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Possible brooding of pterosaur parents

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Science  09 Mar 2018:
Vol. 359, Issue 6380, pp. 1111
DOI: 10.1126/science.aas9153

In his Perspective “How pterosaurs bred” (1 December 2017, p. 1124), D. C. Deeming discussed the remarkable fossils of eggs and developing embryos of the pterosaur reptile Hamipterus tianshanensis, discovered by X. Wang et al. (“Egg accumulation with 3D embryos provides insight into the life history of a pterosaur,” Report, 1 December 2017, p. 1197). Deeming asserts that the thin and flexible-shelled (“parchment-shelled”) ectohydric eggs of lizards (and presumably other reptiles with similar eggshell morphologies) must be buried and covered in a moist substrate to prevent desiccation and embryonic death. Moreover, he suggests that all such developing embryos must rely on ambient environmental sources, presumably the nest material, for heat. He concludes that these constraints preclude contact of the clutch with incubating pterosaur parents because their eggs share this flexible-shelled morphology. These assertions ignore evidence of brooding reptilian parents in a wide variety of extant lineages of lizards and snakes in which mothers coil around or completely cover their ectohydric eggs, using their own bodies as nests or partial nests (1, 2). Some relevant examples include various species of pythonid snakes—obligate brooders that coil around their unburied developing eggs, providing either maternal thermogenic heat, protection from ambient hydric extremes, or a combination of both brooding functions (25).

Nesting behaviors of pterosaur parents may have included brooding their eggs.

ILLUSTRATION: CHUANG ZHOU

Although the discoveries of Wang et al. and others (6) provide some suggestive evidence of posthatching care of neonatal pterosaurs, the conditions of these fossilized nests do not allow determination of the presence of brooding parents covering or partially covering their eggs. If pterosaur parents had the ability to use a body posture or nest construction that prevented them from crushing their eggs with their own weight, they conceivably could have adjusted brooding postures to regulate the amount of water lost or gained by the embryo. More speculatively, if brooding pterosaurs were endothermic (warm-blooded), a physiology supported by equivocal fossil evidence (7), they could have regulated the incubation temperatures of their developing embryos, further controlling growth, mortality, and perhaps hatchling sex ratios. A more accurate understanding of the nesting behaviors of pterosaurs awaits further discoveries of more complete fossil nests under conditions that allow more detailed interpretation of the positions of eggs and attending parents. Current fossil evidence based on eggshell morphology does not exclude the possibility that pterosaur parents brooded their eggs.

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