This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American
The dinosaur group that over the eons has brought us Tyrannosaurus rex and modern birds just got a new member in one of its more perplexing, birdlike families, the alvarezsauridae.
This new 160-million-year-old alvarezsauroid, named Haplocheirus sollers (or "simple, skillful hand"), is described in a new study, which will be published January 29 in the journal Science.
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Haplocheirus' hands appeared to have been skillful, indeed. With a single massive claw on each forearm, these bipedal carnivores were probably expert diggers, according to the researchers behind the new study.
The fossilized skeleton was discovered in 2004, having been well preserved in three dimensions in what is now the Junggar Basin in China. The new dinosaur's age makes it a striking 63 million years older than any of its known close relatives—and 15 million years older than erstwhile bird Archaeopteryx.
Alvarezsauroid dinosaurs, with their short arms, three toes and keeled sternums also look awfully similar to birds. "When we first described members of this group, we thought, 'Oh, bingo! They’re primitive birds,'" lead study author Jonah Choiniere, a PhD candidate at George Washington University, said in an interview with Science.
But as new specimens were found and described, many researchers began to suspect that the small, two-legged alvarezsauroid dinosaurs were of a different theropod line than modern birds.
"What Haplocheirus definitively shows," said Choiniere, "is that alvarezsauroids aren't birds. They're theropod dinosaurs, so they're closely related to birds, but they aren't actually an offshoot of birds themselves."
The ancient age of Haplocheirus "really extends our fossil records," Choiniere said in the interview with Science. As he noted in a prepared statement, "Haplocheirus is a transitional fossil because it shows an early evolutionary step in how the bizarre hands of later alvarezsaurs evolved from earlier predatory dinosaurs." By 160 million years ago, the new genus already had the short arms and long middle claw typical of later alvarezsauroids, but its arms are longer and the claw is a little shorter than the later, Cretaceous species.
The dinosaur would have been about six to seven and a half feet long, the authors of the study wrote, which is quite a bit larger than later alvarezsauroids. Such a size early in the lineage "suggests a pattern of miniaturization for the Alvarezsauroidae," the researchers wrote in their paper—a trend that they note is "relatively rare in dinosaurs."
Thus Haplocheirus is a new glimpse into some of the traits that would later come to define other members of its group. "It's a transition," Choiniere said in the Science interview. "We're seeing the first step in an evolutionary process."
Image or H. sollers reconstruction courtesy of Portia Sloan
