The first dinosaur egg was soft

NORELL, MARK A.; WIEMANN, JASMINA; FABBRI, MATTEO; YU, CONGYU; MARSICANO, CLAUDIA A.; MOORE-NALL, ANITA; VARRICCHIO, DAVID J.; POL, DIEGO; ZELENITSKY, DARLA K.

NATURE

NATURE PUBLISHING GROUP

Año: 2020

0028-0836

Calcified eggshells protect developing embryos against environmental stress and contribute to 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 membrane, an overlying protein matrix containing calcite, and an outermost waxy cuticle. The calcitic eggshell consists of one or more ultrastructural layers that, along with respiratory pore configurations, differs drastically across the three major dinosaur clades. While only hadrosaurid, derived sauropod, and tetanuran eggshells have been discovered to date, missing fossil eggshells covering the phylogenetic gaps between these taxa challenge efforts to homologize eggshell structures across all dinosaurs. We present mineralogical, organochemical, and ultrastructural evidence for an originally non-biomineralised, soft-shelled nature of exceptionally preserved ornithischian Protoceratops and basal sauropodomorph Mussaurus eggs. Statistical evaluation of in situ organic phase Raman spectra obtained for a representative set of hard- and soft-shelled, fossil and extant diapsid eggshells, clusters the originally organic, but secondarily phosphatized Protoceratops and the organic Mussaurus eggshells with soft, non-biomineralised eggshells. Histology corroborates the organic composition of these two soft-shelled dinosaur eggs, revealing a stratified arrangement resembling soft turtle eggshell. An ancestral state reconstruction of composition and ultrastructure compared eggshells from Protoceratops and Mussaurus to those from other archosaurs, and revealed that the first dinosaur egg was soft-shelled. 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.