Abstract Among all of the dinosaurs, the Late Cretaceous oviraptorosaurs, in addition to troodontid dinosaurs, boast the best fossil records ranging from numerous eggs and embryos, seven clutch–adult associations, highly organized egg clutches, two gravid adults and possible colonial nesting grounds, to plenty of species described from skeletons from East Asia. These well-preserved fossils provided insights into the reproductive biology, ecology, behaviour and developmental biology of oviraptorid dinosaurs, contributing to several groundbreaking palaeontological discoveries in the last decade. Some of them received further support, while the others remain controversial, indicating that a comprehensive, rigorous re-examination using all available evidence is required. In a recent review, oviraptorosaurs were considered as an intermediate stage in moving towards the modern avian reproduction, whose reproductive biology can be inferred based on the avian model. With the new insights generated from the shell histology of several embryo-containing eggs and evidence from previous studies, this study offers a synthetic view of the reproductive biology of the Late Cretaceous oviraptorosaurs, illuminating the peculiar reproductive biology of oviraptorosaurs. In conclusion, oviraptorosaurs had an essentially unique reproductive biology lacking modern analogies. Thus, simply applying either avian or reptilian models to infer the biology of extinct animals appears inappropriate in the case of oviraptorosaurs.

Abstract: 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.