The first dinosaur egg was soft


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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Fig. 1: Photographs, histology and Raman spectroscopy of Protoceratops and Mussaurus soft eggshells.
Fig. 2: Biomineralization and evolution of hard- and soft-shelled eggs.

Data availability

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.


  1. 1.

    Erickson, G. M., Zelenitsky, D. K., Kay, D. I. & Norell, M. A. Dinosaur incubation periods directly determined from growth-line counts in embryonic teeth show reptilian-grade development. Proc. Natl Acad. Sci. USA 114, 540–545 (2017).

    CAS  Article  Google Scholar 

  2. 2.

    Romer, A. S. Origin of the amniote egg. Sci. Monthly 85, 57–63 (1957).

    Google Scholar 

  3. 3.

    Sander, P. M. Reproduction in early amniotes. Science 337, 806–808 (2012).

    ADS  CAS  Article  Google Scholar 

  4. 4.

    Hadek, R. The structure of the mammalian egg. Int. Rev. Cytol. 18, 29–71 (1965).

    CAS  Article  Google Scholar 

  5. 5.

    Hou, L. H., Li, P. P., Ksepka, D. T., Gao, K. Q. & Norell, M. A. Implications of flexible-shelled eggs in a Cretaceous choristoderan reptile. Proc. R. Soc. B 277, 1235–1239 (2010).

    Article  Google Scholar 

  6. 6.

    Packard, M. J., Packard, G. C. & Boardman, T. J. Structure of eggshells and water relations of reptilian eggs. Herpetologica 38, 136–155 (1982).

    Google Scholar 

  7. 7.

    Schleich, H. H. & Kästle, W. Reptile Egg-Shells (Gustav Fischer, 1988).

  8. 8.

    Palmer, B. D. & Guillette, L. J. Jr. Alligators provide evidence for the evolution of an archosaurian mode of oviparity. Biol. Reprod. 46, 39–47 (1992).

    CAS  Article  Google Scholar 

  9. 9.

    Coombs, W. P. Modern analogs for dinosaur nesting and parental behavior. Geol. Soc. Am. 238, 21–54 (1989).

    Google Scholar 

  10. 10.

    Varricchio, D. J., Jackson, F. & Trueman, C. N. A nesting trace with eggs for the Cretaceous theropod dinosaur Troodon formosus. J. Vertebr. Paleontol. 19, 91–100 (1999).

    Article  Google Scholar 

  11. 11.

    Mikhailov, K. E. Fossil and Recent Eggshell in Amniotic Vertebrates: Fine Structure, Comparative Morphology and Classification (Special Papers in Palaeontology No. 56) (1997).

  12. 12.

    Isles, T. E. The socio-sexual behaviour of extant archosaurs: implications for understanding dinosaur behaviour. Hist. Biol. 21, 139–214 (2009).

    Article  Google Scholar 

  13. 13.

    Piñeiro, G., Ferigolo, J., Meneghel, M. & Laurin, M. The oldest known amniotic embryos suggest viviparity in mesosaurs. Hist. Biol. 24, 620–630 (2012).

    Article  Google Scholar 

  14. 14.

    Ji, Q. et al. Pterosaur egg with a leathery shell. Nature 432, 572 (2004).

    ADS  CAS  Article  Google Scholar 

  15. 15.

    Wang, X. et al. Egg accumulation with 3D embryos provides insight into the life history of a pterosaur. Science 358, 1197–1201 (2017).

    ADS  CAS  Article  Google Scholar 

  16. 16.

    Chiappe, L. M., Codorniú, L., Grellet-Tinner, G. & Rivarola, D. Argentinian unhatched pterosaur fossil. Nature 432, 571–572 (2004).

    ADS  CAS  Article  Google Scholar 

  17. 17.

    Lü, J. et al. An egg-adult association, gender, and reproduction in pterosaurs. Science 331, 321–324 (2011).

    ADS  Article  Google Scholar 

  18. 18.

    Unwin, D. M. & Deeming, D. C. Pterosaur eggshell structure and its implications for pterosaur reproductive biology. Zitteliana 28, 199–207 (2008).

    Google Scholar 

  19. 19.

    Wiemann, J., Yang, T. R. & Norell, M. A. Dinosaur egg colour had a single evolutionary origin. Nature 563, 555–558 (2018).

    ADS  CAS  Article  Google Scholar 

  20. 20.

    Yang, T. R., Chen, Y. H., Wiemann, J., Spiering, B. & Sander, P. M. Fossil eggshell cuticle elucidates dinosaur nesting ecology. PeerJ 6, e5144 (2018).

    Article  Google Scholar 

  21. 21.

    Araújo, R. et al. Filling the gaps of dinosaur eggshell phylogeny: Late Jurassic Theropod clutch with embryos from Portugal. Sci. Rep. 3, 1924 (2013).

    Article  Google Scholar 

  22. 22.

    Chiappe, L. M. et al. Sauropod dinosaur embryos from the Late Cretaceous of Patagonia. Nature 396, 258–261 (1998).

    ADS  CAS  Article  Google Scholar 

  23. 23.

    Varricchio, D. J. & Jackson, F. D. Reproduction in Mesozoic birds and evolution of the modern avian reproductive mode. Auk 133, 654–684 (2016).

    Article  Google Scholar 

  24. 24.

    Grellet-Tinner, G., Chiappe, L. M., Norell, M. & Bottjer, D. Dinosaur eggs and nesting behaviors: a paleobiological investigation. Palaeogeogr. Palaeoclimatol. Palaeoecol. 232, 294–321 (2006).

    Article  Google Scholar 

  25. 25.

    Horner, J. R. Evidence of colonial nesting and ‘site fidelity’among ornithischian dinosaurs. Nature 297, 675–676 (1982).

    ADS  Article  Google Scholar 

  26. 26.

    Carpenter, K. Eggs, Nests, and Dinosaur Babies: A Look At Dinosaur Reproduction (Indiana Univ. Press, 1999).

  27. 27.

    Norell, M. A. et al. A theropod dinosaur embryo and the affinities of the flaming cliffs dinosaur eggs. Science 266, 779–782 (1994).

    ADS  CAS  Article  Google Scholar 

  28. 28.

    Norell, M. A., Clark, J. M., Chiappe, L. M. & Dashzeveg, D. A nesting dinosaur. Nature 378, 774–776 (1995); erratum 379, 186 (1996).

    ADS  CAS  Article  Google Scholar 

  29. 29.

    Mikhailov, K. E. Eggshell structure, parataxonomy and phylogenetic analysis: some notes on articles published from 2002 to 2011. Hist. Biol. 26, 144–154 (2014).

    Article  Google Scholar 

  30. 30.

    Kielan-Jaworowska, Z. & Dashzeveg, D. New Late Cretaceous mammal locality in Mongolia and a description of a new multituberculate. Acta Palaeontol. Pol. 23, 115–130 (1978).

    Google Scholar 

  31. 31.

    Zelenitsky, D. K. & Therrien, F. Phylogenetic analysis of reproductive traits of maniraptoran theropods and its implications for egg parataxonomy. Palaeont. 51, 807–816 (2008).

    Article  Google Scholar 

  32. 32.

    Zelenitsky, D. K. & Modesto, S. P. Re-evaluation of the eggshell structure of eggs containing dinosaur embryos from the Lower Jurassic of South Africa. S. Afr. J. Sci. 98, 407–408 (2002).

    Google Scholar 

  33. 33.

    Reisz, R. R., Scott, D., Sues, H. D., Evans, D. C. & Raath, M. A. Embryos of an early Jurassic prosauropod dinosaur and their evolutionary significance. Science 309, 761–764 (2005).

    ADS  CAS  Article  Google Scholar 

  34. 34.

    Stein, K. et al. Structure and evolutionary implications of the earliest (Sinemurian, Early Jurassic) dinosaur eggs and eggshells. Sci. Rep. 9, 4424 (2019).

    ADS  Article  Google Scholar 

  35. 35.

    Reisz, R. R. et al. Embryology of Early Jurassic dinosaur from China with evidence of preserved organic remains. Nature 496, 210–214 (2013).

    ADS  CAS  Article  Google Scholar 

  36. 36.

    Wilson, J. A., Mohabey, D. M., Peters, S. E. & Head, J. J. Predation upon hatchling dinosaurs by a new snake from the late Cretaceous of India. PLoS Biol. 8, e1000322 (2010).

    Article  Google Scholar 

  37. 37.

    Mikhailov, K. E. Classification of fossil eggshells of amniotic vertebrates. Acta Pal. Pol. 36, 193–238 (1991).

    Google Scholar 

  38. 38.

    Varricchio, D. J., Jackson, F., Borkowski, J. J. & Horner, J. R. Nest and egg clutches of the dinosaur Troodon formosus and the evolution of avian reproductive traits. Nature 385, 247–250 (1997).

    ADS  CAS  Article  Google Scholar 

  39. 39.

    Kundrát, M., Cruickshank, A. R. I., Manning, T. W. & Nudds, J. Embryos of therizinosauroid theropods from the Upper Cretaceous of China: diagnosis and analysis of ossification patterns. Acta Zool. 89, 231–251 (2008).

    Article  Google Scholar 

  40. 40.

    Wiemann, J. et al. Fossilization transforms vertebrate hard tissue proteins into N-heterocyclic polymers. Nat. Commun. 9, 4741 (2018).

    ADS  Article  Google Scholar 

  41. 41.

    Wiemann, J., Crawford, J. M. & Briggs, D. E. G. Phylogenetic and physiological signals in metazoan fossil biomolecules. Sci. Adv. (in the press).

  42. 42.

    Bonaparte, J. F. & Martin, V. El hallazgo del primer nido de dinosaurios Triásicos, (Saurischia, Prosauropoda), Triásico superior de Patagonia, Argentina. Ameghiniana 16, 173–182 (1979).

    Google Scholar 

  43. 43.

    Pol, D. & Powell, J. E. Skull anatomy of Mussaurus patagonicus (Dinosauria: Sauropodomorpha) from the late Triassic of Patagonia. Hist. Biol. 19, 125–144 (2007).

    Article  Google Scholar 

  44. 44.

    Wiemann, J., Yang, T. R. & Norell, M. A. Reply to: Egg pigmentation probably has an Archosaurian origin. Nature 570, E46–E50 (2019).

    CAS  Article  Google Scholar 

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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.

Author information




M.A.N. designed the project. M.A.N., J.W. and M.F. conceived and designed the experiments. M.A.N., C.Y., C.A.M., D.J.V., D.P. and D.K.Z. contributed material and/or material information. J.W., D.J.V. and A.M.-N. prepared thin sections. J.W. designed the Raman protocol, performed Raman spectroscopy, developed the proxies and analysed the data. M.F. described the clutches, coded eggshells and performed the ancestral-state reconstruction. M.A.N., J.W. and M.F. wrote the manuscript with input from all authors.

Corresponding authors

Correspondence to Mark A. Norell or Jasmina Wiemann or Matteo Fabbri.

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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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Supplementary Information

This file contains Supplementary Note 1, including Supplementary Tables 1-4, Supplementary Figures 1-8 and Supplementary References.

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Norell, M.A., Wiemann, J., Fabbri, M. et al. The first dinosaur egg was soft. Nature 583, 406–410 (2020).

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