[1] The first Japanese fossil eggs were discovered in 2003: Yoichi Azuma documented numerous eggs of the Ratite morphotype in the Kitadani Formation,[2] and Ren Hirayama et al. documented dinosaur and turtle eggshells in the Kuwijima Formation.

In 2016, a team of paleontologists from Japan and Canada collected numerous fossilized eggs at Kamitaki, including the specimens which they would refer to a new oogenus and oospecies: Nipponoolithus rumosus.

[1] Nipponoolithus rumosus is known only from a handful of isolated eggshell fragments ranging from 0.36 to 0.53 mm in thickness, just barely larger than a chicken egg.

These eggs, along with skeletal remains, show that the parents of Nipponoolithus coexisted with numerous other small theropods, as well as an assemblage of ankylosaurs, titanosaurs, hadrosauroids, tyrannosaurs, and therizinosaurs.

The parent of Nipponoolithus probably weighed roughly 15 kilograms (33 lb), comparable to the size of some contemporary small theropods from the Jehol biota in China.