127-Million-Year-Old Baby Bird Fossil Holds Clues To Avian Evolution During Dinosaur Era

ARTIST's IMPRESSION was reconstructed with plumage hypothesized for perinate enantiornithines, though the fossil showed no signs of plumage. RAÚL MARTÍN. 

A baby bird fossil no larger than a pinky finger is providing scientists with a grander view of the avian world in the Age of the Dinosaurs. The creature was found in the wetland deposits of the Las Hoyas fossil site of Spain.

To find the trace of any once-living entity from a past geological age requires some level of serendipity and this discovery is no exception – particularly considering its diminutive size.

The prehistoric chick, which dates back to between 250 and 65 million years ago, is nearly complete, with only its feet, most of the hands, and tail tip missing. The skull is also partially crushed. That degree of degradation might not sound promising, but for paleontologists whose work it is to dig up million-year-old creatures from the bogs of time, it is actually remarkably well preserved.

The researchers estimate that the hatchling was a whopping 5 centimeters (2 inches) in length and weighed 85 grams (3 ounces). This makes it one of the smallest avian fossils ever discovered from the Mesozoic Era. 

However, what makes this discovery a paleontological treasure is not just its teensy size but the fact that it died soon after birth, when its bones were still in the process of development (ossification). The critter fossilized during a little-seen, yet critical stage in the skeletal formation of a bird.

"The stage of this fossil is important because fossils of baby birds from the Age of Dinosaurs provide key information about the early development of primitive birds and yet, they are extremely rare," co-author Luis Chiappe, from the LA Museum of Natural History, told IFLScience.

Phosphorous mapping image and photo of fossil. Dr Fabien Knoll

To see the microstructures of the bones in such fine detail, lead author Dr Fabian Knoll and his team turned to synchrotron radiation. This technology uses "very powerful X-rays that allows for the best imaging resolution using non-intrusive methods," added Chiappe.

They found that the chick’s sternum was still cartilage, not bone, which suggests it was unable to fly. The number of free caudal vertebrae also differed from other juveniles of the same clade.

Based on a few other young fossils, the ossification patterns suggest that the developmental strategies of the ancient avians were more diverse than previously thought. In particular, they varied in size and in the tempo of their skeletal maturation. The chick was found near three other species of Enantiornithes, but to which species this little baby belongs to is difficult to ascertain.

Such research, published in Nature Communications, helps build a picture of how the birds lived during this ancient era. Previous authors have argued that this group of extinct hatchlings were highly precocial – born advanced enough to feed itself soon after birth. That's compared to altricial, which is when the young are relatively helpless for a time being. 

For now, the team say that the lack of bone development doesn’t indicate that it was precocial or altricial, as the line between the two is more of a spectrum.

"This new discovery, together with others from around the world, allows us to peek into the world of ancient birds that lived during the age of dinosaurs," said Chiappe. "It is amazing to realize how many of the features we see among living birds had already been developed more than 100 million years ago."

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