Pterosaurs: Could they fly as soon as they hatched?

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Pterosaurs were the first vertebrates to attain powered flight, and lived between 228 and 66 million years ago. They aren’t on the line to modern birds, which evolved well after pterosaurs appeared, and they appear to have gone extinct without leaving descendants. Often called “pterodactyls” or “flying dinosaurs”, they weren’t really in the group that included dinosaurs.

As you almost certainly know, they looked like this: (from Wikipedia):

While learning about these creatures this morning, I found that some of them were huge-as big as giraffes when they stood upright! One of them, Quetzalcoatlus northropi, had a wingspan of up to 16 meters, or 52 feet! Here’s some diagrams of that creature (the first two are from Wikipedia):

It was a big as a small plane! Here’s a comparison of Q. northropi with a Cessna 172 light aircraft:

Ambling about on four limbs, they are estimated to have been about 3 meters (10 feet) high at the shoulder, and as big as a giraffe from top to bottom:

Members of this species were also heavy, of course: they weighed about 200-250 kg, or 440-550 pounds. No flying bird even gets close to that.

Now could these big puppies fly? Mark Witton, a paleontologist and artist, thinks they could have, and sets out the evidence at this post, which I’ll leave you to read since I want to emphasize a new paper instead. But can you imagine a giraffe-sized reptile flying? That would be something we’d all love to see.

On to the new results. It’s generally accepted that young pterosaurs (unlike most birds, including my ducks) came out of the egg fully ready to fly, even though they still could have hung around the nest and received parental care (there’s no evidence for such care). But not everyone agrees. A two-year-old story in the the Daily Beast describes research that suggested that, because their wing bones weren’t full ossified (turned into hard bone) when they hatched, they couldn’t fly until later:

If pterosaur parents stuck around, perhaps there wasn’t so much of an imperative for the little guys to take to skies immediately after hatching. And there’s evidence from one of the embryos, the authors argue, that they couldn’t if they tried. In a single individual, the researchers found that the thigh bone had mature features and shape (that is to say that it looked like an adult thigh bone, only smaller) while the wing bone had some features still missing or underdeveloped.

“We have made an important progress by showing that the same embryo had the humerus [one of the main bones of the wing] not well ossified, but had the femur very well developed,” Alexander Kellner of the Universidade Federal do Rio de Janeiro, one of the study authors, told The Daily Beast by email. He said the most likely explanation for this mismatched development is that hatchlings could run but not fly.

Well, a new paper in the Proceedings of the Royal Society B (click on screenshot below, reference at bottom), though it’s paywalled, suggests that newly hatched pterosaurs could fly after all, and were as precocious as their modern relatives, the crocodilians, which are born pretty much ready to go, looking like tiny adults but still requiring parental care. Judicious inquiry might yield you a pdf of the paper below:

Unwin and Deemng did extensive analysis of 37 eggs of one pterosaur, Hamipterus tiashanensis, ranging from very small eggs to eggs containing embryos and even a new hatchling. They also looked at 3 other pterosaur species, comprising 19 embryos in total. The analysis is complex, aging eggs by both their size and roundness (eggs get rounder as they develop, and absorb water through the semipermeable “shell”), and examining embryos and newly hatched pterosaurs.

What they concluded is that the flying parts of the pterosaur: the fore- and hindlimb leg bones (remember, the leg bones were part of the airfoil) were sufficiently ossified in very late embryos to be able to fly. Second, earlier studies claiming that the muscle attachments for flight couldn’t have been sufficient to power the wings, aren’t all that convincing. Here’s what the authors say. You can get the point through the jargon (I’ve omitted the references and put the key points in bold.)

Terminal stages of embryonic development, represented by MIC V246, IVPP V 13758, JZMP 03-03-2, and the humeri of a near-term embryo (no. 7) and a hatchling of Hamipterus , have multiple features that point towards flight ability in hatchlings. First, extensive ossification of all elongate structures contributing to the flight apparatus that are likely to have experienced significant loads in bending during flight. These include dorsal and sacral vertebrae, the limb girdles and diaphyses of long bones that form the wing spars. This stiffening of the skeletal components of the flight module is analogous to ossification sequences in Al. mississippiensis, the hatchlings of which are also highly precocial locomotors, but is in sharp contrast to most extant birds where, prior to hatching, only the central region of the diaphysis of long bones is ossified.

Second, inferences regarding the implied lack of development of key flight muscles, based on the absence or poor development of osteological features, are insecure for two reasons: (i) muscle attachment sites do not need to be ossified in order to function effectively. In tension, cartilage can accommodate loads comparable to those for bone ; consequently, it cannot be assumed, a priori, that an incomplete deltopectoral crest directly implies a relatively small m. pectoralis, the principal wing depressor; and (ii) the relative size and shape of the deltopectoral crest of embryos 7, 11-13 and the hatchling is smaller than that of adult Hamipterus, but it is directly comparable in terms of shape and relative size to the deltopectoral crest of other pterosaurs including individuals of Anurognathus and Aurorazhdarcho that are widely considered to have been flight capable

Third, the relative elongation of long bones contributing to the wing spars, their relative proportions to each other and the relative elongation of the fore limb of mid and late term embryos compare closely to the same indices for mature, flight capable individuals of ornithocheirids [ JAC: this is a well-represented pterosaur]. This is in sharp contrast to most birds and all bats where fore limb proportions comparable to those of adults, and flight ability, are only achieved at a relatively late stage of postnatal development.

And here’s a diagram showing that late-stage embryos had well developed “flight bones”: similar to those of hatchlings and immature pterosaurs:

One last question: Did they have parental care, even if hatchlings could fly? The answer is simply, “we don’t know, as there’s a lack of evidence”. As the authors say, “such a behavior is difficult to demonstrate.” Indeed, I’m not sure what would count as evidence for parental care except for hatchlings that were unable to fly and thus unable to feed themselves. But these hatchlings may well have been able to fly.

At any rate, it’s interesting to contemplate a hatching nest of baby pterosaurs, with all of them taking off soon after leaving the egg.

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Unwin David, M. and D. C. Deeming. 2019. Prenatal development in pterosaurs and its implications for their postnatal locomotory ability. Proceedings of the Royal Society B: Biological Sciences 286:20190409.