This study provides a comprehensive investigation of the bone microstructure of multiple bones of the Early Cretaceous filter-feeder, Pterodaustro guinazui, from the Largacito Formation of Central Argentina. We provide information regarding the bone histology of multiple elements from single skeletons, as well as a variety of bones from different individuals. In addition, we analysed changes in bone microstructure through ontogeny in growth series of several long bones of the taxon. Our investigation of skeletal and ontogenetic variation in Pterodaustro gives insights into the developmental growth dynamics of this unusual ctenochasmatid pterodactyloid from early ontogeny through to adulthood and also provides information pertaining to histological variability within and between bones of individuals. This study also documents the presence of what appears to be medullary bone tissue within the medullary cavity of a large femur of Pterodaustro. This suggests that, like birds, reproductively active female pterosaurs may have deposited a special bone tissue (medullary bone) to cope with the demand of calcium during eggshelling. Our study supports the hypothesis that small Jurassic pterodactyloids took several years to reach adult body size. More specifically, we provide data that suggests that Pterodaustro attained sexual maturity at about 2 years of age, and continued to grow for a further 3-4 years doubling in size before attaining skeletal maturity.

Pterosaurs are an extinct group of highly modified flying reptiles that thrived during the Mesozoic. This group has unique and remarkable skeletal adaptations to powered flight, including pneumatic bones and an elongate digit IV supporting a wing-membrane. Two major body plans have traditionally been recognized: the primitive, primarily long-tailed paraphyletic “rhamphorhynchoids” (preferably currently recognized as non-pterodactyloids) and the derived short-tailed pterodactyloids. These two groups differ considerably in their general anatomy and also exhibit a remarkably different neuroanatomy and inferred head posture, which has been linked to different lifestyles and behaviours and improved flying capabilities in these reptiles. Pterosaur neuroanatomy, is known from just a few three-dimensionally preserved braincases of non-pterodactyloids (as Rhamphorhynchidae) and pterodactyloids, between which there is a large morphological gap. Here we report on a new Jurassic pterosaur from Argentina, Allkaruen koi gen. et sp. nov., remains of which include a superbly preserved, uncrushed braincase that sheds light on the origins of the highly derived neuroanatomy of pterodactyloids and their close relatives. A µCT ray-generated virtual endocast shows that the new pterosaur exhibits a mosaic of plesiomorphic and derived traits of the inner ear and neuroanatomy that fills an important gap between those of non-monofenestratan breviquartossans (Rhamphorhynchidae) and derived pterodactyloids. These results suggest that, while modularity may play an important role at one anatomical level, at a finer level the evolution of structures within a module may follow a mosaic pattern.

Life-history parameters of pterosaurs such as growth and ontogenetic development represent an enigma. This aspect of pterosaur biology has remained perplexing because few pterosaur taxa are represented by complete ontogenetic series. Of these, Pterodaustro is unique in that besides being represented by hundreds of individuals with wing spans ranging from 0.3 to 2.5 m, it includes an embryo within an egg. Here we present a comprehensive osteohistological assessment of multiple skeletal elements of a range of ontogenetic sizes of Pterodaustro, and we provide unparalleled insight into its growth dynamics. We show that, upon hatching, Pterodaustro juveniles grew rapidly for approximately 2 years until they reached approximately 53% of their mature body size, whereupon they attained sexual maturity. Thereafter, growth continued for at least another 3-4 years at comparatively slower rates until larger adult body sizes were attained. Our analysis further provides definitive evidence that Pterodaustro had a determinate growth strategy.

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 two juvenile specimens of Pterodaustro guinazui, a medium-sized, filter-feeding pterodactyloid pterosaur from the Lower Cretaceous (Albian) lacustrine deposits of the Lagarcito Formation of central Argentina (Sierra de Las Quijadas, San Luis Province). Both specimens lack the skull, but one is otherwise nearly complete and articulated, and the other is more fragmentary. Their small size (-300 mm of wingspan), the lack of fusion of several postcranial bones, the minimal epiphyseal ossification, and the porous appearance of the periosteal surfaces indicate the specimens died at a very early stage of postnatal development. The presence of derived characters unique, within Pterodactyloidea, to P. guinazui (e.g., caudal vertebrae exceeding 16 elements) supports the proposed specific identification of the specimens.The new material provides anatomical information previously unknown for P. guinazui and sheds light on allometric transformations during the growth of this pterosaur. Comparisons with more mature and much larger individuals support two major allometric trends: (i) the negative allometric growth of the proximal portion of the forelimb, and (ii) the positive allometric growth of the metacarpal IV.Résumé : Nous soumettons un article sur deux spécimens juvéniles de Pterodaustro guinazui, un ptérosaurien ptérodactyloidé, de taille moyenne et qui se nourrit par filtrage, dans les dépôts lacustres de la Formation de Lagarcito (Crétacé inférieur -Albien) du centre de l'Argentine (Sierra de Las Quijadas, province de San Luis). Aucun des deux spécimens n'a de crâne mais l'un des deux est presque complet et articulé alors que l'autre est plus fragmenté. Leur petite taille (envergurẽ 300 mm), le manque de fusion de plusieurs os postcrâniens, l'ossification épiphysaire minimale et l'apparence poreuse des surfaces du périoste indiquent que les spécimens sont morts à un stage très précoce de développement postnatal. La présence de caractères dérivés, uniques à Pterodaustro guinazui chez les ptérodactylidés, (p. ex. des vertèbres caudales à plus de 16 éléments) supporte l'identification spécifique proposée des spécimens. Le nouveau matériel fournit de l'information anatomique auparavant inconnue pour Pterodaustro guinazui et nous renseigne sur les transformations allométriques au cours de la croissance de ce ptérosaurien. Des comparaisons avec des individus plus adultes et beaucoup plus grands supportent deux tendances allométriques majeures : (1) la croissance allométrique négative de la portion proximale du membre antérieur et (2) la croissance allométrique positive du 4 e métacarpien.[Traduit par la Rédaction] Codorniú and Chiappe 18

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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