Fossilized fragments of a skeleton, hidden in rock the size of a grapefruit, have helped overturn one of the oldest hypotheses about the origins of modern birds.
Researchers from the University of Cambridge and the Natuurhistorisch Museum Maastricht have found that one of the main skull features that characterizes 99% of modern birds – a movable beak – evolved before the mass extinction that killed all large dinosaurs , 66 million years ago.
This discovery also suggests that the skulls of ostriches, emus and their relatives evolved “inside out”, returning to a more primitive state after the appearance of modern birds.
Using computed tomography techniques, the Cambridge team identified the bones of the palate, or roof of the mouth, of a new species of large ancient bird, which they named Janavis finalidens. It lived at the very end of the dinosaur era and was one of the last toothed birds to ever live. The arrangement of its palate bones shows that this “dino-bird” had a mobile and dexterous beak, almost indistinguishable from that of most modern birds.
For more than a century, it had been assumed that the mechanism for a movable beak evolved after the extinction of the dinosaurs. However, the new discovery, reported in the journal Naturesuggests that our understanding of the origin of the modern bird skull needs to be reassessed.
Each of the approximately 11,000 species of birds on Earth today is classified into one of two main groups, based on the arrangement of their palate bones. Ostriches, emus, and their relatives are classified as paleognaths, or “ancient jawbones,” which means that, like humans, their palate bones are fused together into a solid mass.
All other groups of birds are classified in the neognathic or “modern jaw” group, which means that their palate bones are connected by a movable joint. This makes their beaks much more dexterous, useful for nest building, grooming, foraging and defence.
Both groups were originally categorized by Thomas Huxley, the British biologist known as “Darwin’s Bulldog” for his vocal support of Charles Darwin’s theory of evolution. In 1867 he divided all living birds into “ancient” or “modern” jaw groups. Huxley’s hypothesis was that the “ancient” jaw configuration was the original condition of modern birds, with the “modern” jaw appearing later.
“This hypothesis has been taken as given ever since,” said Dr Daniel Field of Cambridge’s Department of Earth Sciences, the paper’s lead author. “The main reason this hypothesis has held is that we haven’t had well-preserved fossil bird palaces from the period when modern birds originated.”
The fossil, Janavis, was found in a limestone quarry near the Belgian-Dutch border in the 1990s and was first studied in 2002. It dates to 66.7 million years ago, during the last days of the dinosaurs. Since the fossil was encased in rock, scientists at the time could only base their descriptions on what they could see from the outside. They described the pieces of bone sticking out of the rock as fragments of skull and shoulder bones, and returned the unremarkable-looking fossil to storage.
Nearly 20 years later, the fossil was loaned to Field’s group in Cambridge, and Dr. Juan Benito, then a Ph.D. student, began to give it another look.
“Since this fossil was first described, we started using CT scans on the fossils, which allows us to see through the rock and see the whole fossil,” said Benito, now a researcher. postdoctoral fellow at Cambridge and lead author of the article. “We had high hopes for this fossil – it was originally said to have cranial material, which is not often preserved, but we couldn’t see anything resembling a skull in our CT scans, so we abandoned and put the fossil aside.”
During the early days of the COVID-19 lockdown, Benito took the fossil out again. “Previous descriptions of the fossil just didn’t make sense – there was one bone that really intrigued me. I couldn’t see how what was first described as a shoulder bone could actually to be a shoulder bone,” he said.
“It was my first in-person interaction in months: Juan and I had a socially distanced outdoor meeting, and he passed the mysterious bone fossil on to me,” said Field, who is also a curator at the ornithology at the Cambridge Zoological Museum. “I could see it wasn’t a shoulder bone, but there was something familiar about it.”
“Then we realized we had seen a similar bone before, in a turkey skull,” Benito said. “And because of the research we’re doing at Cambridge, we have things like turkey skulls in our lab, so we pulled one out and the two bones were almost identical.”
The realization that the bone was a skull bone, not a shoulder bone, led researchers to conclude that the unfused state of the “modern jawbone”, which turkeys share, evolved before the state of the “old jaw” of ostriches and their relatives. For some unknown reason, the fused palates of ostriches and relatives must have evolved at some point after the establishment of modern birds.
Two of the key features we use to differentiate modern birds from their dinosaur ancestors are toothless beaks and movable upper jaws. While Janavis finalidens still had teeth, making it a pre-modern bird, its jaw structure is that of the modern, mobile genus.
“Using geometric analyses, we were able to show that the shape of the fossil palate bone was extremely similar to that of living chickens and ducks,” said study co-author Pei-Chen Kuo. Added co-author Klara Widrig, “Surprisingly, the palate bones of birds that look the least like Janavis come from ostriches and their relatives.” Kuo and Widrig hold doctorates. students from Field’s laboratory in Cambridge.
“Evolution doesn’t happen in a straight line,” Field said. “This fossil shows that the movable beak – a condition that we had always thought to postdate the origin of modern birds, in fact evolved before modern birds existed. We have been completely backward in our assumptions about how from which the modern bird skull evolved for well over a century.”
The researchers say that while this discovery doesn’t mean the entire bird family tree needs to be redrawn, it does rewrite our understanding of a key evolutionary characteristic of modern birds.
And what happened to Janavis? Like large dinosaurs and other toothed birds, it did not survive the mass extinction at the end of the Cretaceous. Researchers say this may be due to his large size: Janavis weighed around 1.5 kg and was the size of a modern vulture.
It’s likely that smaller animals – like the “miracle chicken”, identified by Field, Benito and colleagues in 2020, which comes from the same region and lived alongside Janavis – had an advantage at this point in the history of Earth because they needed to eat less to survive. This would have been beneficial after the asteroid hit Earth and disrupted global food chains.
Daniel Field, Cretaceous ornithurine supports neognathic crown bird ancestor, Nature (2022). DOI: 10.1038/s41586-022-05445-y. www.nature.com/articles/s41586-022-05445-y
Provided by the University of Cambridge
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