Feathered fossil still stirs debate

Pity poor little Caudipteryx, an animal that undoubtedly would be suffering an identity crisis if it were still around.

Researchers who first described the 120-million-year-old fossils of the turkey-size species in 1998 claimed the animal’s ancestors were meat-eating dinosaurs (SN: 6/27/98, p. 404: https://www.sciencenews.org/sn_arc98/6_27_98/fob1.htm). Even though Caudipteryx obviously had feathers on its short tail and forelimbs, it lacked many other typical features of birds. Other scientists vehemently argued that the species was neither a dinosaur nor a link between dinosaurs and birds. Instead, they claimed, it was a flightless descendent of more-ancient birds.

More than 2 years later, the debate still rages about where to place the animal in life’s family tree. A new analysis presented in the Aug. 17 Nature, however, seems to bolster the view that Caudipteryx was a bird has-been and not a bird wanna-be.

A team of researchers led by Terry D. Jones, now a paleobiologist at Stephen F. Austin State University in Nacogdoches, Texas, measured fossils and used previously published data to calculate the ratio of leg length to body length for 38 types of bipedal dinosaurs. They did the same for 24 types of flightless birds, including living species, such as roadrunners and ostriches, and extinct ones, such as moas and elephant birds.

Jones’ team found that, as a group, flightless birds had significantly longer legs than bipedal dinosaurs with the same body length. When the researchers examined Caudipteryx, they found that its leg proportions matched those of flightless birds and were distinctly different from those of the bipedal dinosaurs.

Other clues also suggest that Caudipteryx was a flightless bird, Jones says. Almost all bipedal dinosaurs have a long tail to counterbalance the weight of their upper body, and they typically walk by swinging the entire leg about the hip joint.

On the other hand, Jones says, Caudipteryx has a very short tail—possibly shorter than that of any bipedal dinosaur. As a result, the animal’s center of gravity was located forward of the hips, a typical body structure of flightless birds, he notes. When such birds stride, they typically swing only the portion of the leg below the knee while holding their upper legs motionless.

Jones also points out that Caudipteryx‘s leg bones lack indications that the upper leg muscles were major sources of locomotion. Jones contends it’s unlikely that Caudipteryx‘s ancestors would have abandoned a body structure and style of walking that proved successful among all other bipedal dinosaurs.

Instead, he says, the most likely explanation for the animal’s distinct body proportions is that Caudipteryx is a bird that for some reason lost the ability to fly. Says Jones: “It’s almost ludicrous to think that this animal reorganized its body so that it could run like a bird.”

Thomas R. Holtz Jr., a vertebrate paleontologist at the University of Maryland at College Park, describes the analysis as “interesting but somewhat less than convincing.”

“The body proportions, short tail, and other features of the upper leg bones show only that Caudipteryx was a transitional animal with a very ‘birdy’ gait,” Holtz says. It will be interesting, he adds, to see whether recently discovered fossils of animals related to Caudipteryx also show the relatively longer legs that Jones associates with flightless birds.