Early birds like Archaeopteryx were far too heavy to sit on their eggs without cracking them. The conclusion holds true for non-bird dinosaurs too, leading to fresh doubts about how to interpret spectacular fossils that appear to show dinosaurs brooding their eggs.
Most birds today lay eggs with strong, hard shells. This strength is necessary because 鈥� meaning the adult rests its body weight directly on the eggs. But just because modern bird eggs can support the weight of a brooding adult it doesn鈥檛 necessarily follow that ancient bird eggs could, says at the University of Lincoln, UK.
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To investigate, Deeming and his colleague 鈥� at the Senckenberg Research Institute and Natural History Museum Frankfurt, Germany 鈥� looked at fossils of 21 species of bird that lived alongside the dinosaurs during the Jurassic and Cretaceous.
For each species, they studied the bones of pelvis and estimated the size of egg that the bird could have comfortably laid. A recent study suggests there is a , which means Deeming and Mayr could predict what sort of load the eggs of their ancient birds could have withstood before cracking.
Getting broody
Then the two researchers needed to work out the load the eggs would have experienced during brooding 鈥� in other words, how heavy the ancient adult birds would have been. suggested there is a predictable relationship between body weight and the length of certain arm and wing bones. Deeming and Mayr measured the relevant bones in each fossil bird and used the figures to estimate body weight.
The results suggested that all of the ancient birds would have struggled to sit on their eggs safely. 鈥�Archaeopteryx was over four times heavier than its predicted egg would hold,鈥� says Deeming.
This is not because ancient birds were particularly large and heavy 鈥� Archaeopteryx, for instance, was about the size of a . It鈥檚 more because their eggs were strikingly small, says Deeming.
He says ancient birds had an O-shaped bony ring in their pelvis through which the eggs had to pass during laying. Because this O-shape had a small diameter, the ancient birds had to lay small eggs 鈥� and smaller eggs are weaker. Modern birds are different: the pelvic bones form a flexible, open C-shape that doesn鈥檛 limit egg size. This means modern birds can comfortably lay larger, stronger eggs.
Many non-bird dinosaurs also had the closed O-shape pelvis. Researchers think , but Deeming says these eggs were too small and fragile to bear an adult鈥檚 weight. This seems to be contradicted by fossils of dinosaurs like Oviraptor brooding eggs like a modern bird would 鈥� but Deeming thinks those fossils have been misinterpreted. He argues they really show dinosaurs that had carefully buried their fragile eggs beneath dirt or vegetation, and were simply squatting on the nest mound to protect it from predators 鈥� as some crocodiles do today.
Tiny eggs
鈥淚t is unwise to impose the behaviour of modern birds on extinct species,鈥� says Deeming.
鈥淚 found their results shocking,鈥� says at Montana State University in Bozeman. What really astonishes Varricchio is that the results suggest some early birds laid eggs about 12 per cent the size a modern bird would lay. 鈥淭he numbers are so shockingly small that it makes me wonder if something is wrong with their calculations. But I have no evidence for that.鈥�
Earlier this week a different research team . Its size 鈥� just 5 centimetres long 鈥� suggests it did indeed hatch from a tiny egg, perhaps just 1.6 cm wide, says Deeming.
Varricchio agrees that ancient birds and non-bird dinosaurs don鈥檛 appear to have reproduced in the same way as modern birds. But he thinks there is strong evidence that some non-bird dinosaurs like Oviraptor and Troodon really did brood their eggs. His research suggests their eggs were about 50 per cent the size a modern bird would lay. 鈥淚f their numbers are correct, then it might be that there is a lot of diversity in reproduction among Mesozoic birds 鈥� some burying eggs and others brooding.鈥�
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