The 75-centimeter-long (30-inch-long) dinosaur likely hunted lizards, insects, and other small prey, possibly including mammals, according to Chiappe.
The authors argue that the animal provides evidence that some early dinosaurs developed feathers faster than their relatives.
"The fact that Juravenator lacks any of these feathers," they write, "indicates that these animals may have differed greatly in the extension of their feathery covering."
The most primitive known feathered dinosaur is Sinosauropteryx, which lived 120 to 150 million years ago.
"These animals look quite similar to one another, and that makes the lack of feathers in Juravenator most interesting," Chiappe said in an interview.
Fossils of several other early theropod dinosaurs have shown evidence of feathers, including a species described by Xu in 2004 that was an ancestor of Tyrannosaurus rex. (Read "New Dinosaur Discovered: T. Rex Cousin Had Feathers")
Theories differ as to why some dinosaurs that weren't birds had feathers. Some suggest that the plumage may have been used for keeping warm or for display.
"The earliest structures interpreted as feathers do not show aerodynamic specializations—they lack vanes and they don't form an airfoil," Chiappe said, explaining that the feathers could not have been used for flight.
"It is also possible that feathers evolved in the context of display to attract mates or to help the identification of suitable mates of a same species," he added.
Unique Feature
Because feathers are a unique feature, they are thought to be characteristic of many theropod dinosaurs, including tyrannosaurs.
Given its position in the dinosaur family tree, Juravenator "should bear filamentous feathers," Xing Xu said in an interview.
In seeking to explain its lack of feathers, he says it's possible the species didn't grow feathers on areas of its body where the skin was preserved.
The new species may have been more scaly than modern birds, which have scales only on their legs, he added.
A scaly Juravenator, Xu said, could be a "starting point for feather evolution."
But Chiappe's theory differs.
"If the absence of feathers in Juravenator can be explained by arguing that feathers had not yet evolved, the animal needs to be even more primitive than [tyrannosaurs], and I find this hard to believe," Chiappe said.
For his part, Xu notes that some "otherwise beautifully preserved" specimens of the first bird, Archaeopteryx, had been found in which feathers didn't survive.
"Feather preservation is extremely rare," he added.
But Chiappe says the new fossil didn't seem to bear any physical evidence of feathers, missing or not.
"You could expect to see follicle [in the skin], small pits that contain feather buds. We don't see them in Juravenator," Chiappe said.
Xu proposes that feathers may have evolved independently several times in similar groups of dinosaurs.
"Even after the discoveries of so many fossils of feathered dinosaurs, we still have only a patchy picture of feather distribution," he said.
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