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Re: Idle question on Cretaceous chronostratigraphy

The Cretaceous is the longest period of the Phanerozoic, yet also the
only one divided into only two epochs (Lower and Upper Cretaceous) in
the ICS chart. Is there a rational explanation for this somewhat
surprising state of affairs, or did it just "happen"?

It did just happen.

Glut's Encyclopedia recognizes a "Middle Cretaceous", apparently without knowing that this kind of nomenclature is regulated and can't just be introduced by usage. It consists of the Albian and Cenomanian, which makes a lot of sense -- there were mass extinctions at both ends, but IIRC none between the Albian and the Cenomanian.