To be able to fly soon after hatching from the egg, a bird or pterosaur must have well-developed wings. Studies of small-bodies pterosaur species from the Jurassic period showed that their babies already had large wings when they hatched and they could have wobbled into flight within a few days of birth. But did this work for the later pterosaurs which were much larger in size? In the Cretaceous period, pterosaurs usually had wingspans of 5 m, and some even reached 10-15 m, the size of a small glider.
Yang et al. show that large-bodied pterosaurs, such as the giant pterosaur Pteranodon (shown here), probably practiced parental care. Image credit: James Robins / UCC.
“This was a difficult project. We needed examples of pterosaurs where we had at least one hatchling or very young specimen as well as adults so we could study their growth rates. But baby pterosaurs are really rare,” said Dr. Zixiao Yang, a paleontologist at University College Cork.
“Luckily, we were able to use some classic specimens from the Jurassic of Europe and the Cretaceous of North America, together with new finds from China,” added Nanjing University’s Professor Baoyu Jiang.
“By measuring the skulls, backbones, wings, and hind legs, we were able to test for differences in the relative growth of different parts of the body.”
In their study, Dr. Yang, Professor Jiang and their colleagues focused on testing the allometry, or how the creatures’ characteristics changed with size.
“We are all familiar with allometry in human babies, puppies and kittens — their heads, eyes and knees are huge, and the rest of the body grows faster to get to adult proportions,” Dr. Yang said.
“It’s the same with many animals, including dinosaurs and pterosaurs. The babies have cute faces, with short noses, big eyes, and big heads.”
“The small, bird-sized, Jurassic pterosaurs were born with large wings and strong arms and legs, evidence that the babies could fly from birth.”
“As they grew from baby to adult, their arms and legs showed negative allometry, meaning they started large and were then growing more slowly than the rest of the body.”
“But it was different for the Cretaceous giants. They also started as small babies, but the key limb bones show positive allometry through growth, suggesting a very different developmental model.”
“This means that the pterosaur giants had sacrificed low-input childcare to the need to grow huge eventually as adults.”
“Minimal childcare makes sense in the early evolutionary history of these ancient reptiles because it saves energy.”
“But to grow huge, the larger pterosaurs had a problem — it basically took much longer to become an adult, and therefore parents needed to protect their young from accidents.”
“The babies of all pterosaurs, large and small, were small because of the limitations of egg size.”
“The bird-sized Jurassic pterosaurs were born with relatively large wings and critically, strong arms and legs, evidence that the babies could fly from birth,” said Yunnan University’s Professor Xu Xing.
“As they grew from baby to adult, their arms and legs showed negative allometry, meaning they started large and were then growing more slowly than the rest of the body.”
“But it was different for the Cretaceous giants. They also started as small babies, but the key limb bones show positive allometry through growth, suggesting a very different developmental model,” said University College Cork’s Professor Maria McNamara.
“This means that the pterosaur giants had sacrificed low-input childcare to the need to grow huge eventually as adults,” said Queen Mary College London’s Dr. David Hone.
“Minimal childcare makes sense as a starting point for these ancient reptiles because it saves them energy. But to grow huge, the larger pterosaurs had a problem, which was the much longer time it took to become an adult, and the need to protect their young from accidents.”
“The babies of all pterosaurs, large and small, were small because of the limitations of egg size. Investing in childcare by having non-flying babies was offset in evolutionary terms by releasing a constraint on becoming truly huge.”
“We see the same thing in birds and mammals today,” Dr. Yang said.
“Some birds fly very young, and of course some mammals like cattle and antelopes are on their feet the day they are born.”
“But this kind of behavior is risky for the babies because they are often clumsy and are easy targets for predators; it’s costly also for the mother because the babies have to have highly developed wings or legs at the point of birth.”
“So, we see the same thing in extinct pterosaurs. They were restricted in maximum body size until the end of the Jurassic, at which point their parental care behavior changed, and then they could achieve huge sizes.”
The results were published in the journal Proceedings of the Royal Society B.
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Zixiao Yang et al. 2023. Allometric wing growth links parental care to pterosaur giantism. Proc. R. Soc. B 290 (2003): 20231102; doi: 10.1098/rspb.2023.1102
