A new dinosaur fossil find with unusual four-digit hands added new evidence to unravel one of the mysteries that how birds descended from dinosaurs.
Xu Xing, a principal investigator at the Chinese Academy of Sciences (CAS) Institute of Vertebrate Paleontology and Paleoanthropology, led an international team found to show the dinosaur-to-bird evidences at the latest issue of the academic journal of Nature, published Thursday.
Xu's team discovered two skeletons of a beaked, plant-eating dinosaur, which they named Limusaurus, in the Junggar Basin in Xinjiang Uygur Autonomous Region, northwest China.
The new fossil, belonging to the theropods, had four digits, equal to human's thumb, index, middle and ring fingers. Its "thumb" became greatly reduced and the size of its index finger increased.
Paleontologists who supported the dinosaur-bird theory long faced a problem on fingers. Previous fossils of theropod dinosaurs showed earlier dinosaurs had five fingers and later ones gradually lost their ring finger and little finger.
The wings of birds developed from the forelimbs of their ancestors and biologists found skeletal traces of former fingers, which were index, middle and ring ones.
The conflicting finds of modern birds and dinosaur fossils became one of the most important pieces of evidence against the dinosaur-bird hypothesis.
"The new limusaurus indicated a different pattern of finger reduction. It was the thumb that was lost, instead of the ring finger," Xu told Xinhua Thursday in an interview.
"This new animal is fascinating by itself, and when placed into an evolutionary context it offers intriguing evidence about how the hand of dinosaurs evolved into the hand of birds," said James Clark, co-author of the paper who teaches at the George Washington University in Washington D.C..
The team reexamined hand skeletons of some previous fossils of theropods. "We realized the pattern of their finger reduction was much more complicated than people had thought," Xu said.
They believed the changes seen in limusaurus represented the beginning of a trend.
They also presented a new hypothesis to explain why advanced theropods, from which birds were believed to have evolved, had the second, third and fourth fingers but they looked like the first three ones, as some fossils showed.
"Homeotic changes might happen on dinosaurs, which means the index finger has the feature of a thumb," Xu said. "Such changes happened on humans. For example, a person usually has seven cervical vertebraes but, for some people, the first thoracic vertebrae has the feature of cervical ones."
Alan Feduccia, one key critic of the dinosaur-to-bird theory, said to Xinhua, "If confirmed, this would be the only early dinosaur with a hand composed of the middle three digits, which isthe same pattern as that of modern birds, albeit dramatically different in overall hand morphology, as well as having a highly abbreviated forelimb."
"The new fossil answers many questions and raises still another set of unknowns," the University of North Carolina at Chapel Hill paleobiologist said.
Guenter Wagner, a renowned Yale evolutionary biologist, responded to a Xinhua email interview, "There is very strong developmental evidence that there was a homeotic change in digit identity in the evolution of the bird lineage.
"I find it intriguing that there is more similarity to the 2,3,4 digits proximal, and more similarity to 1, 2, 3 digit identities distally," he said.
Wagner, however, remained neutral on the question of whether the morphology of this animal is a derived feature that may not be informative of the stages the bird lineage went through.
Mark Norell, chairman and curator-in-charge of Fossil Reptiles, Amphibians, and Birds with the American Museum of Natural History, agreed with Xu and his colleagues that the homeotic change is possible and an explanation for the change pattern of dinosaur fingers. "But the theory is difficult to test," he told Xinhua also in an email.
The new dinosaur, living about 159 million years ago, lacked teeth and had a bird like beak and gizzard stones that helped break plants down for digestion. But it is an early member of a carnivorous theropod dinosaur family called ceratosaurs.
"This paper provides evidence congruent with this theory. Nevertheless, the findings indicate that the evolution of birds from other dinosaurs was a complex pattern of changes," Norell said.
"This find will fill in important details as we do not yet fully understand how the transformation took place, not whether the transformation happened," Wagner said.