The first dinosaur egg was soft

NORELL, M A; WIEMANN, J.; FABBRI, M.; POL, D.

Congreso; 79° Annual Meeting of the Society of Vertebrate Paleontology; 2019

The calcified eggshell protects a developing embryo against environmental stress and thereby contributes to the parent?s reproductive success. Since modern crocodilians and birds lay hard-shelled eggs, this eggshell type has also been inferred for nonavian dinosaurs. Known dinosaur eggshell is characterized by an innermost shell membrane, an overlying protein matrix that contains calcite, and an outermost waxy cuticle. The egg calcite may occur in a single or multilayered ultrastructure which, alongside respiratory pore configurations, differs drastically across the three major clades of dinosaurs. While only hadrosaurid, derived sauropod, and tetanuran theropod eggshells have been definitely identified to date, missing intermediate shell types challenge efforts to homologize eggshell across these highly derived taxa. Here we present mineralogical, organochemical, and ultrastructural evidence for a soft-shelled nature of exceptionally preserved eggs of the ornithischian Protoceratops and basal sauropodomorph Mussaurus. Statistical evaluation (UPGMA, rho, unconstrained, one-way) of in situ organic phase Raman spectra obtained for a representative set (n=40) of hard and soft eggshells from fossil and extant diapsids clusters the organic, but phosphatized Protoceratops and the fully carbonaceous Mussaurus eggshells with soft-shelled eggs. Histology corroborates the organic composition of these two soft-shelled dinosaur eggs, and reveals a stratified arrangement resembling soft turtle eggshell. An ancestral state reconstruction of eggshell composition and ultrastructure compared eggshells from Protoceratops and Mussaurus to those of other archosaurs, and revealed that the first dinosaur egg was soft. The calcified dinosaur egg evolved at least three times independently throughout the Mesozoic, explaining the bias towards eggshells of highly derived dinosaurs in the fossil record.