Most modern birds have highly flexible skulls, an adaptation that allows them to absorb shock and reduce weight. But just when these ‘bendy’ skulls first appeared in birds has been an open question until now.
In a paper just released in the Proceedings of the National Academy (USA), scientists from the University of New England and Chinese Academy of Sciences have digitally reconstructed parts of a 120 million year old ancient bird’s skull to answer this question.
Birds evolved from dinosaurs at least 150 million years ago and are their only surviving descendants. In the 65 million years since dinosaurs were wiped out, birds have become extraordinarily successful. A number of special features are key to the success of birds, among these is the ability to bend their light, fragile skulls to resist shock when biting.
This adaptation, known as cranial kinesis, is present in the vast majority of living birds and helps them to minimise weight, an obvious advantage for any flying animal.
Cranial kinesis is made possible by reducing and changing the shape of bones in the roof of the mouth (palate). The dinosaur ancestors of birds are thought to have had stiff, rigid skulls.
However, to date, no one has been sure about how or when this transition to a bending skull in birds took place. This is because in the oldest fossil birds, bones in the palate are typically poorly preserved, if present at all.
Among living bird species a few, such as emus and ostriches, have rigid skulls, but it’s not clear whether these birds come from a group of birds that once had bending skulls which later evolved to be rigid again, or, whether they never evolved cranial kinesis at all.
Some of the oldest birds discovered so far are from China. Among these is the 120 million year old Sapeornis. Thanks to exceptional fossil preservation, CT scanning and computer-based 3D reconstructions, a team including Chinese and Australian scientists have reconstructed an important bone, called the vomer, in the palate of Sapeornis.
They then compared this to that of many living birds and that of a troodontid dinosaur called Sinovenator. Although small, troodindids like Sinovenator were highly effective sharp-toothed predators with very large, sickle-like claws, and they are thought to be close to the base of the evolutionary tree that included birds.
The results of this work show that Sapeornis had a rigid skull and that the small group of living birds with rigid skulls have hung on to this ancient dinosaurian feature, not lost then re-evolved it. This suggests that the remarkably flexible skull of most living birds is a relatively recent adaptation that helped them to diversify into the extraordinary range of ecological niches that have made them so successful.
The research team was led by Dr. Han HU from University of New England, Australia and Dr. Zhonghe ZHOU from the Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences. The research was published in the latest issue of PNAS, in a paper titled “Evolution of the vomer and its implications for cranial kinesis in Paraves”.
