Artist impression of Archaeopteryx lithographica by Carl Buell

Today’s birds’ ancestors descended from non-avian dinosaurs according to a recent paper that thoroughly studied Anchiornis (155 million years ago) and Archaeopteryx (between 152 million and 168 million years ago) – the oldest known feathered dinosaurs.

Archaeopteryx is so far the oldest bird-like dinosaur found thus far, and since it was first discovered almost 150 years ago, it was thought that it could fly around just like other bird today.  In the meantime, studies have shown that it is not a primitive bird, but just another feathered dinosaur. Recently, Yale University paleontologist Nicholas Longrich and colleagues Jakob Vinther and Anthony Russell have shown that  Archaeopteryx, as well as its cousin Anchiornis, had a most curious plumage, very much different from that encountered in modern birds, which warrants a new way of envisioning them.

Around the time of their discovery, in the mid-1800s, Darwin’s theory of evolution was sweeping controversy through out Europe, and people easily assumed that these archaic birds function much in the same way as modern birds. We now need to re-think this.

Feather by feather

For starters, the birds had small feathers on its hind legs. Upon closer inspection it was found that compared to the single layer of feathers that give modern birds their dextrous flight abilities, the fossils exhibited a peculiar plumage pattern in which the feathers are stacked in layers on top of each other, almost like two-ply tissues.

“I realized you couldn’t really get from what the fossils showed to the way people were drawing it,” Longrich said. “People have been drawing the wing this one way for more than 100 years, and had this particular idea about what the wing would look like. And this is coming along after more than a century and saying we got it wrong.”

Bird ancestors gliders, not flyers

This feature was encountered in both feathered dinosaurs, but the two weren’t the same. While as Archaeopteryx had multiple layers of long flight feathers,  Anchiornis had an abundance of simple, strip-like feathers that overlap, like penguins today. What this means is that most likely these multiple overlapping layers wouldn’t have made the dinosaurs very much efficient flyers. The two dinosaurs most likely relied on gliding to escape predators and move over large distances.

“Modern birds have the ability to separate their wing feathers sort of like a Venetian blind,” Longrich said. “This allows them to raise the wing rapidly, and seems to be critical to flapping flight at low speeds.”

“The feather arrangement in Archaeopteryx and Anchiornis wouldn’t let them do this,” he added, “so it may have made takeoff from the ground and flapping at low speeds more difficult.”

Still, those feathers sure came in handy when averting predators.

“Gliding is a fast way to move from tree-to-tree. Instead of climbing down one tree and running up the next, you just glide quickly from one to the other,” Longrich explained.

“I would imagine that the dinosaurian ancestors of birds were living in the trees,” he noted, “probably to find food-like insects, lizards and mammals, and to avoid becoming food for other dinosaurs.”

Arkhat Abzhanov, associate professor of organismic and evolutionary biology at Harvard University, said that the study sheds new light on both species and the evolution of wing design. Longrich and colleagues believe wing feather arrangement seen in modern birds may have evolved within a period spanning a few tens of millions of years and then remained largely unchanged for the past 130 million years.

“Interpreting these fossils is really complicated, and there’s this interplay between having an interpretation going into it that will help guide you to understand the specimen, but also ignorance or openminededness, like a child would have, where you’re just kind of seeing what’s there and interpreting what’s there,” Longrich said. “That kind of naïve interpretation and the ability to look at the fossils that way, even after you’ve seen a lot of them, is really important.”

The findings were published in the journal Current Biology.

source: Boston