Sunday, June 3, 2018

The Ancestor of All Modern Birds Was a Ground Dweller

New research shows that global mass deforestation caused by the asteroid that wiped out the dinosaurs led to the extinction of all the ancient tree-dwelling birds. The study, led by the Milner Centre for Evolution, suggests all modern tree-dwelling birds evolved from a partridge-like common ancestor that lived on the ground.

The new study says all modern birds evolved from the same ground-dwelling, common ancestor, after an asteroid slammed into Earth millions of years ago and devastated the planet’s forests.

“Everyone knows an asteroid the size of Manhattan caused a mass extinction 66 million years ago. The composition and distribution of life on Earth today cannot be understood except in light of this cataclysm,” said Yale professor of geology and geophysics Jacques Gauthier, co-author of a study published May 24 in Current Biology. Gauthier is curator of vertebrate paleontology and vertebrate zoology at the Yale Peabody Museum of Natural History.

“Although all the giant dinosaurs disappeared, a few small flying dinosaurs — namely, birds — survived,” Gauthier said. “We propose that widespread destruction of forests following the asteroid impact favored ground-dwelling birds over tree-dwelling birds.”

An artist’s rendering of a bird fleeing from a burning forest after the asteroid strike that wiped out non avian dinosaurs.
Credit:  Phillip M. Krzeminski

An international team of researchers from the U.S., England, and Sweden pieced together evidence from the plant fossil record and ecology of ancient and modern birds, in order to conduct the study. The lead author is Daniel Field, of the Milner Centre for Evolution at the University of Bath.
  • Ancient birds, called Enantiornithes or “opposite birds” were once widespread and diverse but were completely wiped out by the Cretaceous-Paleogene (K-Pg) mass extinction, caused by an asteroid impact 66 million years ago.
  • Study shows the asteroid also caused global deforestation and extinction of most flowering plants, destroying the habitats of the tree-dwelling opposite birds
  • Reconstruction of the evolutionary tree of birds shows all modern birds evolved from a small ground-dwelling bird similar to modern partridges
  • Study connects for the first time the extinction of the arboreal opposite birds, with the observation of an explosion of diversity in birds after the K-Pg event
  • Birds didn’t move back into the trees again until the forests recovered thousands of years later
Gauthier said the study’s hypothesis explains the disappearance of the common tree-dwelling birds of the Cretaceous period. “It also explains the fact that although many birds live in trees today, their earliest relatives emerging from the wake of the asteroid impact were long-legged ground dwellers,” he said.
Credit: University of Bath

The international team of UK, US, and Swedish researchers, publishing in the journal Current Biology, pieced together evidence from the plant fossil record and ecology of ancient and modern birds, and found that the only birds to survive the Cretaceous-Paleogene (K–Pg) mass extinction event lived on the ground.

“We drew on a variety of approaches to stitch this story together,” said lead author Dr Daniel Field, Prize Fellow at the Milner Centre for Evolution.

“We concluded that the devastation of forests in the aftermath of the asteroid impact explains why tree-dwelling birds failed to survive across this extinction event. The ancestors of modern tree-dwelling birds did not move into the trees until forests had recovered from the extinction-causing asteroid.”

Paleobotanist Dr Antoine Bercovici from Smithsonian Institution analysed the plant fossil record before and after the K-Pg boundary.

He said: “Our study examined the fossil record from New Zealand, Japan, Europe and North America which showed there was a mass deforestation across the globe at the K-Pg boundary. This was followed by a mass colonisation of so-called ‘disaster plants’, such as ferns, whose spores were able to survive the asteroid hit and germinate rapidly to fill the habitats left empty by the extinction of the trees.

“The recovery of canopy-forming trees such as palms and pines happened much later, which coincides with the evolution and explosion of diversity of tree-dwelling birds.”

The researchers found that once the forests had recovered, birds began to adapt to living in trees, acquiring shorter legs than their ground-dwelling ancestors and various specialisations for perching on branches.

Dr Field added: “Today, birds are the most diverse and globally widespread group of terrestrial vertebrate animals—there are nearly 11,000 living species.

“Only a handful of ancestral bird lineages succeeded in surviving the K–Pg mass extinction event 66-million-years-ago, and all of today’s amazing living bird diversity can be traced to these ancient survivors.

“We are working hard to shed new light on this murky portion of the fossil record, which promises to tell us a lot about how birds and other animal groups survived - then thrived -following the extinction of T. rex and Triceratops.”

Co-authors of the study are Jacob Berv of Cornell Lab of Ornithology, Regan Dunn of the Field Museum of Natural History, Tyler Lyson of the Denver Museum of Nature and Science, David Fastovsky of the University of Rhode Island, Vivi Vajda of the Swedish Museum of Natural History, and Jacques Gauthier of Yale University.

Additional co-authors of the study are Antoine Bercovici of the Smithsonian Institution, Jacob Berv of the Cornell Lab of Ornithology, Regan Dunn of the Field Museum of Natural History, Tyler Lyson of the Denver Museum of Nature and Science, David Fastovsky of the University of Rhode Island, and Vivi Vajda of the Swedish Museum of Natural History.

Contacts and sources: 
Vicky Just
University of Bath

Jim Shelton
Yale University

Citation: “Early Evolution of Modern Birds Structured by Global Forest Collapse at the End-Cretaceous Mass Extinction” Daniel J. Field, Antoine Bercovici, Jacob S. Berv, Regan Dunn, David E. Fastovsky, Tyler R. Lyson, Vivi Vajda and Jacques A. Gauthier (2018)   Current Biology DOI: 10.1016/j.cub.2018.04.062.

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