Tiny fossil reveals what happened to birds after dinosaurs went extinct

By Carolyn Gramling   

The fossils of a tiny bird found on Native American land in New Mexico are giving scientists big new ideas about what happened after most dinosaurs went extinct. The 62-million-year-old mousebird suggests that, after the great dino die-off, birds rebounded and diversified rapidly, setting the stage for today’s dizzying variety of feathery forms.

“This find may well be the best example of how an unremarkable fossil of an unremarkable species can have enormously remarkable implications,” says Larry Witmer, a paleontologist at Ohio University in Athens who was not involved in the research.

The newly discovered fossils, described online today in the Proceedings of the National Academy of Sciences, are a scrappy collection of bits and pieces rather than a complete skeleton. But certain tell-tale characteristics—such as its fourth toe, which it could turn around forward or backward to help it climb or grasp—convinced the team that it was an ancient mousebird. Researchers unearthed the fossils in New Mexico on ancestral Navajo lands, in rocks dating to between 62.2 million and 62.5 million years old. They named the creature Tsidiiyazhi abini—Navajo for “little morning bird.” Its mousebird descendants—about the size of a sparrow and marked by their soft, grayish or brownish hairlike feathers—still dwell in trees in sub-Saharan Africa 

today.

But it’s the age of the fossil that is particularly interesting. It’s just a few million years after an asteroid struck Earth and brought the age of dinosaurs to an abrupt end 66 million years ago. Groups such as mammals and frogs are known to have rebounded rapidly after that event, diversifying into multiple new forms as they occupied newly available niches—a process evolutionary biologists called adaptive radiation. But there has been scant fossil evidence for what happened to birds—the only dinosaurs to survive the extinction—in its aftermath.

Paleontologists have suspected birds made a quick rebound. But bird fossils from the early Paleogene period immediately after the extinction—particularly those of small, tree-dwelling animals—are rare. So researchers have used genetic studies to suggest that “a few lineages survived extinction and had a really fast radiation right afterwards,” says Daniel Ksepka, a paleo-ornithologist at the Bruce Museum in Greenwich, Connecticut, and the lead author on the paper.

This new find clinches that notion with fossil evidence, and helps flesh out the fate of birds during this crucial time period. The team combined the new fossil evidence with previously collected genetic data from living birds to update the phylogenetic tree of bird evolution. Previous trees used these data to differentiate the birds into different groups, but weren’t able to determine when they had diverged. Now, with the new fossils so precisely dated, the team could determine when exactly different bird lineages split off from one another. As a result, Ksepka and colleagues estimate that the ancestors of some nine major land bird lineages—from mousebirds to owls to raptors like hawks and eagles—must have emerged in quick succession, all practically in the shadow of the extinction event.

“There’s just basically 3.5 million years for all of these groups to start splitting off,” Ksepka says. He adds that other recent finds suggest that water birds such as penguins did the same thing: Earlier this year, researchers reported finding a 61-million-year-old fossil of a 1.5-meter-tall penguin in what is today New Zealand.

T. abini “is a significant find” that shifts the fossil record of tree-dwelling birds significantly back in time, says paleontologist Gerald Mayr of the Senckenberg Research Institute in Frankfurt, Germany, who led the team that reported on the penguin fossils.

The new fossil has “tremendous value,” agrees paleobiologist Helen James, the curator of the division of birds at the Smithsonian Institution in Washington, D.C., who was also not involved in the study. “Firmly resolving the relationships of birds continues to be a headache, whether using genetic or morphological data, or both,” she says. “The paper fortifies the evidence for an early, explosive radiation of modern birds.” 

The study also gives paleontologists new reason to scrutinize early Paleocene rocks, not to mention existing museum collections, for signs of other representatives of modern bird groups, Witmer says. “This little fossil mousebird signals that those groups must have been there—we just need to find them.”