Our knowledge of the eggs and embryos of pterosaurs, the Mesozoic flying reptiles, is sparse. Until now, the recent discovery of an ornithocheirid embryo from 121-million-year-old rocks in China constituted the only reliable evidence of an unhatched pterosaur. Here we describe an embryonic fossil of a different pterosaur from the Early Cretaceous lacustrine deposits of Loma del Pterodaustro (the Lagarcito Formation, which is about 100 million years old) in central Argentina. This new fossil provides insight into the eggshell morphology, early growth and nesting environments of pterosaurs.

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We report on a diminutive pterosaur specimen (MIC-V246), from the Lower Cretaceous Lagarcito Formation, which has anatomical features and general proportions that agree with those of other juvenile specimens of the filter-feeding pterosaur Pterodaustro guinazui. MIC-V246 is nearly complete, with the majority of its bones in natural articulation. The specimen is preserved within a small oval surface inferred to demarcate the outline of an egg. It includes remains of the skull and mandible, some cervical, dorsal, sacral and caudal vertebrae, ribs, gastralia, both shoulder girdles, part of the ilium, and the forelimbs and hindlimbs. The skeleton is arranged in a position similar to that of avian embryos, with the wings folded sideways, the hindlimbs flexed, and the skull tucked beneath a wing. The bones are partially covered by an irregularly distributed substance differing in texture and colour from the bony elements, and identified as remnants of the eggshell. The diminutive size of the skeleton, along with its arrangement and morphology, support the identification of this specimen as one of the very few pterosaur embryos known worldwide. Its similarity in size to other Pterodaustro specimens interpreted as hatchlings suggests that MIC-V246 was near hatching when it died.

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