Dino-killing asteroid also took out the trees — and the birds that lived in them

The impact flattened the trees as well as the dinosaurs

Tree-dwelling birds went the way of the dinosaurs 66 million years ago, because the trees they dwelled in were wiped out for a millennium.

A new study led by Canadian paleontologist Daniel Field has thrown light on how forests were devastated after the impact of an asteroid or comet 66 million years ago. This resulted in the extinction of tree-dwelling birds, leaving only ground-dwelling birds as the survivors. It was these few ground-dwelling survivors that gave rise to the diversity of modern birds we see today.

• Research Paper in Current Biology

Birds are descended from dinosaurs — paleontologists often refer to them as "avian dinosaurs" — but their lineage had fully separated from their forebears well before the extinction impact 66 million years ago. Even by that time, birds had considerable evolutionary diversity and seem to have occupied the niches that modern birds do today. There were primitive aquatic birds, ground-dwelling birds, and tree-dwelling birds living alongside Triceratops and Tyrannosaurus rex.

However the tree-dwelling birds were particularly vulnerable to the devastation of the dinosaur killing impact event.

Field, a Prize fellow at the Milner Centre for Evolution at the University of Bath, worked with paleoecologists to understand what conditions were like in the aftermath of the impact. They studied pollen and spore fossils, which are plentiful in sedimentary samples and can give a rich picture of the plants that existed at the time.

Trees were replaced by ferns for a millennium

What they found, according to Field, was that before the impact, there was a broad diversity of plant life — including lots of trees, but that changed drastically.

"Forests worldwide were devastated by the asteroid impact, and we found that for a period of about 1,000 years, after the impact, forests around the world were really, really hammered."

The forests may have been killed by the either the global firestorm, or the chaotic climatic conditions that followed the impact. Their reconstruction suggest that post-impact ferns, on the other hand were abundant. Ferns are known to thrive in open and disturbed environments, and are often the first plants to recover after disasters like fires wipe out forests. 

The loss of trees, according to Field, would have been disastrous for many birds.

"Any birds that were specialized for tree-dwelling, that existed prior to the asteroid impact, were likely wiped out," he says. "If you're a bird that's dependent on dwelling in trees, either in order to find a place to build your nest or to forage for fruits or tree-dwelling insects, a thousand years is obviously a long time -— perhaps enough to drive you to obliteration or at least prevent you from reproducing." 

In particular, Field says a large and very diverse group of mostly tree-dwelling primitive birds called the Enantiornithes, or "opposite birds," completely disappeared from the fossil record at the extinction boundary.

Modern tree-dwellers evolved from ancient ground-dwellers

DNA evidence, according to Field, supported the idea that ground-dwelling birds survived the extinction. He says that family trees built by comparing the DNA of modern birds indicate that they descended from ancient ground-dwelling birds.  

"Only a small number of some of the deepest branches within the living bird tree of life, crossed the end Cretaceous extinction event," says Field. "And they were almost certainly non-arboreal in their ecology."

These survivors would have had thick, sturdy legs, suitable for walking or running, and while they would have been able to fly, much of their activity would have been on the ground. Field says modern analogues would have been partridges or the wild relatives of our domestic chickens.

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These ground-dwelling birds seem to have given rise, through evolution, to the full modern diversity of birds, including new species of tree-dwelling birds. As forests recovered a thousand years after the impact, this niche would have opened up, and the birds would once again have taken to the trees.