From dinosaur to bird – the transition was more complex than we thought

  Last updated May 17, 2018 at 12:00 pm

Topics:  

New, well-preserved fossils of a dinosaur-era beaked bird fill in some gaps


A CT-scan-based skull restoration and life reconstruction of the toothed stem bird Ichthyornis dispar show that the first form of the avian beak was a precision pincer-tip probably used for fine manipulation.
Credit: Michael Hanson and Bhart-Anjan S. Bhullar.


Until now it’s been difficult to determine exactly how, and in what order, modern birds’ features have developed from their dinosaur forebears.


The big problem has been the generally poor preservation of fossil bird skulls.


That’s why the description of four new fossils of the early bird Ichthyornis dispar is so important.


Ichthyornis was a toothy, tern-like seabird with a 60-centimetre wingspan, which lived around 100–66 million years ago in what is now North America.


But what makes Ichthyornis so important for the study of the transition from the dinosaurs is its close relation to modern birds while retaining many ancestral features, including sharp, curved teeth.


The bird is not new to science. It was discovered in 1870, but the heads of the first specimens were incomplete and badly crushed.


First complete skull found


And these are the first specimens uncovered since then, but they have been worth the wait, being particularly well-preserved three-dimensional fossils, including one extraordinarily complete skull.


Bhart-Anjan Bhullar from Yale University, and his colleague, Kevin Padian of the University of California, Berkeley, also report two previously overlooked elements from other Ichthyornis specimens.


From three-dimensional scans, the authors have reconstructed the head of the bird.


As with dinosaurs, Ichthyornis sports holes in its skull for large jaw-closing muscles. But overall it has a largely modern-style, articulated skull — with a small, primitive beak on its jaw tips.


These features would have enabled preening and object manipulation after arms became wings, and reveal that the feeding apparatus of living birds evolved earlier than previously thought.


The findings place Ichthyornis between the earliest birds and living birds.


“The beak was small, had not yet evolved a bony shelf structure in the palate and was limited to the tip of the jaw,” writes Padian. “However, the probable mobility of the Ichthyornis skull seems to be more like that of living birds.


New findings raise new questions


“The brain would have been much like those of today’s birds, but the cheek region, bounded by bones of the skull roof and the side of the skull, has characteristics that are closer to those of dinosaurs, such as the retention of a large bony chamber for the adductor muscles that close the jaw.”


He says that means several key features of the brain and palate evolved before the jaw muscles became reduced and the familiar features of the beak of living birds evolved.


Padian says the study also raises many questions.


“Were there functional changes that went along with reducing the jaw muscles from the ancestral dinosaurian condition? Did this change reflect a change in diet? And what ecological habits are correlated with the loss of teeth from the front part of the upper jaw and the evolution of the horny beak that covers it?”


He believes Ichthyornis was probably a surface skimmer or perhaps a shallow plunger like a tern or gull.


The report was published in Nature.




About the Author

Bill Condie
Bill is Head of Publishing at the Royal Institution of Australia. Previously he was Publisher of the popular science magazine, Cosmos, based in Melbourne, Australia. Bill has been a journalist for more than 30 years and his work has been published in Cosmos magazine, The Guardian, The Observer, The Times, The Daily Telegraph and the London Evening Standard.