Looking up the history, the original character was a drawing of a kneeling person, with their head emphasized, which is why かしら is one of the readings. The kanji data should probably include “head” as a meaning. This is why it shows up in so many body part kanji related to the head.
The fact that it is now written with the bottom part looking like 貝 is a fluke of history. It never had any relation to shellfish originally, it just got transformed from “drawing of a kneeling person” to 頁 over time, and people came to call that おおがい because it looks like 貝 but it’s bigger.
I’ll have to look for more info later why it came to mean page.
Maybe @Jonapedia knows. That is, assuming it means page in Chinese at all, which might not be the case.
EDIT: Oh, it shared a reading with 葉 (leaf), so it was used in its place at some point, and the leaf meaning morphed to page. Not sure when or where those changes took place. In any case, the page meaning is just a result of borrowing a sound-alike character, not because the shape related to page.