Birds are the only living amniotes with coloured eggs1,2,3,4, which have long been considered to be an avian innovation1,3. A recent study has demonstrated the presence of both red-brown protoporphyrin IX and blue-green biliverdin5—the pigments responsible for all the variation in avian egg colour—in fossilized eggshell of a nonavian dinosaur6. This raises the fundamental question of whether modern birds inherited egg colour from their nonavian dinosaur ancestors, or whether egg colour evolved independently multiple times. Here we present a phylogenetic assessment of egg colour in nonavian dinosaurs. We applied high-resolution Raman microspectroscopy to eggshells that represent all of the major clades of dinosaurs, and found that egg colour pigments were preserved in all eumaniraptorans: egg colour had a single evolutionary origin in nonavian theropod dinosaurs. The absence of colour in ornithischian and sauropod eggs represents a true signal rather than a taphonomic artefact. Pigment surface maps revealed that nonavian eumaniraptoran eggs were spotted and speckled, and colour pattern diversity in these eggs approaches that in extant birds, which indicates that reproductive behaviours in nonavian dinosaurs were far more complex than previously known3. Depth profiles demonstrated identical mechanisms of pigment deposition in nonavian and avian dinosaur eggs. Birds were not the first amniotes to produce coloured eggs: as with many other characteristics7,8 this is an attribute that evolved deep within the dinosaur tree and long before the spectacular radiation of modern birds.
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The authors declare that all Raman data supporting the findings of this study are available within the paper (pigment maps and depth profiles), and its Supplementary Information and Source Data.
Publisher’s note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
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We thank D. E. G. Briggs for advice and assistance with the manuscript. M. Fabbri, N. Mongiardino Koch, J. Gauthier, K. Zykowski and R. Prum made helpful suggestions. Y.-N. Cheng, Y.-F. Shiao, X. Wu, T. Töpfer, P. M. Sander and K. Zykowski provided eggshell specimens. This research was supported by the Steven Cohen Award of the Society of Vertebrate Paleontology (J.W.), the Macaulay Family Endowment (M.A.N.) and the Division of Paleontology, American Museum of Natural History.
Nature thanks D. Zelenitsky and the other anonymous reviewer(s) for their contribution to the peer review of this work.
Extended data figures and tables
Extended Data Fig. 1 Spectral plots that validate the Raman bands indicative of biliverdin and protoporphyrin IX relative to PFPs, based on three representative samples.
Spectral plots are 200–1,600 cm−1 ± 2 cm−1, from 6 accumulations; spectra are baselined and normalized. Each spectrum was repeated three times independently, and yielded similar results. The lowermost spectral plot depicts blue spectra for biliverdin (from D. novaehollandiae eggshell), red spectra for protoporphyrin IX (from G. domesticus) and brown spectra for fossil proteinaceous soft tissue (from A. fragilis bone; see previous study26). The uppermost dark-brown spectra represent an in situ measurement for the H. huangi (NMNS CYN-2004-DINO-05) eggshell that is known to preserve both pigments (biliverdin and protoporphyrin IX). Blue asterisks label bands of biliverdin that differ from PFP; red asterisks label bands of protoporphyrin IX that are absent in fossil soft-tissue remains. Overall, 22 Raman bands are identified that can be generated only by original unaltered pigments, and not by PFPs that are simultaneously present.
Extended Data Fig. 2 Spectral close-ups of the pigment fingerprint region for nineteen archosaur eggshells.
Spectra are at 1,500–1,700 cm−1 ± 2 cm−1 from 6 accumulations, and are baselined and normalized. a, Unpigmented eggshell samples (Fig. 1). b, Eggshell samples containing only protoporphyrin IX. Red bands label the set of bands that is diagnostic for protoporphyrin IX eggshell pigment. c, Biliverdin-rich eggshell samples. Blue bands label the set of bands that is diagnostic for biliverdin eggshell pigment. Note the characteristic spectral differences between unpigmented and pigmented eggshell samples. 1, A. mississipiensis; 2, M. peeblesorum (YPM VP PU 023464); 3, South American saltasaurid; 4, French titanosaurid; 5, G. domesticus; 6, Chinese troodontid; 7, Mongolian microtroodontid (MAE 14-40); 8, P. rothschildi; 9, Mongolian enantiornithine; 10, Mongolian troodontid (AMNH FARB 6631); 11, Mongolian troodontid (IGM 100/1003); 12, D. noveahollandiae; 13, R. americana; 14, D. antirrhopus; 15, Mongolian microtroodontid (IGM 100/1323); 16, H. huangi.
Extended Data Fig. 3 Eggshell net enrichment plots resulting from subtraction of the sediment spectral functions from their corresponding eggshell spectral functions.
Spectra are at 1,500–1,700 cm−1 ± 2 cm−1 from 6 accumulations, and are base-lined and normalized. Positive values indicate a net enrichment of fossil eggshells in organic compounds relative to their sediment surroundings, whereas negative values indicate a net depletion. Red bands are diagnostic for protoporphyrin IX, and blue bands are diagnostic for biliverdin. a, M. peeblesorum (YPM VP PU 023464). b, South American saltasaurid. c, French titanosaurid. d, H. huangi. e, Mongolian microtroodontid (IGM 100/1323). f, Mongolian microtroodontid (MAE 14-40). g, North American troodontid. h, Mongolian troodontid (AMNH FARB 6631). i, Mongolian troodontid (IGM 100/1003). j, Chinese troodontid. k, D. antirrhopus. l, Mongolian enantiornithine.
a, Whole spectra-based PCA of all modern (n = 4 biologically independent samples) and fossil (n = 14 biologically independent samples) eggshell and sediment samples (n = 12 independent samples). Sediment samples cluster together (green), and do not overlap the cluster of eggshell samples (grey-to-yellow). Eggshell biomolecules and sediment organics are distinct. Within the eggshell cluster, extant material is separated from fossil material. The proximity of unpigmented, protoporphyrin IX-bearing and biliverdin-rich extant eggshell material emphasizes that the taphonomic signal overprints the pigment signal at the level of a whole spectrum-based PCA. PC1 (73.116%) and PC2 (10.977%) are characterized by high support values and explain most of the variation in the eggshell and sediment sample set. b, PCA based on the pigment fingerprint region (1,500–1,650 cm−1 ± 2 cm−1) of protoporphyrin IX and biliverdin in fossil eggshell material (n = 12 biologically independent samples). The exclusion of fresh and ‘subfossil’ (P. rothschildi and ornithoid ratite) eggshell material and the extraction of the pigment fingerprint region enable a chemo-space separation based on the eggshell pigment signal. Pigmented fossil eggshells form a cluster (yellow) that is separated from the cluster (grey) of unpigmented fossil eggshells (Extended Data Fig. 5). The pigment fingerprint region includes—besides protoporphyrin IX and biliverdin—structurally similar PFPs as well as pigment fossilization products, which account for the distribution of samples within each cluster of fossil eggshells (pigmented and unpigmented). PC1 represents sample variation based on pigment type, concentration and PFP contents, and PC2 separates samples based on the presence or absence of eggshell pigments (Extended Data Fig. 5).Source data.
These loading plots are for the PCA shown in Extended Data Fig. 4. a, Loadings per wavelength and functional group for PC1 (57.118%). Negative loadings are associated with C–N functions, while positive loadings are associated with C–O functions. PC1 separates samples in the chemospace based on pigment type (protoporphyrin IX ratio of C–N to C=O) > biliverdin (ratio of C–N to C=O)), concentration, and PFP contents. b, Loadings per wavelength and functional group for PC2 (23.841%). Negative loadings are associated with prominent PFP bands, whereas positive loadings are associated with eggshell pigments. Thus, PC2 separates samples in the chemo-space on the basis of the presence or absence of eggshell pigments. c, Loadings per wavelength and functional group for PC3 (9.258%).
n = 12 independent samples. Measurement parameters are identical to those used to process eggshell samples, to guarantee comparability (300–2,000 cm−1 ± 2 cm−1, 6 accumulatations, 20 s of exposure, base-lined and normalized). Each sediment measurement was repeated three times independently and yielded similar results. The coloured dots label potential pigment-band positions (biliverdin, blue; protoporphyrin IX red). For comparison, a pigment-positive spectrum of H. huangi eggshell (1) is provided at the bottom, followed by sediments associated with M. peeblesorum (YPM VP PU 023464) (2); M. peeblesorum (YPM PU 22523) (3); South American saltasaurid (4); French titanosaurid (5); H. huangi (6); Mongolian microtroodontid (7); North American troodontid (8); Chinese troodontid (9); Mongolian troodontid (10); D. antirrhopus (11); Mongolian enantiornithine (12); and North American ornithoid ratite (13) eggshells. None of the sediments contains substantial amounts of eggshell pigments.
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Nature Communications (2019)