Abnormal, multilayered eggshells are frequently reported in fossil specimens. However, previous reports of this pathological condition pertain to taxonomically unidentified fossil eggs or eggshell fragments rather than in situ clutches. A survey of 393 in situ clutches, referable to titanosaur sauropod dinosaurs, from Upper Cretaceous rocks in northwestern Patagonia, Argentina, revealed that six of these clutches contain both normal eggs and abnormal, multi- layered eggs within the same clutch. Excavation of one clutch exposed 30 eggs, distributed in three levels, including 27 normal eggs and 3 multilayered eggs. The three abnormal eggs occupied the highest level within the clutch and represent the last eggs laid by the female sauropod. The innermost eggshell layer in multilayered eggs from all six clutches exhibits typical megaloolithid calcite structure. The structure of the overlying, abnormal shell layer(s), however, varies among the clutches and within a single egg. Three types of abnormal eggshell morphology are documented. All previous studies that link abnormal eggshells to dinosaur physiology and the terminal Cretaceous extinction event are incomplete in terms of taphonomy, taxonomic identification, and lack rigorous statistical methods.