古生物学：柔らかい殻の卵の進化を示す「硬い」証拠

Two studies that shed light on the evolution of amniote eggs are published in Nature this week. One study suggests that the first dinosaurs may have laid soft-shelled eggs, a finding at odds with the prevailing view that dinosaurs laid hard-shelled eggs. The second study describes a large, soft-shelled egg from Antarctica, the first known fossil egg ever to have been found in the continent.

Amniotes—the group that includes birds, mammals and reptiles—produce eggs with an inner membrane or ‘amnion’ that helps to prevent the embryo from drying out. Some amniotes, such as lizards or turtles, lay soft-shelled eggs, whereas others, such as birds, lay eggs with hard, heavily calcified shells. These variations present different evolutionary trajectories. The evolution of calcified eggs, which offer increased protection against environmental stress, represents a milestone in the history of the amniotes, as it likely contributed to reproductive success and thus the spread and diversification of this clade. However, soft-shelled eggs are rare in the fossil record, which makes it difficult to study the transition from soft to hard shells.

Mark Norell and colleagues studied embryo-containing fossil eggs belonging to two species of dinosaur, Protoceratops and Mussaurus, and found that the eggs were soft-shelled. They suggest that hard-shelled, calcified eggs evolved independently at least three times in dinosaurs, and probably developed from a range of ancestral soft-shelled types. The soft-shelled eggs were probably buried in moist soil or sand and then incubated with heat from decomposing plant matter, as with some reptiles today.

In the second study, Julia Clarke and colleagues describe an almost-complete, football-sized soft-shelled egg from approximately 66 million-year-old Cretaceous deposits of Antarctica. It is one of the largest eggs ever described; second only in size to those laid by the extinct elephant bird from Madagascar. The dimensions and thin shell of the egg, which lacks a crystalline outer layer, hint at an ovoviviparous lifestyle, in which a ‘vestigial’ egg develops inside the mother and then hatches immediately after it is laid. The egg is ascribed to a new taxon, Antarcticoolithus bradyi; although the mother of the egg remains a mystery, the authors suggest it could have been laid by a giant marine reptile such as a mosasaur. An alternative explanation, suggested in an accompanying News and Views article, is that the egg was laid by a dinosaur. This assumption is made because the estimated weight of the egg is close to that of the largest eggs known from birds and nonavian dinosaurs, and both of these groups left fossils behind in Antarctica.