“Despite being malformed, the egg is excellently preserved, including parts of the eggshell that are rarely seen in the fossil record, such as traces of the egg membrane and the cuticle, which are mostly made of proteins and other organic materials.”
Dr Jingmai O’Connor, of the same research institute, said: “This type of cuticle morphology would be expected for birds that partially bury their eggs, as it has already been proposed to be the case for enantiornithines.
“Finding this morphology in Avimaia also supports the hypothesis that a cuticle with protective spherules represents the ancestral condition for avian eggs.
“Female birds about to lay eggs deposit a unique bone tissue found inside the empty spaces of their skeleton, which serves as a calcium reservoir for the developing eggshell.”
Article source: https://www.express.co.uk/news/science/1102894/dinosaur-discovery-bird-bones-egg-fossil-evolution