Gamera Chronicles Extra Chapter

40 Million Years Ago in the Eocene, Gamera's ancestors arose from an early linage of snapping turtles. They retreated into the hollow earth when facing competition with the more faster mammals. Millions of years later within the hollow earth, gamera's ancestors gradually grew bigger as a result of the energetic ecosystem of the hollow earth. They would often conflict with the ancestors of Godzilla as the two apex predators often clash for dominance but over time the godzilla ancestors would take to the oceans while gamera's ancestors would take to the skies...or rather empty spaces between the lands and seas of the hollow earth. 

Millions of years later, apox. 10,000 BCE. Early Holocene Epoch, Quaternary Period, Cenozoic Era.

It was a new dawn in the hollow earth, titanic fish swam in the lakes and rivers while warbats stalk around the various hills and mountains searching for prey. A Titanodontognathus bassi bursts from the north-side of the hollow earth. Resembling a cross between a sturgeon and a flying fish escapes the water from a hungry Godzilla's jaws. The fish flapped its powerful flight fins to escape from the massive radioactive dinosaurs jaws. Godzilla returned back to the water's cold depths as the flying bass escaped.

The kaiju bass starts swimming in the air to feed on the flying bubble seeds. It then quickly joins a whole school of Titanodontognathus as they swim through the space between. A loud scream echoed across the hollow earth, the Titanodontognathus school suddenly switched directions as a massive set of tusked jaws came swooping to gobble the bass kaiju whole