A giant soft-shelled egg from the Late Cretaceous of Antarctica

Abstract

Egg size and structure reflect important constraints on the reproductive and life-history characteristics of vertebrates1. More than two-thirds of all extant amniotes lay eggs2. During the Mesozoic era (around 250 million to 65 million years ago), body sizes reached extremes; nevertheless, the largest known egg belongs to the only recently extinct elephant bird3, which was roughly 66 million years younger than the last nonavian dinosaurs and giant marine reptiles. Here we report a new type of egg discovered in nearshore marine deposits from the Late Cretaceous period (roughly 68 million years ago) of Antarctica. It exceeds all nonavian dinosaur eggs in volume and differs from them in structure. Although the elephant bird egg is slightly larger, its eggshell is roughly five times thicker and shows a substantial prismatic layer and complex pore structure4. By contrast, the new fossil, visibly collapsed and folded, presents a thin eggshell with a layered structure that lacks a prismatic layer and distinct pores, and is similar to that of most extant lizards and snakes (Lepidosauria)5. The identity of the animal that laid the egg is unknown, but these preserved morphologies are consistent with the skeletal remains of mosasaurs (large marine lepidosaurs) found nearby. They are not consistent with described morphologies of dinosaur eggs of a similar size class. Phylogenetic analyses of traits for 259 lepidosaur species plus outgroups suggest that the egg belonged to an individual that was at least 7 metres long, hypothesized to be a giant marine reptile, all clades of which have previously been proposed to show live birth6. Such a large egg with a relatively thin eggshell may reflect derived constraints associated with body shape, reproductive investment linked with gigantism, and lepidosaurian viviparity, in which a ‘vestigial’ egg is laid and hatches immediately7.

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Fig. 1: The holotype specimen of A. bradyi.
Fig. 2: Scaling relationships of egg traits in amniotes and lepidosaurs, with A. bradyi pictured for comparison.
Fig. 3: Evolution of the relative thickness of the calcareous layer in amniotes.

Data availability

CT scan data are available from Open Science Framework at https://osf.io/gx8fq/. All other data generated or analysed here are included in the Supplementary Information (Supplementary Tables 13). The code used for all statistical analyses is available on GitHub at https://github.com/LucasLegendre/Antarcticoolithus_project.

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Acknowledgements

We thank J. Singh, T. Etzel, G. Rojas, V. Lynch and N. Miller for technical assistance with thin-sectioning, scanning electron microscopy, CT scanning, X-ray diffraction and inductively coupled plasma mass spectrometry, respectively (Supplementary Methods); L. Scheinberg (California Academy of Sciences), D. Kizirian (American Museum of Natural History), T. LaDuc and K. Bader (University of Texas at Austin) for specimen access; C. Gutstein and R. F. Jimenez for assistance with fossil collection and sampling; and M. Cloos, R. Martindale, C. Kerans, C. Torres, M. Leppe, N. Crouch and S. Hood for discussions. Select taxon silhouettes in Figs. 2, 3 were modified from files under Public Domain licence from phylopic.org. This work was supported by a grant to the University of Texas at Austin from the Howard Hughes Medical Institute through the Science Education Program (GT10473 to J.A.C. and L.J.L.) and by an ANID-PIA Anillo grant (ACT172099 to D.R.-R., R.A.O. and A.O.V.).

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Contributions

J.A.C., D.R.-R. and L.J.L. conceived the study; D.R.-R. and R.A.O. collected the specimen; D.R.-R., R.A.O. and A.O.V. prepared the specimen and contributed to its description; L.J.L. collected the data and performed experiments and statistical analyses; S.N.D. sampled the specimen and contributed to its description; G.M.M. contributed to the description and line drawings of the specimen; L.J.L. and J.A.C. wrote the paper; all authors discussed and contributed to the final draft of the manuscript.

Corresponding authors

Correspondence to Lucas J. Legendre or Julia A. Clarke.

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The authors declare no competing interests.

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Peer review information Nature thanks Johan Lindgren, Jasmina Wiemann 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 The holotype specimen of A. bradyi in bottom view.

This side of the specimen, deeply flattened, is referred to as the inferior side in the Supplementary Discussion. a, Photograph of SGO.PV 25.400, showing the sampling locations of eggshell and sediment matrix fragments used for experiments (Supplementary Methods and Extended Data Fig. 4). b, Line drawing of a, showing the relative arrangement of the eggshell (s) and the infilling sediment matrix (m) at the surface of the specimen. The matrix is visible through large creases (yellow arrows), corresponding to zones of local collapsing and infolding of the eggshell.

Extended Data Fig. 2 Scanning electron microscopy images of the eggshell of A. bradyi.

a, The outer surface of the membrana testacea (mt) shows intersecting fibres (blue arrows). bd, The smooth inner surface of the eggshell, identified as the boundary layer (bl in b), can be locally broken off, showing the structures consistent with remnants of protein fibrils seen in the membrana testacea of extant taxa (arrows in b, red triangles in d), and revealing an underlying, dense, irregular globular pattern, identified as part of the membrana testacea (b, c). Pyrite crystals, present in most samples within the membrana testacea, show a framboidal (arrows in c) or octahedral (blue arrows in d) structure. Eggshell sampling locations: 2 (c) and 4 (a, b, d) in Extended Data Fig. 1. Further detail on the eggshell structure and methods is found in the Supplementary Methods, Supplementary Discussion, Fig. 1 and Extended Data Figs. 1, 3, 4.

Extended Data Fig. 3 Microstructure of A. bradyi and several species of extant lepidosaurs.

ac, Observed under an optical microscope; dh, observed by scanning electron microscopy (SEM). mt, membrana testacea; bl, boundary layer; cl, calcareous layer; rm, rock matrix. ac, Histological thin sections of the eggshell of Antarcticoolithus (a), a common kingsnake (Lampropeltis getula, b) and a gold tegu (Tupinambis teguixin, c), showing their similarities in microstructure. Dashed lines in a delimit the inner and outer surface of the eggshell, as well as the less-obvious boundary between layers in the shell. Arrows in c indicate layering between protein fibrils, which is slightly less conspicuous than in ab. d, Surface of the sediment matrix of Antarcticoolithus, showing a granular texture distinct from that of the eggshell (see inset at top right corner). e, f, Inner surface of the eggshell of Antarcticoolithus, locally showing fibrous structures (arrows in e) that might correspond to protein fibrils, and a dense globular pattern (f). g, h, Inner surface of the eggshell of two extant lepidosaurs: an ornate tree lizard (Urosaurus ornatus, g) and a common side-blotched lizard (Uta stansburiana, h). The boundary layer is partially missing, revealing the protein fibrils (arrowheads in h) in the membrana testacea. Calcified globules (arrows in g, h) are also present among the fibrillar structures, with some being isolated and deposited at the surface of the boundary layer (probably during mounting before SEM sampling). Sampling locations for Antarcticoolithus (from Extended Data Fig. 1): matrix sample, 3 (d); eggshell samples, 2 (e, f) and 3 (a). See also Fig. 1, Extended Data Figs. 1, 2, Supplementary Methods and Supplementary Discussion.

Extended Data Fig. 4 Spectra for the eggshell and matrix of A. bradyi.

a, c, e, Spectra obtained using energy-dispersive X-ray spectroscopy; cps, counts per second. b, d, f, Corresponding sampling locations on the specimen, indicated by blue circles. a, b, Shell sample 3 (inner surface), showing high amounts of calcium (Ca), oxygen (O) and phosphorus (P), characteristic of apatite. c, d, Shell sample 4 (inner surface), showing apatite as well as iron (Fe) and sulfur (S), indicating the presence of pyrite crystals (labelled Py in d). e, f, Matrix sample 3 (outer surface), showing a combination of apatite and silicates, the latter containing silicon (Si) and oxygen. Traces of potassium (K), aluminium (Al) and sodium (Na) can also be detected. Scale bar, 500 μm. See Extended Data Fig. 1 for sampling locations.

Extended Data Fig. 5 Distribution of eggshell thickness corrected for egg length for extant lepidosaurs in our sample and for A. bradyi.

The extant lepidosaurs are listed in Supplementary Methods, dataset 1. Antarcticoolithus is represented by a black ovoid; its eggshell thickness/egg length (ET/L) ratio is intermediate between the averages obtained for oviparous and viviparous lepidosaurs. A phylogenetic one-way analysis of variance performed on oviparous and viviparous taxa in our sample identified a significant difference in ET/L ratio between the two groups (F = 6.076968; P = 0.007) (Supplementary Methods). The small sample size of viviparous species is due to the scarcity of studies on eggshell thickness in viviparous lepidosaurs. Percentiles of box plots are available in Extended Data Table 1. See also Supplementary Discussion and Supplementary Table 1.

Extended Data Fig. 6 Scaling of egg volume with body mass for our sample of extant lepidosaurs.

See dataset 1 in the Supplementary Methods (n = 241); each clade is labelled. The regression line corresponds to a PGLS fit (r2 = 0.6851; P < 2.2 × 10−16). For Antarcticoolithus, estimates of body mass (BM) from both PEMs and PGLS are provided. The PEM estimate is very low for a giant marine reptile of more than 10 m in length (see text), which is probably due to the lack of taxa within the 106 g order of magnitude usually inferred for large marine reptiles. Conversely, the PGLS estimate is surprisingly high, which might be linked to the large gap in egg-volume values between Antarcticoolithus and extant lepidosaurs, as suggested by the similar results we obtained with body-length estimations (Fig. 2, Supplementary Methods and Supplementary Table 1).

Extended Data Fig. 7 Element mapping of the eggshell of A. bradyi and extant lepidosaurs, obtained using inductively coupled plasma mass spectrometry.

a, Counts per second (cps) for major elements in the eggshell of A. bradyi. bl, boundary layer; cl, calcareous layer; rm, rock matrix. Two and a half cycles are shown, each cycle corresponding to laser ablation and measurement of elemental relative density through the whole eggshell from the inside to the outside, through an outer layer of sediment, and through the surrounding epoxy of the thin section. Calcium and phosphorus are present at high concentrations in the eggshell, and silicon and aluminium are more prominent in the sediment; uranium, an element present in low amounts in both shell and sediment, is shown for comparison. bd, Eggshell thin section of A. bradyi (b), with corresponding element map (c) and scale (d) showing relative amounts of calcium (43Ca) in the eggshell. Calcium is abundant in the eggshell, but absent in the outer layer of sediment (rm); ep, epoxy resin. eg, Eggshell thin section of a gold tegu (T. teguixin; e), with corresponding element map (f) and scale (g) showing relative amounts of calcium (43Ca) in the eggshell. Calcium is mostly present in the calcareous layer, but also featured in the lower portion of the membrana testacea, probably through calcite globules, as documented in many lepidosaur species (see, for example, ref. 5). h, i, Relative density maps for calcium (43Ca) in A. bradyi (h) and T. teguixin (i), corresponding to side views of the maps shown in c and f, respectively. Peaks indicate a high amount of calcium, and flat surfaces and depressions indicate a lower amount. In both maps, the calcareous layer tends to show the highest amount, suggesting its preservation in A. bradyi and the general structural similarity between the two thin sections. Both maps present a single, very high peak in their centre, which could also be identified in all element maps for each section, and most probably corresponds to a contamination artefact. Colour gradients for h and i are the same as in d and g, respectively. See also Supplementary Methods and Supplementary Discussion.

Extended Data Fig. 8 Powder X-ray diffraction spectra of the eggshell of A. bradyi.

The two best matches for these spectra are francolite (a) and fluorapatite (b). Lin, linear intensity. See the Supplementary Methods for a detailed description of the protocol, and Supplementary Discussion for an interpretation of the results.

Extended Data Table 1 Sample sizes and percentiles of box plots

Supplementary information

Supplementary Information

This file includes: Supplementary Methods; Supplementary Discussion; Supplementary Tables 1–3; and associated references.

Reporting Summary

Supplementary Video 1 | CT scan of SGO.PV 25.400, showing the general inner and outer structure of the specimen

The three main sides of the specimen are shown and labelled in Fig. 1 (see also Main Text and Supplementary Discussion). The small ammonite mentioned in the Supplementary Discussion can be seen in the cut-through animation (00:27 to 00:29).

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Legendre, L.J., Rubilar-Rogers, D., Musser, G.M. et al. A giant soft-shelled egg from the Late Cretaceous of Antarctica. Nature (2020). https://doi.org/10.1038/s41586-020-2377-7

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