A fossilized bird embryo dating to 121 million years ago has features that suggest that the species’ young, unlike those of many modern birds, could walk about and feed themselves right after they hatched.
The specimen, tucked in an oval shape about 35 millimeters long, was extracted from a layer of shale in northeastern China. Although the relic doesn’t include any evidence of eggshell, the paleontologists who describe the find in the Oct. 22 Science contend that it represents the embryo of a bird that’s almost ready to hatch.
For one thing, the tiny skeleton is tightly curled in a posture that matches the position assumed by embryos of modern birds just before they break out of their shells, say Zhonghe Zhou and Fucheng Zhang of the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing. If the ancient bird had already hatched, its remains wouldn’t have been preserved in such a compact posture. The vanes of the fossil’s feathers were tightly furled, another clue that the creature was still eggbound, the researchers suggest.
The nearly complete feather structures, plus the embryo’s large skull and well-ossified bones, indicate that hatchlings of the species were precocial, or able to move and feed independently soon after they emerged from their eggs.
Previously, paleontologists had unearthed fossil embryos in Argentina that didn’t preserve any feathers. Zhou and Zhang contend that those specimens, which date from about 75 million years ago, and the new find together bolster the notion that early birds were precocial but then evolved to produce chicks that usually hatch helpless and featherless and are sometimes coated with down.
The scientists who described those Argentine fossils in 2002 have some reservations about that interpretation. Their fossils could represent embryos that hadn’t reached a developmental stage in which they would have sported feathers, says Mary H. Schweitzer of North Carolina State University in Raleigh. Moreover, she notes, the coarse-grained material that surrounded those South American fossils might not have preserved evidence of feathers even if they had been present.
The new remains, though of an embryo, reveal some clues about habits of the adult. For example, the embryo’s claws were long and curved, a hint that the bird would have spent much of its time in trees rather than on the ground.
Getting more specific than that might not be possible. It’s hard to match embryos and hatchlings to a particular species of bird because “they pretty much all look the same,” says paleontologist Luis M. Chiappe of the Natural History Museum of Los Angeles County in California.