Biogenic origin of secondary eggshell units in dinosaur eggshells elucidates lost biomineralization process in maniraptoran dinosaurs

Secondary eggshell units, though rarely observed in modern avian eggshells, are marked structures in non-avian dinosaur eggshells that offer valuable paleobiological insights. Despite their significance, the origins of secondary eggshell units remain understudied, leading to debates in paleontology, including the hypothesis of an abiogenic origin for these structures. Here, we demonstrate that secondary eggshell units in non-avian dinosaur eggshells are biogenic in nature, based on analyses using advanced microscopic techniques. The structural characteristics of these units suggest a formation mechanism similar to that of turtle and crocodile eggshells, with matrix fibers likely initiating their development. Furthermore, a diminishing presence of secondary eggshell units in non-avian maniraptoran dinosaurs points to the evolution of a more refined physiological process for eggshell formation in this lineage. These findings shed light on the evolutionary trajectory of eggshell biomineralization in dinosaurs and their close relatives.