Palaeohistology reveals an unusual periodontium and tooth implantation in a filter-feeding pterodactyloid pterosaur, Pterodaustro guinazui, from the Lower Cretaceous of Argentina.

Pterosaurs are an extinct group of Mesozoic flying reptiles, which exhibited high diversity with regard to their dentition. Although morphological features of pterosaur dentition have been described in detail in several contributions, the histology of tooth and tooth attachment tissues (i.e. periodontium) has been scarcely analysed to date for this clade. Here we describe and interpret the microstructure of the tooth and periodontium attachment tissues of Pterodaustro guinazui, a filter-feeding pterodactyloid pterosaur from the Lower Cretaceous of Argentina. The histological analysis of the lower jaw and its filamentous teeth verifies that the geometry of the implantation corresponds to an aulacodont condition (i.e. teeth are set in a groove with no interdental separation). This pattern departs from that recorded in other archosaurs, being possibly also present in other, non-closely related, pterosaurs. Regarding tooth attachment, in contrast to other pterosaurs, there is no direct evidence for gomphosis in Pterodaustro (i.e. the absence of cementum, mineralized periodontal ligamentum and alveolar bone). Nevertheless, the current evidence for ankylosis is still not conclusive. Contrary to that reported for other archosaurs, replacement teeth are absent in Pterodaustro, which is interpreted as evidence for monophyodonty or diphyodonty in this taxon. Most of the microstructural features are possibly related to the complex filter-feeding apparatus of Pterodaustro and does not appear to represent the general pattern of pterosaurs.

A Jurassic pterosaur from Patagonia and the origin of the pterodactyloid neurocranium.

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.

Palaeobiological implications of the bone histology of Pterodaustro guinazui.

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.

Palaeobiology: Argentinian unhatched pterosaur fossil.

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.