This trait is widespread in modern birds, but is very rare among Mesozoic fossil eggs.

It was formerly believed that triple-layered eggshells were unique to neognath birds, but some three-layered eggs have been found containing enantiornithines and non-avian theropods, indicating that the presence of a third layer is plesiomorphic among dinosaurs.

[1] Also, the presence of Nippononaia ryosekiana and the types of charophytes suggest a Barremian age.

Indeed, Plagioolithus is the oldest known trace of bird eggs in the fossil record.

[1] At the Kitadani Formation, numerous other dinosaurs are known to have coexisted with Plagioolithus, including Fukuiraptor, Fukuisaurus, Fukuititan, Koshisaurus, and an unidentified type of non-avian theropod.