Board Thread:General Discussion/@comment-6057702-20130730010329/@comment-136273-20130907114146

I second the notion of the top 4 Espada (#1-#4) being Vasto Lorde before conversion, it would tie in nicely with that Resurrección restriction in Las Noches. We saw their pre-Arrancar forms and all looked the part.

Nevertheless, while very probable, it's just a well-grounded speculation without an outright official confirmation... Which kind of sucks.