Newly Discovered Fossil Bird Fills in Gap Between Dinosaurs and Modern Fliers

A skeleton from the Cretaceous found in Japan reveals an early bird with a tail nub resembling the avians of today

Birds are ancient creatures. Every hawk, sparrow, pigeon and penguin alive today has ancestral roots dating back to the Jurassic, when the first birds were just another form of raptor-like dinosaur. Dozens of fossils uncovered and described during the last three decades have illuminated much of this deep history, but the rock record can still yield surprises. A fossil recently found in Japan is one such unexpected avian that raises questions about what else may await discovery.

The skeleton, named Fukuipteryx prima, was described by Fukui Prefectural University paleontologist Takuya Imai and colleagues today in Communications Biology. And while numerous birds of similar geologic age have been named in the past few decades, the details of these bones and where they were found have experts a-flutter.

The 120 million-year-old fossil was discovered in the summer of 2013 while searching for fossils at Japan’s Kitadani Dinosaur Quarry. “One of my colleagues at Fukui Prefectural Dinosaur Museum spotted tiny bones in a block of siltstone,” Imai says. At the time, it wasn’t clear what creature the bones belonged to, but once the encasing rock was chipped away, the structure of the fossil became clear. The skeleton was an early bird, and an unusual one at that.

Small bodies and hollow bones have made birds relatively rare finds in the fossil record. Only a few unique fossil deposits, like China’s 125 million-year-old Jehol Biota or the United States’ 50 million-year-old Green River Formation, allow paleontologists to get a good look at ancient avians. To find a well-preserved fossil bird outside such places of exceptional preservation represents a noteworthy paleontological discovery, and Fukuipteryx in Japan adds another significant spot on the map for fossil birds.

More than that, the skeleton of Fukuipteryx is preserved in three dimensions, meaning the bird’s bones are close to their shape in life and have not been compressed over the course of time. “To be honest, we were not expecting to find such good material from a fossil bird at our site,” Imai says. The paleontologists had hoped for fragments and got most of a well-preserved skeleton. The fossil differs from those found in China’s Jehol Biota, which are smooshed and look like bony pancakes. Given that Fukuipteryx is roughly the same age as those flattened fliers, the skeleton offers experts a clear look at avian skeletal anatomy during the Early Cretaceous.

In overall form, Imai and coauthors write, Fukuipteryx looks very similar to some of the earliest birds that evolved about 30 million years earlier during the Jurassic. Fingers ending in claws, for example, is a trait Fukuipteryx shares with one of the earliest known birds, Archaeopteryx. But the tail of Fukuipteryx is short and ends in a skeletal structure called a pygostyle. The bony structure is an anchor point for muscle and tail feathers, seen in modern birds and considered an important trait that birds evolved along their transition from raptor-like dinosaurs to the fliers we know today.

The combination of characteristics put Fukuipteryx in an unexpected place among early birds. “Our analysis revealed it is the most primitive among the Early Cretaceous birds,” Imai says. Fukuipteryx shared a great deal in common with the earliest birds while having the flashy tail associated with more modern species.

Rather than being a strange case, Fukuipteryx underscores a common theme in evolution. “As early parts of evolutionary [diversification] become better sampled, it doesn’t surprise me when we see unexpected combinations of characteristics,” says Stony Brook University paleontologist Alan Turner. “Evolution rarely proceeds in a linear manner,” Turner adds, with features—like a pygostyle—sometimes showing up in combinations not seen before.

Future discoveries will test the idea, but this one bird may indicate that the early proliferation of birds through the late part of the Jurassic and early part of the Cretaceous took on more varied forms than experts now recognize. “I think as new localities are found with previously unsampled birds, we should expect surprises,” Turner says, adding that experts have only just scratched the surface of ancient bird diversity.

Fukuipteryx won’t be the last fossil flapper to surprise scientists. “I feel it is merely one of many currently unknown birds awaiting to be discovered in the future outside of China,” Imai says. There is an entire world of early birds waiting in the wings.