This baby bird fossil gives a rare look at ancient avian development

The lack of complete breastbone suggests diversity in how some early birds developed

TINY FOSSIL The breastbone of this fossilized ancient baby bird was still mostly cartilage, rather than bone, when the bird died, meaning it wouldn’t yet have been able to fly, an analysis suggests.

F. Knoll et al/Nature Communications 2018

By Carolyn Gramling

March 7, 2018 at 11:42 am

This baby bird had barely hatched before it died 127 million years ago — and its lack of fully developed bony breastbone, or sternum, suggests it couldn’t yet fly. The tiny fossil, just a few centimeters long, is giving paleontologists a rare window into the early development of a group of extinct birds called Enantiornithes, researchers report March 5 in Nature Communications.

Previous studies of juvenile Enantiornithes have shown that the sternums of these birds ossified in a pattern different from modern and other ancient birds. The sternum’s ossification — a process in which the cartilage is replaced by bone — is a prerequisite to withstand the stresses of flight. But which parts of the sternum fuse first varies widely among modern birds. Those patterns are reflected in modern birds’ life histories, such as how soon birds can fly and how long they rely on their parents after hatching.

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illustration of a baby Enantiornithes — **LANDLOCKED** A baby *Enantiornithes*, which might have looked like this artist’s illustration, was born about 127 million years ago. The hatchling would have been less than 5 centimeters in length, roughly the size of a large cockroach. Raúl Martín

Similar diversity existed in how Enantiornithes developed too, the new study suggests. The baby bird’s sternum was still mostly cartilage at death, but some parts were beginning to turn to bone, which fossilized. That ossification pattern differed markedly from patterns in other known juvenile Enantiornithes, the researchers found.

It’s harder to say how these developmental features might have related to behavior. Although the baby bird couldn’t yet fly, it still might have been able to leave the nest. That’s also true of certain modern birds: Some plover chicks can walk and feed themselves shortly after hatching, but take a little longer to fly.