Hard Facts for Soft Balls
So far, scientists have agreed: dinosaurs laid hard-shelled eggs, while some of their relatives gave birth to sea-living young. Findings of fossil eggs once encased in a soft shell are shattering these views.
The appearance of the so-called amniotic egg is considered a key event in the evolutionary history of vertebrates. Its decisive adaptation advantage lies in the eponymous amnion: the innermost embryonic covering that prevents the offspring from drying out. The next step in development occurred when a tough outer shell offered protection and mechanical stability. Only then could the first reptiles conquer the mainland more than 300 million years ago and pave the way for the rise of birds and mammals.
Hardened by crystalline calcium carbonate, hard-shelled eggs, such as those of birds, are well preserved as fossils. In contrast, the leathery outer shells of the specimens, which are laid by most lizards and snakes, usually disintegrate very quickly. Since modern-day crocodiles and birds produce hard-shelled eggs, it was previously assumed that their close relatives, the dinosaurs, also produced such shells. Fossils of soft-shelled eggs millions of years old are now challenging this prevailing view of reproduction in both dinosaurs and marine reptiles…