To date, only a small handful of pterosaur eggs with a well-preserved 3-D structure and embryo inside have been found and analyzed -- three eggs from Argentina and five from China. This sparse sample size was dramatically increased upon the discovery of 215 eggs of the pterosaurs species Hamipterus tianshanensis from a Lower Cretaceous site in China.
Xiaolin Wang et al. used computed tomography scanning to peer inside the eggs, 16 of which contain embryonic remains of varying intactness. The most complete embryo contains a partial wing and cranial bones, including a complete lower jaw. The samples of thigh bones that remain intact are well-developed, suggesting that the species benefited from functional hind legs shortly after hatching.
However, the structure supporting the pectoral muscle appears to be underdeveloped during the embryonic stage, suggesting that newborns were likely not able to fly. Therefore, the authors propose that newborns likely needed some parental care. Based on growth marks, the team estimates one of the individuals to be at least 2 years old and still growing at the time of its death, supporting the growing body of evidence that pterosaurs had long incubation periods.
Lastly, the fact that a single collection of embryos exhibits a range of developmental stages hints that pterosaurs participated in colonial nesting behavior, the authors say. Denis Deeming discusses these findings in a related Perspective.
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