Enigmatic 29cm Antarctic Late Cretaceous soft-shell egg

Legendre et al. (6 co-authors) 2020
report on an enigmatic egg they cannot identify. They nicknamed it “The Thing”. Without knowing anything else about it, my first guess, based on “giant” and “leathery or soft” is a giant azhdarchid (Fig. 1; first imagined in 2012). Let’s see if any clues guide us toward or away from that initial guess.

Quetzalcoatlus embryo and egg.

Figure 1. Hypothetical Quetzalcoatlus embryo and egg imagined in 2012. Compare to figure 2. The elongated shape and soft, thin shell were needed to encompass the elongated beak, neck and metacarpals. The long axis is ~35cm. See figure 4 for images of the mother.

Excerpts from the abstract
“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.”

As in the azhdarchid hypothesis (Fig. 1).

“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).

As in the azhdarchid hypothesis (Fig. 1; Peters 2007).

“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.”

Is taxon exclusion a factor here?

“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 birth.”

Perhaps taxon exclusion is a factor here. I will need to see the list of 259 lepidosaur species to see if it includes any pterosaurs.

“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 immediately.”

As in the azhdarchid hypothesis (Fig. 1).

Now let’s look at the supplemental data
(writing this in real time as I do the research).

Specimen name: Antarcticoolithus bradyi.
The long axis is 29cm (Fig. 2). The short axis is estimated at 15cm. Compare that to the imagined 2012 azhdarchid egg (Fig. 1) with a long axis of 35cm. Just curl the embryo a bit and the guess = the discovery. The wider, but shorter Antarcticoolithus egg gives the developing azhdarchid? embryo a bit more room to move about. By the look of the egg, it appears to have a slit in it, as if it hatched already.

Figure 2. Antarcticoolithus bradyi from Legendre et al 2020.

Figure 2. Antarcticoolithus bradyi from Legendre et al 2020.

Figure 2b. Is that a slit in the egg shell? I am still awaiting the text of the study.

Figure 2b. Antarcticoolithus bradyi from Legendre et al 2020. Side two. Is that a slit in the egg shell from arrow to arrow? I am still awaiting the text of the study. (turns out to be a crack in the rock)

From the Supplemental Data:
“The first known remains of Late Cretaceous Antarctic pterosaurs were recently described (Kellner et al. 2019) however, the largest known pterosaur eggs with known taxonomic affinities (Pterodaustro guiñazui, egg length: ~60 mm; Fig. 3) belonged to a species with a ~2.5 m adult wingspan102. Hence, if the 290 mm-long Antarcticoolithus was a pterosaur egg, it would have been laid by a species with a wingspan of over 12 m, which is much larger than the maximum wingspan of 4–5 m described in known Antarctic pterosaurs.”

The known Antarctic pterosaurs include bits from one or two specimens (Fig. 6).

Figure 2. Original interpretations (2 frames black/white) vs. new interpretations (color).

Figure 3. Original interpretations (2 frames black/white) vs. new interpretations (color).

Now let’s check out the mother’s pelvis
(Fig. 4). Looks like 10cm in the short axis was about the maximum, unless the ischia were free to expand during egg-laying. It is also possible that the pliability of the egg itself might have enabled Antarcticoolithus to pass through a hypothetical pelvis of a giant Q. northropi, if similar in proportion to the small Q. species, which is no sure thing in these flightless giants., wingspan ~11m.

Quetzalcoatlus eggs

Figure 4. Quetzalcoatlus northropi (left) nd Q. sp. (right) to the same scale alongside hypothetical eggs and hatchlings. The egg-layer of Antarcticoolithus, if azhdarchid pterosaurian, might have had a larger cloacal opening than shown here.

Finally, let’s consider those Antarctic pterosaurs. What were they?
Hard to say because they are such small parts of the pterosaur wing (Fig. 5).

Figure 6. Antarctic pterosaur bones from Kellner et al. 2019. The elements appear to be too gracile to fit the hypothetical outline provided.

Figure 6. Antarctic pterosaur bones from Kellner et al. 2019. The elements appear to be too gracile to fit the hypothetical outline provided.

Conclusion:
Don’t overlook the possibility of a giant azhdarchid egg layer for Antarcticoolithus.

Legendre et al. report,
“Interestingly, the two specimens of pterosaurs in our sample fall within the range of soft-shelled lepidosaur eggs, despite one of them showing a prismatic calcareous layer.”

We’ve known since Peters 2007 that pterosaurs are lepidosaurs.

“Pterosaur eggs have been repeatedly described as soft-shelled due to the thin and pliable aspect of their eggshell. The first detailed description of a pterosaur egg microstructure, however, showed a conspicuous prismatic layer. Another specimen was reported to lack a calcareous layer, and be most similar in structure to a lepidosaur eggshell, but no description of its microstructure using microscopy techniques was provided, preventing a clear identification of a soft-shelled structure. Since these first descriptions, more specimens of exceptionally preserved eggs have been described for a handful of pterosaur species – some hard-shelled (Grellet-Tinner et al. 2014) some soft-shelled.”

Pterodaustro eggs (Fig. 3) can hardly be called ‘hard-shelled’ contra Grellet-Tinner et al. 2014. Eggs with deep infolds, like those of Antarcticoolithus are not filled to bursting with full-term embryos, as is formerly empty, sediment-filled egg shown in figure 2.

“There is currently no consensus on whether such a soft eggshell was widespread among pterosaurs, nor on the relationship of the structure of that soft eggshell to that of lepidosaur eggshells.”

No consensus, for reasons listed earlier, but Peters 2007 was the first worker to nest pterosaurs within lepidosauria simply by adding taxa.

“More studies on pterosaur eggshells are thus necessary to assess their potential microstructural similarity with extant soft-shelled eggs. While the possibility of Antarcticoolithus being a fossilized pterosaur egg cannot definitely be ruled out, it should be noted that no remains of giant pterosaurs likely to have laid such a large egg are known from Antarctic deposits, contrary to giant marine reptiles.”

Leave the options open. Always a good idea. This egg may belong to something else entirely, like a mosasaur (see NPR online below). As more information arrives, I will add data to this blogpost.


References
Grellet-Tinner, G. et al. 2014. The first pterosaur 3-D egg: Implications for Pterodaustro guinazui nesting strategies, an Albian filter feeder pterosaur from central Argentina. Geoscience Frontiers 5, 759–765.
Kellner AWA et al. 2019. Pterodactyloid pterosaur bones from Cretaceous deposits of the Antarctic Peninsula. Anais da Academia Brasileira de Ciências91,e20191300.
Legendre LJ, et al. (6 co-authors) 2020.
A giant soft-shelled egg from the Late Cretaceous of Antarctica. Nature Jun 17 https://doi.org/10.1038/s41586-020-2377-7
Peters D 2007.
The origin and radiation of the Pterosauria. In D. Hone ed. Flugsaurier. The Wellnhofer pterosaur meeting, 2007, Munich, Germany. p. 27.

At ResearchGate.net:
A_new_lepidosaur_clade_the_Tritosauria

From NPR with mosasaur baby illustration
https://www.npr.org/2020/06/17/877679868/scientists-find-the-biggest-soft-shelled-egg-ever-nicknamed-the-thing

https://static-content.springer.com/esm/art%3A10.1038%2Fs41586-020-2377-7/MediaObjects/41586_2020_2377_MOESM3_ESM.mov

https://pterosaurheresies.wordpress.com/2012/02/21/an-egg-for-quetzalcoatlus/

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