From China, A Flock Of Fossils

Our picture of bird evolution has changed dramatically over the past three decades, thanks to an avian fossil jackpot in northeastern China. These ancient remains, dating back 120–131 million years ago, are part of a diverse assembly of animal and plant fossils collectively known as the Jehol Biota. (The term is a historic reference to a region ruled centuries ago by the Khitan Empire.)

Throughout most of the 20th century, available fossil evidence suggested that birds didn’t flourish until after their close cousins, the dinosaurs, succumbed to a space rock. But a bonanza of specimens from the Jehol “have radically transformed our understanding of the lives of birds,” write the authors of Birds of Stone, a new book that focuses primarily on the Jehol avifauna.

Researchers dug up the first Jehol fossils in the late 19th century, but it wasn’t until the 1980s that paleontologists found avian remains in the region. The discovery of feathered dinosaur fossils in the 1990s then set off a frenzy of digging, and hunters have since uncovered thousands of bird fossils in fine shale deposits across western Liaoning Province, northeastern Hebei Province, and southeastern Inner Mongolia.

“Numerous fossils of Mesozoic birds have been unearthed from sites around the world but nowhere in such abundance, diversity, or superb preservation as in northeastern China,” the authors write.

The pristine fossil deposition stems from a rare confluence of environmental conditions that occurred over millions of years. Lakes interlacing the region swallowed many birds (and other organisms) when they died—sometimes en masse, in volcanic eruptions. Fine sediment quickly encased the carcasses in a benthic tomb, hindering their decomposition by microbes and scavengers. As a result, most of the bird fossils are near complete, with large portions of plumage spraying from their skeletons and, sometimes, the outline of skin etched into the rock.

Science Friday recently chatted with paleontologist Luis Chiappe, one of the authors of Birds of Stone and the vice president for research and collections at the Natural History Museum of Los Angeles County, to learn more about the stories that these shale deposits tell about avian evolution.

The Jehol birds flourished during the first half of the Cretaceous Period. Can you paint a picture of what their environment was like in general?
The deposits that contained the fossils are lakebeds, so we know that there were a number of lakes in the area, probably connected by streams, and we know that there were forests, because of the paleobotanical record that we have. We also know that there was volcanic activity in the area, so you could imagine a mountain range of some size. The latitude was approximately where it is now, in the low 40 degrees, so you’re looking at a temperate setting with warmer summers and cool winters. Of course, the cast of species would have been different. For example, 120 million years ago, flowering plants—the dominant plants that we see today—were just beginning to evolve. So you would have had a lot more conifers and ferns in the undergrowth of the forest.

Is there a bird species that’s most common to the Jehol fossils?
In terms of a species—the way we understand it in modern biology, like Homo sapiens—there’s Confuciusornis sanctus [which translates to, “sacred bird of Confucius”]. Thousands of specimens of that particular species have been collected from the Jehol.

Why is there such a preponderance of that particular bird?
That’s not an easy question to answer. There are aspects of the ecology and behavior that could have contributed. Perhaps these animals lived by the lakeshore [where lake sediment could have preserved their carcasses], and perhaps they were more gregarious and formed big flocks. But there’s also a fact in nature that not all species have the same abundance. Look at how many pigeons, sparrows, and starlings you find in New York City. When you go to Central Park, you’re going to find a lot more of those three species of birds, and then you’re going to find the occasional hawk or the occasional woodpecker. And it could be that the abundance of fossils that we have of Confuciusornis reflects a real abundance of that species, for whatever reason, in the environment in which it lived. But that’s very difficult to know.

Feathers, of course, are a defining characteristic of birds. How well have feathers been preserved in the Jehol fossils, and how do they compare with those of modern bird feathers?
The feathers are beautifully preserved. You get to see the barbs, you get to see the shafts. Some of the fossils also preserve the ultra-structure of these feathers, like, for example, small organelles called melanosomes—the little sacs that contain the pigment melanin that gives color to many feathers. Depending on the concentration of the different kinds of melanosomes that you find in a feather, you could infer the color of the fossil feather.

Some of the feathers that are found in the Jehol don’t exist in birds anymore—they’re essentially extinct as a type. That’s not surprising; animal traits have evolved throughout hundreds of millions of years, and some traits that are found in the fossil record are no longer in existence.

The earliest known bird fossils in the Jehol Biota are nearly 20 million years younger than the fossils of the oldest bird we know of, Archaeopteryx, which lived in what is today southern Germany. Why haven’t we found any bird fossils from those intervening years? Where could they be?
That’s the million-dollar question. The fossil record and the rock record has gaps, and we just don’t know of any rock that dates back to between 150 and 130 million years ago that has avian remains. There are no obvious places where you would go and explore. They may exist somewhere in vast, unexplored regions of our planet. Those places don’t need to be enormous—they could be as simple as a small pond in some national park in the American West. That doesn’t mean that next week someone won’t find them.

It seems like that would be quite a find.
It would. Those fossils could answer a number of questions. If you take the avian fossil record at face value, you have Archaeopteryx 150 million years ago, and it’s a poorly diverse avifauna. Some people argue that maybe there’s more than one species of Archaeopteryx, but the specimens are all very similar. And then you fast-forward 20 million years into the Jehol, and you have an enormous diversity of avians. That tells you that those 20 million years are critical for understanding the pattern of that explosion of avian diversity.

What are some other distinctive features that are present in the Jehol bird fossils that modern birds are missing?
The most obvious one? Teeth. The majority of the Mesozoic birds were toothed, but there’s no toothed bird alive today. Interestingly, you can, in the lab, create some of the early phases of tooth formation by stimulating developmental genes in bird embryos that would then produce tooth buds.

I can’t imagine having a toothed bird.
It would put your lovebird bite to a new level of pain.

There were multiple lineages of birds that existed during the time of the Jehol avifauna, but only ornithuromorphs—the group that includes all living birds—survived. What were some of the significant traits that kept them going?
No one has a good answer for that. From an anatomical point of view, there’s some evidence of modernity that you see in the skeletons of the birds that are very close to living birds in an evolutionary context, yet they’re not that far from some of the others that became extinct. It has been proposed that the ecology of the basal ornithuromorphs may have played a role in their survival. Many were aquatic birds and perhaps fared better with the catastrophic event that led to the extinction of many of the terrestrial land animals, like the large dinosaurs. But that’s a hypothesis. I think that there’s a lot of work that needs to be done to answer that question.

Numerous recent fossils, and many from the Jehol Biota, have provided pretty convincing evidence that birds descended from a group of carnivorous dinosaurs. What do you think is a common misconception that people still have about the relationship between birds and dinosaurs?
I think that, for most people, when they hear the word ‘dinosaur,’ they see a T-rex, a Triceratops. They don’t realize that there are many dinosaurs that have been found in the fossil record that are very bird-like, that would have been the size of a chicken or something like that, with feathers, some of them even with beaks, and without teeth. The fossil record tells you that these dinosaurs are the closest relatives to birds, and they are not like T-rex.

We talked about the 20 million-year fossil gap between Archaeopteryx and the Jehol avifauna. Are there any other major gaps that hinder our understanding of bird evolution?
You have a really long period of time—70–80 million years—after the Jehol in which we do have records of birds, but not with the preservation quality and abundance that we know for either the Jehol or a combination of sites that are in Europe and North America [that date back to around 50 million years]. The fossils are broken up, they’re single bones. Occasionally there’s a better preserved skeleton, but by no means are there the feathers, the soft tissues, all the complete skeletons and so on that you have in the other deposits. So that’s a big gap.

What, in your mind, is the most important revelation about bird evolution that the Jehol Biota has afforded us?
It’s hard to say just one. But the enormous paleontological wealth that we have in the Jehol—and that continues to grow—has provided us with a perspective of avian diversity during the Mesozoic that we had no idea existed. The quality of those fossils has provided information about the evolution of the biology of birds in ways that we did not know before, from aspects that relate to how these animals grew up, to their physiology, to their diet, to their flight performance, and to the way they looked. You’re looking at a critical junction in the story of how birds became birds.

This interview has been edited for space and clarity.