Calcified eggshells protect developing embryos against environmental stress and contribute to reproductive success1. As modern crocodilians and birds lay hard-shelled eggs, this eggshell type has been inferred for non-avian dinosaurs. Known dinosaur eggshells are characterized by an innermost membrane, an overlying protein matrix containing calcite, and an outermost waxy cuticle2,3,4,5,6,7. The calcitic eggshell consists of one or more ultrastructural layers that differ markedly among the three major dinosaur clades, as do the configurations of respiratory pores. So far, only hadrosaurid, a few sauropodomorph and tetanuran eggshells have been discovered; the paucity of the fossil record and the lack of intermediate eggshell types challenge efforts to homologize eggshell structures across all dinosaurs8,9,10,11,12,13,14,15,16,17,18. Here we present mineralogical, organochemical and ultrastructural evidence for an originally non-biomineralized, soft-shelled nature of exceptionally preserved ornithischian Protoceratops and basal sauropodomorph Mussaurus eggs. Statistical evaluation of in situ 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-biomineralized eggshells. Histology corroborates the organic composition of these soft-shelled dinosaur eggs, revealing a stratified arrangement resembling turtle soft eggshell. Through an ancestral-state reconstruction of composition and ultrastructure, we compare eggshells from Protoceratops and Mussaurus with those from other diapsids, revealing that the first dinosaur egg was soft-shelled. The calcified, hard-shelled dinosaur egg evolved independently at least three times throughout the Mesozoic era, explaining the bias towards eggshells of derived dinosaurs in the fossil record.
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All relevant Raman spectra and eggshell codings are available within this paper and its Supplementary Information. Materials are available from the corresponding authors upon reasonable request.
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We thank M. Ellison for the Protoceratops clutch photography; G. Watkins-Colwell and K. Zyskowski for providing eggshell specimens from the Yale Peabody collections; and D. E. G. Briggs for comments on the manuscript. J. Headden, S. Hartman, E. Willoughby and M. Witton created the PhyloPic silhouettes used in Fig. 2. A grant to D.P. from the National Geographic Society (grant 8860-10) funded the collection of Mussaurus eggshells.
The authors declare no competing interests.
Peer review information Nature thanks Johan Lindgren and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.
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Extended data figures and tables
Extended Data Fig. 1 Hierarchical cluster analysis of biomineralization signatures preserved in eggshell proteins (extant samples) and their fossilization products (fossil samples).
The topology represents a cluster analysis of n = 24 selected eggshell protein and PFP bands (Methods). Sampling of both biomineralized proteins (in situ analysis) from hard-shelled eggs and extracted, non-biomineralized membranes from soft and decalcified hard-shelled (Caiman, Alligator, Emys, Mesoclemmys, Phrynops and Gallus) eggs avoids phylogenetic attraction of the included fossil samples, and thereby allows eggshell clustering on the basis of the protein and PFP biomineralization signal. Two separate clusters of biomineralized and non-biomineralized eggshell proteins/PFPs are recovered. Pink nodes illustrate biomineralized egg proteins/PFPs, and blue nodes represent non-biomineralized eggshell proteins/PFPs. The egg icons illustrate whether samples represent originally hard or soft eggshell. One spectrum only was used for Mussaurus, as there is not much compositional variation across the eggshell (Fig. 1e), whereas all three eggshell spectra were sampled for Protoceratops, owing to the differences in composition across the egg section (Fig. 1d). Hard-shelled Alligator and turtle eggshells were excluded from this biomineralization analysis, as they do not produce any substantial organic signal with the spectroscopy protocol used (Supplementary Information and ref. 44). Both Protoceratops and Mussaurus eggshells are nested within the cluster of originally non-biomineralized eggshell proteins/PFPs.
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Norell, M.A., Wiemann, J., Fabbri, M. et al. The first dinosaur egg was soft. Nature 583, 406–410 (2020). https://doi.org/10.1038/s41586-020-2412-8