Fossil “turns over a century of knowledge” about bird evolution

Artist’s reconstruction of the last known toothed bird, Janavis finalidens, in its original environment (Phillip Krzeminski)

A new analysis of a 66.7-million-year-old fossilized bird skeleton found on the Dutch-Belgian border in the 1990s has ‘upended’ one of the key hypotheses about the evolution of modern birds.

Until now, the mobile, agile beaks of modern birds — which can move independently of their heads — were thought to have evolved more recently, with birds like ostriches and emus, which have fixed upper mandibles, emerging first.

However, the new fossil reveals that mobile beaks had already evolved before the mass extinction event that killed off the dinosaurs.

It means that instead of bridging an evolutionary gap, birds like ostriches and emus “evolved backwards” to get a simpler beak design.

Today, approximately 99% of all birds have movable beaks.

Researchers from Cambridge University and the Natuurhistorisch Museum Maastrich used CT scanning techniques to identify bones from the roof of the mouth of fossilized bones that come from a previously unknown species of large ancient bird, which they named Janavis finalidens.

The team said it lived at the very end of the dinosaur era and was one of the last toothed birds ever to exist.

The arrangement of its palate bones shows that this “dino-bird” had a mobile beak, “almost indistinguishable from that of most modern birds,” they said.

The research team said their work “suggests that our understanding of how the skull of a modern bird came to be needs to be reevaluated.”

Currently, each of the approximately 11,000 bird species living on Earth today is classified into one of two general groups, based on the arrangement of their palate bones.

Ostriches, emus, and their relatives are classified in the paleognathic group, or “ancient mandible,” meaning that, like humans, their palate bones are fused together into a solid mass.

All other groups of birds are classified in the neognath, or “modern jaw” group, meaning that their palate bones are connected by a movable joint.

Having a more agile beak is useful for nest building, grooming, food gathering, and defense.

But the emergence of a solid beak after the emergence of the more mobile jointed upper mandible could require a significant rewriting of the history books.

The research team noted that the two beak classifications were originally made by Thomas Huxley, a British biologist who became known as “Darwin’s Bulldog” for his resounding support of Charles Darwin’s theory of evolution.

In 1867, he divided all living birds into “ancient” or “modern” jaw categories. Huxley’s hypothesis was that the “ancient” jaw configuration was the original condition for modern birds, with the “modern” jaw arising later.

“This assumption has been taken for granted ever since,” said Dr. Daniel Field of Cambridge’s Department of Earth Sciences, senior author of the paper.

“The main reason this hypothesis has persisted is that we haven’t had palates of well-preserved fossil birds from the period in which modern birds originated.”

Palate of Janavis finalidens compared to that of a pheasant and an ostrich.  The palate anatomy of Janavis probably approximates that of the most recent common ancestor of all living birds, and is more similar to that of chicken- and duck-like birds, such as pheasants, than to birds such as ostriches and emus, which previously thought to exhibit ancestral condition of birds (Juan Benito and Daniel Field, University of Cambridge)

Palate of Janavis finalidens compared to that of a pheasant and an ostrich. The palate anatomy of Janavis probably approximates that of the most recent common ancestor of all living birds, and is more similar to that of chicken- and duck-like birds, such as pheasants, than to birds such as ostriches and emus, which previously thought to exhibit ancestral condition of birds (Juan Benito and Daniel Field, University of Cambridge)

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 back to 66.7 million years ago, during the last days of the dinosaurs. Because the fossil is encased in rock, scientists at the time could only base their descriptions on what they could see from the outside. They described the bone fragments protruding from the rock as fragments of skull and shoulder bones, and put the unremarkable-looking fossil back into storage.

But nearly 20 years later, the fossil was loaned to Dr. Field’s group at Cambridge, and Dr. Juan Benito, then a doctoral student, began taking another look at it.

“Since this fossil was first described, we’ve started using CT scanning on fossils, which allows us to see through the rock and visualize the entire fossil,” said Dr. Benito, now a postdoctoral researcher at Cambridge and lead author of the paper.

“We had high hopes for this fossil – it was originally said to contain cranial material, which is often not preserved, but we couldn’t see anything that looked like it came from a skull in our CT scans, so we gave up and set the fossil aside.

During the first Covid-19 lockdown, Dr. Benito took the fossil out again. “The previous descriptions of the fossil just didn’t make sense – there was one bone that really puzzled me. I couldn’t see how what was initially described as a shoulder bone could actually be a shoulder bone,” he said.

Dr Field said: ‘It was my first in-person interaction in months: Juan and I had a socially distanced outdoor meeting and he passed me the mysterious fossil bone.

“I could see it wasn’t a shoulder bone, but there was something familiar about it.”

“Then we realized we’d seen a similar bone before, in a turkey skull,” said Dr. Benito. “And because of the research we do in Cambridge, we happen to have things like turkey skulls in our lab, so we took one out and the two bones were almost identical.”

They said this realization led the team to conclude that the ‘modern’ unfused jaw condition, which turkeys share, evolved earlier than the ‘ancient’ jaw condition of ostriches and their relatives.

For an as yet unknown reason, the fused palates of ostriches and relatives must have evolved at some point after modern birds had already established themselves.

“Evolution doesn’t happen in straight lines,” said Dr. Field.

“This fossil shows that the mobile beak, a condition we had always thought postdates the origin of modern birds, actually evolved before there were modern birds. We have been completely behind in our assumptions about how the skull of a modern bird evolved for 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 feature of modern birds.

The team said the Janavis species did not survive the mass extinction event that marked the end of the Cretaceous period, and so did all large dinosaurs and other toothed birds.

The species would have weighed around 1.5 kg and was the size of a modern vulture. The lack of food due to the collapse of food chains likely prevented the survival of the species, the team said.

The research is published in the journal Nature.

Leave a Reply

Your email address will not be published. Required fields are marked *