Prenatal development in pterosaurs and its implications for their postnatal locomotory ability

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.

Unique near isometric ontogeny in the pterosaur Rhamphorhynchus suggests hatchlings could fly

“…However, this is challenged by their near isometry, the similarities in forelimb and hindlimb ossification, and the presence of so many small animals found out to sea. Work on the development of embryonic pterosaurs from China also suggests they were capable of aerial locomotion very soon after hatching (Unwin & Deeming, 2019) and supports this contention here for Rhamphorhynchus. This is not necessarily universal for pterosaurs, however.…”

Interpretation of fossil embryos requires reasonable assessment of developmental age

“…For example, Chapelle et al (2020) used tomographic data of Centrochelys sulcata , Gallus gallus , and Crocodylus niloticus embryos to help interpret well-preserved embryos of the early-branching sauropodomorph dinosaur Massospondylus carinatus . Unwin and Deeming (2019) used descriptions of ossification in Alligator and the quail Coturnix coturnix to interpret pterosaur embryos. By contrast, Horner and Weishampel (1988, 1996) used a more qualitative approach to interpret the poorly ossified terminal ends of small limb bones of Maiasaura as representing altricial development.…”

“…The size of a specimen in ovo is often deemed to “fill” the egg (Xing et al 2022), which is seen as another piece of evidence that the embryo was close to hatching. Rarely (Deeming et al 1993; Unwin and Deeming 2019) do reports of fossilized embryos offer any quantitative assessment of age in terms of development. Therefore, how reasonable is it to assume that specimens like YLSNHM01266 (Xing et al 2022) and other oviraptorosaur embryos (Norell et al 1994, 2001; Weishampel et al 2008; Wang et al 2016; Pu et al 2017) are of a size that is “close to hatching”?…”

“…Deeming et al (1993) used qualitative observations of small ichthyosaur skeletons in conjunction with quantitative data from other ichthyosaur fossils and from extant alligator embryos to suggest that these small skeletons were indeed embryos rather than cannibalistic prey items. A similar approach has been applied to pterosaur embryos, whereby four separate quantitative aproaches allowed for a more precise identification of the developmental stages of skeletons in ovo and probable hatchlings (Unwin and Deeming 2019).…”

Multi-Aged Small Pterosaur Track Assemblage from the Upper Cretaceous of Korea

“…Moreover, many works regarding embryo fossils, growth rate, glide performance, and bone strength of pterosaur identi ed the possibility of precocial ight ability (e.g. [42][43][44][45][46] ), and gregarious nesting (e.g. 46 ).…”