An Early Jurassic soft eggshell provides insights into the evolutionary history of dinosaur eggs

Early eggshell fossils offer invaluable insights into evolutionary innovations in over 300 million years of amniote egg history. However, the incompleteness of the dinosaur eggshell fossil record hinders ancestral state reconstruction and leads to uncertainty about whether the first dinosaur egg was soft-shelled, semi-rigid-shelled, or rigid-shelled. Herein, we present a fossilized soft eggshell (Mutabilisoolithus dawaensis) from an Early Jurassic basal sauropodomorph. We find that M. dawaensis has a soft, plastic-like deformation, a thick cuticle, a thin calcareous layer formed by non-interlocking shell units, and a significantly thick shell membrane. Its shell membrane is similar to that of avian eggs in structure, diameter, and arrangement of the fibers, as well as in the distinctions among outer, inner, and limiting membrane layers, but it is much thicker than that of avian eggs. In dinosaurian egg evolutionary history, M. dawaensis provides evidence of early Jurassic soft-shelled eggs, may represent the earliest stage of biomineralization, and bridges the gap between non-biomineralized membranous soft eggshells and biomineralized semi-rigid eggshells. Fossil dinosaur eggs are extremely rare, and there is uncertainty about their ancestral anatomy and evolution. A newly described Early Jurassic sauropodomorph soft-shelled egg offers valuable insights into the reproductive biology of dinosaurs.