Researchers at University College London have found that the evolution of bird skulls slowed down after dinosaurs were wiped out. This contradicts the long-standing theory that bird skull diversity accelerated after non-avian dinosaurs were killed at the end of the Cretaceous 66 million years ago.
Modern birds have an incredible amount of diversity in skull shape and size, and a team of scientists led by evolutionary biologist Ryan Nicholas Felice set out to investigate the history of bird skull morphology.
The researchers used 3D geometric morphometrics to map the shape of 354 living and 37 extinct avian and non-avian dinosaurs in unprecedented detail.
The team investigated the evolutionary relationships among the various species to look for a shift in the pace of skull diversification after modern birds emerged.
The study revealed that all regions of the skull evolved more rapidly in non-avian dinosaurs than in birds. There were certain regions that showed rapid bursts of evolution in particular lineages, and the experts identified the reasons behind these spurts.
In non-avian dinosaurs, for example, rapid evolutionary changes in the jaw joint were associated with changes in the animals’ diet. In addition, accelerated evolution of the roof of the skull occurred in lineages that sported bony ornaments such as horns.
Among birds, the most rapidly evolving part of the skull was the beak. The study authors attribute this to the need to adapt to different food sources and feeding strategies.
“In contrast to the appendicular skeleton, which has been shown to evolve more rapidly in birds, avian cranial morphology is characterised by a striking deceleration in morphological evolution relative to non-avian dinosaurs,” wrote the study authors.
“These results may be due to the reorganisation of skull structure in birds—including loss of a separate postorbital bone in adults and the emergence of new trade-offs with development and neurosensory demands.”
“Taken together, the remarkable cranial shape diversity in birds was not a product of accelerated evolution from their non-avian relatives, despite their frequent portrayal as an icon of adaptive radiations.”
The study is published in the journal PLOS Biology.