The first dinosaur egg was soft

Abstract

Calcified eggshell protect developing embryos against environmental stress and contributes toreproductive success. Since modern crocodilians and birds lay hard-shelled eggs, this eggshelltype has also been inferred for nonavian dinosaurs. Known dinosaur eggshell is characterized byan innermost membrane, an overlying protein matrix containing calcite, and an outermost waxycuticle. The calcitic eggshell consists of one or more ultrastructural layers that, along withrespiratory pore configurations, differs drastically across the three major dinosaur clades. Whileonly hadrosaurid, derived sauropod, and tetanuran eggshells have been discovered to date,missing fossil eggshells covering the phylogenetic gaps between these taxa challenge efforts tohomologize eggshell across all dinosaurs. We present mineralogical, organochemical, andultrastructural evidence for an originally non-biomineralized, soft-shelled nature of exceptionallypreserved ornithischian Protoceratops and basal sauropodomorph Mussaurus eggs. Statisticalevaluation of in situ organic phase Raman spectra obtained for a representative set of hard- andsoft-shelled, fossil and extant diapsid eggshells, clusters the originally organic, but secondarilyphosphatized Protoceratops and the carbonaceous Mussaurus eggshells with soft eggshells.Histology corroborates the organic composition of these two soft-shelled dinosaur eggs,revealing a stratified arrangement resembling soft turtle eggshell. An ancestral statereconstruction of composition and ultrastructure compared eggshells from Protoceratops andMussaurus to those from other archosaurs, and revealed that the first dinosaur egg was softshelled.The calcified dinosaur egg evolved at least three times independently throughout theMesozoic, explaining the bias towards eggshells of highly derived dinosaurs in the fossil record.