A new fossil has once again fired up the debate over the
theory of the dinosaur origin of birds. The
theory states that birds arose around 150 million years ago from fast-running,
warm-blooded dinosaurs, known as theropods.
The find was made in Liaoning province in northeastern China, an area
that has produced other non-avian theropod dinosaurs with feather-like
structures. The fossil, reported in
the April 26, 2001 issue of Nature, is that of a dromaeosaur, a group of small
theropods, which includes Velociraptor, made famous by Jurassic Park.
Although it obviously could not fly, it is distinctive due to what
appears to be a covering of downy feathers.
The discovery team was headed by Qiang Ji of the Chinese Academy of
Sciences in Beijing and Mark Norell of the American Museum of Natural History in
New York. These paleontologists and
others have pointed to the earlier finds of feathered theropods in the Yixian
Formation (dated at 125 million years ago) of Liaoning Province to support the
dinosaur-bird ancestral link. One, known as Sinosauropteryx, sported a downy fringe
along its neck and backbone. Critics,
led by ornithologists such as Alan Feduccia, dismissed these as frayed internal
collagen fibers. Two others, dubbed
Caudipteryx and Protoarchaeopteryx, displayed what supporters
consider true feathers [see ScienceWatch, Jan./Feb. 1999 or "Birds(?)
of a Feather??" for a complete description of these fossils].
The latest find is that of an animal, about three feet
long, with a rigid tail, curved wrist bone and claw-like toe, all characteristic
of an ordinary dromoaeosaur. However,
the observer is struck by a delicate halo of what looks like down feathers
surrounding the animal, especially at the head. Microscopic examination has revealed that these fibers are
definitely attached to the skin. Many
other fine-structured fossils have been taken from the Yixian Formation, a
region formed from a series of lake deposits and layers of volcanic ash during
the Early Cretaceous age. The
fossil consists of two slabs, perfect impressions of the left and right halves
of the skeleton. Its head is large
for its size, leading the team to conclude it was a juvenile animal.
The appearance of a downy covering on an animal that clearly could not
fly supports the theory that feathers originally arose in warm-blooded dinosaurs
for insulation, and were adapted for flying only later on.
Dromaeosaur skeletons resemble primitive birds more so than
any other dinosaur, and as such they are considered to be their closest
relatives, sharing many features, most important of which is a furcula or
wishbone. In the past critics of
the theory have noted that no feathered dromaeosaurs were known.
So the fossil is a setback for them because now they have lost that piece
of their argument. However, diehards to the last, they contend that the
down-like imprints could be skin.
What is most amazing about this story is that through July 31 you can see this fossil on display (the original, not a copy), as I did at the American Museum of Natural History, and judge for yourself.*
*If
you can’t get to the museum and wish to see a photo of the fossil, visit the
museum’s website at: www.amnh.org/science/specials/dinobird.html.