Bird skulls evolved surprisingly slowly

It’s quite remarkable that emus, pelicans, hummingbirds, hawks and woodpeckers are all part of the same class of animals given their broad diversity, including the shape and size of their skulls.

As the only dinosaurs to survive the last mass extinction 66 million years ago, it’s theorised that birds’ success resulted from a sudden surge in their skull’s evolution – but the reality was quite the opposite, according to a study published in the journal PLOS Biology.

“Surprisingly, the skull evolves much slower in birds than non-avian dinosaurs,” says lead author Ryan Felice from University College London, UK, part of a team from the UK and across the US.

“This means that birds did not achieve their amazing diversity of skull shapes through rapid change following the demise of the dinosaurs but through slow and steady variation.”

His team took high resolution 3D scans and mapped the skull shape of 354 living bird and 37 extinct dinosaur species, including famed specimens Sophie the stegosaurus and Dippy the diplodocus, to trace their evolution over more than 200 million years.

They found faster evolution in dinosaurs, before they evolved into myriad bird species, in nearly all cranial regions; most notably at the top of the skull, or braincase.

That region is constrained in birds but has high variation in non-avian dinosaurs, in which it evolved rapidly into many different forms.

These evolutionary changes included bony ornaments, such as horns or crests in ceratopsians, and feeding strategies such as large jaw closing muscles, which are particularly massive in the bone-crushing Tyrannosaurus rex.

In birds, the braincase is crowded with their large eyes and brains and is decorated with feathers instead. Beaks were their fastest evolving (and most variable) feature, which the authors attribute to different food sources and feeding strategies.

The researchers note their discovery highlights the “mosaic evolution” of birds, in which other features such as wings and hindlimbs quickly diverged.

They say this is the largest and most detailed mapping of dinosaur and bird skull evolution to date, as part of an ambitious project, led by Anjali Goswami from London’s Natural History Museum, to map those of all vertebrates and understand what shapes species diversity.

So far, the team has published results using this approach in birds, snakes, lizards, frogs, salamanders, and caecilian amphibians. Next, watch out for studies of crocodiles, turtles and mammals.

The 3D scans are freely available for download on the museum’s Phenome10K.org database.