The first discovery of non-avian dinosaur egg clutch (Macroolithus yaotunensis, Elongatoolithidae) from the Upper Cretaceous Qiupa Formation of Tantou Basin

The Upper Cretaceous of Tantou Basin in western Henan has yielded many vertebrate fossils, which are featured by several non-avian dinosaurs. Meanwhile, studies on their eggs were yet inadequate though many eggshells have been reported. The newly discovered material 41HⅤ0199 was excavated from the Upper Cretaceous Qiupa Formation in 2021. The block preserves eight complete eggs arranged in two partial rings that form a partial clutch, and there are some scattered eggshells preserved closely with the block, showing a concave-up to concave-down ratio of 54.5 : 45.5, which indicates that the scattered eggshells come from the clutch and the clutch had been partially broken before it was buried. Based on morphological and microstructural characteristics, the eggs and eggshells can be assigned to Macroolithus yaotunensis (Elongatoolithidae), an oospecies known to be related to oviraptorids, which leads Yulong mini to be its probable producer. Besides, some eggshells show microstructural signs indicating egg retention, which marks the second example of egg retention in the oofamily Elongatoolithidae.