fossil bird egg native small smooth shelled elongated eggs discovered 1960s early 70s series fossil hunting expeditions desert described minor major eggs probably laid colonial nesting sites banks rivers lakes minor unusual frequently found skeletons bird embryos developed wings suggest able fly soon hatching unlike modern birds distribution found desert specifically fossils found dated upper cretaceous history numerous fossils including specimens discovered collected polish fossil hunting expeditions desert soviet expeditions fossils brought warsaw moscow eggs classified analyzed polish described developed bird embryos fossil eggs housed warsaw identified similar scientists began referring eggs russian introduced modern system classify fossil eggs eggs formal classification scheme assign basic type believed eggs probably considered year polish reviewed entire collection fossil eggs discovered polish expeditions including named eggs following referred tentatively time ongoing studies similar eggs found soviet expeditions cast doubt classification working divided eggs informal groups containing smaller eggs including ones embryos described larger eggs years later went classify eggs new containing single minor major corresponding respectively described new genus species bird based embryos eggs classified larger egg specimens previously assigned major moved new description eggs small smooth shelled shaped similar modern bird eggs end distinguished mainly size major ranges long thickness minor thick thoroughly studied heavy specimens makes difficult examine structure system consists possibly structural layers inner layer called layer half thickness outer continuous layer outside specimens outer layer simply true external zone layer present bird eggs rare non specimens unaffected layer rule possibility originally layers external layer easily separate rest specimens reveal system means pores low density long narrow straight fossil egg differs larger thicker smaller layer eggs larger thicker layer embryos minor specimens contain remains genus embryos skeletons implying late stage development died wings shoulders especially developed modern little exhibit comparable degree arm shoulder bones likely like little able fly soon hatching embryos known major eggs usually assumed laid similar type bird nests eggs probably laid open nests banks rivers lakes frequently flood nesting areas bury eggs distribution eggs suggests long term colonial nesting site locality locality eggs typically arranged scattered randomly oriented nearly vertically substrate solitary eggs laid buried individually similar nesting habits modern possible originally laid flooding separated deposited vertically water level dropped water damage explain shells frequently heavily classification according system classify fossil eggs classified turn classified called basic type analysis performed pictured found sister taxon considered different warrant
Gobioolithus