An asteroid redirected bird evolution

by Lucas Joel
Friday, August 3, 2018

A ground-dwelling bird tiptoes through burning forests in the aftermath of the asteroid impact about 66 million years ago at the Cretaceous-Paleogene boundary. Credit: ©Phillip Krzeminski, Cornell Laboratory of Ornithology.

When an asteroid hit Earth 66 million years ago, it helped wipe out all the dinosaur lineages save one: the birds. But birds didn’t completely dodge the cataclysm the asteroid triggered. Recent research suggests that forests around the planet were devastated. With forests gone, bird species that called trees home went extinct alongside their nonavian dinosaur cousins. This means that the birds that we see living in trees today evolved from lineages that, in the aftermath of the impact, were ground-dwelling.

The asteroid impact triggered wildfires and sunlight-blocking dust clouds that spread around much of the planet, decimating forest communities. The arboreal havoc appears in the rock record in the form of a so-called “fern spike” following the impact boundary layer, explains Regan Dunn, a paleobotanist at the Field Museum of Natural History in Chicago and a co-author of a new study in Current Biology. The rapid rise in the proportion of ferns represented among fossil plant remains after the boundary suggests that as trees vanished from landscapes, ferns supplanted them and became pervasive.

The fern spike lasted about 1,000 years before forests began recovering, but these emerging forests didn’t look the same as their predecessors had. Study co-author Antoine Bercovici, a palynologist at the Smithsonian Institution in Washington, D.C., examined pollen abundances in the aftermath of the impact, finding that the kinds of trees present were drastically different before and after the fern spike. “Many angiosperms — flowering plants — that were common in the Cretaceous disappeared,” Bercovici says. Then, after the spike, once-rare trees like pines and palm trees became common, he says.

Researchers have known about the fern spike for years, but the new study connects the fate of forest ecosystems after the impact with the story of bird evolution. Bird fossils from before the extinction display traits — like toes adapted for perching on branches — that disappear in bird fossils immediately following the extinction, says Daniel Field, a paleontologist at the University of Bath in England and lead author of the study. “The groups of birds that exhibit feet like that failed to survive the mass extinction event,” he says.

In 2015, Field and another team of researchers published a phylogenetic tree depicting the evolutionary history of all birds. The tree displays the timing of major branching events and reveals that “many of the deepest divergences within the living bird tree of life apparently sprung forth quite quickly after the impact 66 million years ago,” right around the time of the fern spike, Field says. Then, a key pattern emerged: “All of those branches lead to groups that today are dominated by ground-dwelling birds,” he says. This suggested to the team that the modern arboreal lineages that arose must have also emerged from groups that were ground-dwelling after the extinction.

So, the birds that flit around in trees today owe their existence to an asteroid impact and the ecological catastrophe that it wrought, Field says. But exactly how and when the terrestrial birds that survived the extinction evolved and moved into the trees that flourished after the fern spike is not clear, says Daniel Ksepka, a paleontologist at the Bruce Museum of Arts and Science in Greenwich, Conn., who was not involved in the new research. “It would be interesting to see how quickly the birds got to the trees,” he says.

The thing to do next, Field says, is to hunt for fossils of tree-dwelling birds in rock layers just above the extinction boundary, which could help reveal the story of how modern arboreal birds came to be.

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