Guys, guys, you are trying to tie into the "real world" too tightly. This is a world where, and I never get tired of stating it, a ten-year-old human can go on explorations of a cave, find a hole, leap to the next floor below to a ground that is ~10m away, or even leap down some whole floors, and pretty much instantly walk away, unscratched. What a friggin' "mouse", "dragonfly" or "sea lion" would have evolved up like during countless generations to have forced humans to be like this
? We are allowed to scale "normal" things up quite a good bit; expected to if we are explicitly comparing things to the real world
, I'd say.
Specific bones that grow up to become a detachable part is doable. Then again, growing a partial exoskeleton is even more doable. We do have cases like Scyther where one part of the body does not grow like the others. The stress from detaching a part of their body for the first time, perhaps under instigation from other members of their species, would be an important element in the rite of passage for such Pokémon.
For an (nonexclusive) alternative and if nothing else, the shell could be the remains / former home of a Clamperl that are collected by the adults as a foundation of heirloom or even as a form of currency. It even make sense that individuals of the species wanted to keep a specific "coin" over the others they may have collected. Youth of the species may build skill and confidence by betting and trading such "coins" in battles.
Anyway, makes me think (perhaps I shouldn't have got back to drinking coffee
). Giving such Pokémon as Cubone bred in captivity a helmet made of (say) plastic and then letting Trainers have those Pokémon, while it works for the individual, does not scale well because we'd have to state what the League considers a Cubone with a water-resisting plastic helmet entering a tournament to be like, and that's only the simpler case (I'm thinking of Wormadam
living near demolition sites for a complicated case right now), not to mention it breaks the semantics of their rituals in the wild (I mean, it's not going to be nice when such a released Cubone decided to leave his Uranium-enriched club at the species's graveyard).
Meh... you guys go with the article. I need to go empty my head.