: Finally, it would also explain why Dino Piranha was already out of their egg in Super Mario Galaxy´s ending, but then in Super Mario Galaxy 2, Dino Piranha is inside their egg in Boss Blitz Galaxy.