Abstract
Recent fossil finds in China and Argentina have provided startling new insights into the reproductive biology and embryology of pterosaurs, Mesozoic flying reptiles. Nineteen embryos distributed among four species representing three distinct clades have been described and all are assumed to be at, or near, term. We show here how the application of four contrasting quantitative approaches allows a more precise identification of the developmental status of embryos revealing, for the first time to our knowledge, the presence of middle and late developmental stages as well as individuals that were at term. We also identify a predicted relationship between egg size and shape and the developmental stage of embryos contained within. Small elongate eggs contain embryos at an earlier stage of development than larger rounder eggs which contain more fully developed embryos. Changes in egg shape and size probably reflect the uptake of water, consistent with a pliable shell reported for several pterosaurs. Early ossification of the vertebral column, limb girdles and principal limb bones involved some heterochronic shifts in appearance times, most notably of manus digit IV, and facilitated full development of the flight apparatus prior to hatching. This is consistent with a super-precocial flight ability and, while not excluding the possibility of parental care in pterosaurs, suggests that it was not an absolute requirement.
Highlights
Pterosaurs, Mesozoic flying reptiles, have been known since the late 1700s, but fossil evidence for their prenatal development only dates back to 2004
Embryos and/or eggs have been reported in four species of pterosaur representing three distinct Late Jurassic –Early Cretaceous clades ([1,2,3,4,5,6,7,8]; electronic supplementary material, table S1)
We have argued that pterosaurs were capable of flight soon after hatching and probably did not require parental care [4,14,15], implying a profoundly different life-history mode for pterosaurs, compared to that of extant fliers
Summary
Pterosaurs, Mesozoic flying reptiles, have been known since the late 1700s, but fossil evidence for their prenatal development only dates back to 2004. Cross-comparison of the degree of ossification of skeletal structures of four pterosaur embryos, calibrated against a range of hatchling and immature individuals (electronic supplementary material, table S1), was used to capture data on developmental patterns of skeletogenesis that reflect the unique skeletal morphology of pterosaurs including manus digit IV, the ‘wing-finger’. The absence in Hamipterus embryos 11–13 of a series of skeletal structures and teeth (figure 3) is argued here to reflect a lack of ossification/eruption, rather than their loss owing to taphonomic processes This argument is supported by the observation that, originally, many eggs including those containing embryos appear to have been complete [8] with embryonic bones that are visible having been exposed by collection and preparation. It seems much more probable that this pattern reflects lack of ossification of a particular set of elements that mineralize relatively late (i.e. sacral and caudal vertebrae, ribs, gastralia, pelvis, carpals and tarsals), rather than taphonomic processes that are likely to have resulted in a random assortment of elements
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