Some studies, however, cast doubt on the appropriateness of this term. Some authors promoting this view have referred to baby pterosaurs as ‘flaplings’ (e.g. Overall, their development recalls that of precocial sauropsids rather than the altricial offspring of neoavian birds, and it seems reasonable to interpret juvenile pterosaurs as neither nest-bound, nor helpless and dependent upon their parents. These lines of evidence indicate that juvenile pterosaurs were capable of powered flight early in life, plausibly within days or hours of hatching. Furthermore, soft tissues preserved in embryos show that flight membranes were present even before hatching 7. Among the more surprising conclusions of these studies are that even embryonic pterosaurs were well-ossified and adult-like in skeletal proportions, differing only in a few aspects of bone fusion and proportions. More recently, the discovery of eggs, embryos and bone beds have improved our knowledge of the earliest phases of pterosaur growth 6, 7, 8, 9, 10, 11. Comprehensive assessments published since the 1990s have shown that very young pterosaurs can be identified on the basis of both skeletal proportions and the identification of features indicative of osteological immaturity (e.g. The earliest stages of pterosaur life history have long been shrouded in mystery, a fact due mostly to the rarity or absence of eggs and embryos for the majority of taxa, but also to the difficulty inherent in distinguishing hatchlings from small adults. We propose on the basis of these conclusions that pterosaur species occupied distinct niches across ontogeny. Juveniles appear to have been adapted for flight in cluttered environments, in contrast to larger, older individuals. The wing forms of very young juveniles differ significantly from larger individuals, meaning that variation in speed, manoeuvrability, take-off angle and so on was present across a species as it matured. We further show that young juveniles were excellent gliders, albeit not reliant on specialist gliding. We therefore reject the ‘fly-late’ and ‘glide-early’ models. The humeri of pterosaur juveniles are similar in bending strength to those of adults and able to withstand launch and flight wing size and wing aspect ratios of young juveniles are also in keeping with powered flight. We argue that a young Sinopterus specimen has been mischaracterised as a distinct taxon. We test these models by quantifying the flight abilities of very young juvenile pterosaurs via analysis of wing bone strength, wing loading, wingspan and wing aspect ratios, primarily using data from embryonic and hatchling specimens of Pterodaustro guinazui and Sinopterus dongi. A ‘flap-early’ model proposes that hatchlings were capable of independent life and flapping flight, a ‘fly-late’ model posits that juveniles were not flight capable until 50% of adult size, and a ‘glide-early’ model requires that young juveniles were flight-capable but only able to glide. Competing views exist on the behaviour and lifestyle of pterosaurs during the earliest phases of life.
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