Illustration by Julius T Csotonyi / HKU Vertebrate Palaeontology Laboratory
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A baby bird known as an enantiornithine leaves the nest shortly after birth in an illustration of lakeshore life about 125 million years ago.

Illustration by Julius T Csotonyi / HKU Vertebrate Palaeontology Laboratory

Prehistoric birds were born ready to run, fossil shows

A new look at a very old animal backs up the notion that dinosaur-era birds had to be quick on their feet.

By shining lasers on one of the smallest avian fossils ever discovered, researchers have revealed that the ancient chick was likely born ready to run and maybe even fly—a discovery that offers fresh insights into the nesting behaviors of prehistoric birds.

This strategy may have proved handy 127 million years ago in the early Cretaceous period, when hatchlings sometimes needed to make a swift exit to avoid a plethora of small carnivorous dinosaurs ready to snap them up as a snack. (Also this week, researchers found the first fossil bird preserved with an unlaid egg.)

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The chick, which measures just 1.2 inches in length, is a kind of bird from a prehistoric lineage called the enantiornithines, which had teeth in their beaks and small wing claws, but otherwise looked very similar to modern birds. They were the most common type of bird during the Cretaceous period. (See a baby enantiornithine found trapped in amber.)

This specimen was found more than a decade ago in northern Spain and was revealed to the world last year. At the time, the study of the fossil found no evidence of feathers, and that team came to the conclusion that the chick had poorly developed bones and was unlikely to have been able to fly shortly after hatching.

Up to two thirds of birds today, such as pigeons, falcons, and parrots, have a so-called altricial nesting strategy, in which hatchlings emerge immobile with their eyes shut and remain in the nest under their parents’ care. (See a rare picture of a baby pigeon.)

“If the chick had no feathers, it tells us that this hatchling was altricial, which means that when it was born, it was naked and helpless and relied on its parents to look after it,” says Michael Pittman, coauthor of the new analysis detailed this week in the journal Scientific Reports.

Lighting up feathers

Other ancient birds from the same fossil site in Spain, known as Las Hoyas, do preserve feathers. So when Pittman and his Foundation for Scientific Advancement colleague Thomas Kaye saw the specimen, they began to wonder if it might have had feathers after all, but they were just poorly preserved and not easily visible.

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“We wanted to scan it, because we know that these birds from Las Hoyas preserve feathers and the fact this hatchling has no feathers made me wonder,” Pittman says. “It has brown marks around it that look like iron stains on a regular slab of rock … We wanted to look at these to see if they were hiding something.”

Along with colleagues from the Autonomous University of Madrid, the pair employed a method called laser-stimulated fluorescence, which uses a laser to excite atoms in the surface of a fossil, making some substances glow and revealing hidden features invisible to the naked eye.

Though previous analyses of the fossil using ultraviolet light and synchrotron beams had failed to find any feathers, the new method detected wisps around the chick’s neck and wings suggestive of downy feathers, as well as what appears to be a much bigger feather with a central vane on the left wing.

The size of that feather makes it possible that the chick’s “flight feathers were already well developed at the time of birth,” argues Madrid-based co-author Jesús Marugán-Lobón.

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The 125-million-year-old fossil bird was preserved in two slabs of rock, seen here at either side under normal light. When researchers found the fossil at the Las Hoyas site in Spain, initial analysis didn't show any signs of feathers. But seeing the fossil with laser-stimulated fluorescence (middle) revealed the presence of downy covering after all.

Born ready

Finding feathers on the Las Hoyas chick suggests that rather than being altricial, as a lack of feathers might have suggested, it instead used the precocial nesting strategy seen today in some modern birds, such as ducks and chickens, which hatch fully feathered.

“This is in line with what we know about enantiornithine chicks in general,” Pittman says. “The fossils all indicate this out-of-the-egg running, possibly flying, lifestyle and point away from a conclusion of an altricial lifestyle.”

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Today, baby chickens also hatch feathered and ready to move.

“Virtually all studies of eggs, nests, and embryos show Cretaceous birds had precocial young and breeding strategies that were fundamentally different from their [living] relatives,” says Gerald Mayr, an ornithologist and expert on bird evolution at the Senckenberg Research Institute in Frankfurt, Germany, who was not involved in the research.

For instance, fossilized clutches reveal that enantiornithine eggswere buried and not incubated, and that young enantiornithines were “super precocial,” hatching at a very advanced stage of development, he says.

The discovery also underlines the fact that “just because feathers aren’t preserved, doesn’t mean they weren’t there,” comments Jingmai O’Connor, a paleontologist at the Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences in Beijing.

In this case, the authors were “able to show feathers are there and you just couldn’t see them, which might also be the case for a lot of other fossil specimens,” she says, noting the find adds to evidence that “all enantiornithines were super-precocial, born fully fledged and ready to fly.”

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