Yale Bulletin and Calendar

January 26, 2001Volume 29, Number 16

Analysis of this fossil recently found in the Gobi Desert of southern Mongolia revealed surprising information about ancient birds.

Fossil sheds light on rare branch of birds' evolutionary tree

The discovery in Mongolia of the fossil of a new bird, Apsaravis Ukhaana, that lived about 80 million years ago, sheds new light on the evolution of birds, say scientists from Yale and the American Museum of Natural History.

The nearly complete specimen of the pigeon-sized bird was found at the locality Ukhaa Tolgod in the Gobi Desert of southern Mongolia as part of the ongoing joint expeditions of the American Museum of Natural History in New York and the Mongolian Academy of Sciences.

The find, announced this week in the Jan. 10 issue of the journal Nature, was analyzed by Julia Clarke, a doctoral candidate in vertebrate paleontology at Yale, and Mark Norell, chair and curator of the Division of Paleontology at the American Museum of Natural History. The discovery is particularly important because the new bird comes from a part of the evolutionary tree close to the origin of all living birds and that is not well represented in the fossil record.

"All of the birds living today have a most recent common ancestor that they share," says Clarke, who is in Yale's Department of Geology. "This fossil is just outside the group, or 'clade,' that includes the descendants of that common ancestor. It is the best preserved specimen of a fossil from close to the radiation of all living birds discovered in over 100 years."

Specifically, the finding is the most significant from this part of the avian tree since the discovery of specimens of the closest relative to living birds, Ichthyornis, first discovered more than 100 years ago in Kansas, she adds. These specimens of Ichthyornis are part of the permanent collection at Yale's Peabody Museum of Natural History.

Clarke says the fossil dispels the notion that the Ornithurines, or nearest relatives to today's existing birds, were restricted to near-shore environments while the interior was dominated by another lineage of fossil birds known as the "opposite birds" or Enantiornithes. The Enantiornithes are comparatively abundant in terrestrial deposits in the fossil record of the Mesozoic era, but became extinct at the end of the Cretaceous period, 65 million years ago.

"The new find suggests there was no reason to believe that there was a restriction of the nearest relative of living birds to coastlines by this lineage of 'opposite birds,'" Clarke says. "Here is a near relative of living birds that is from a locality in a continental interior that was buried adjacent to abundant sand dunes."

The Apsaravis specimen from these terrestrial deposits reveals that the nearest relatives of the lineage including living birds were already occupying diverse environments and ecologies as different as those of seagulls and pigeons today, she notes.

The finding also has important implications in the current thinking about the evolution of birds after the origin of flight. The new fossil has a feature on a bone of its hand that indicates a muscle arrangement connecting the movement of the hand to movement of the forearm. This muscle arrangement, seen in living birds, performs a key role in transition from the upstroke to the downstroke.

"This automated part of the flight stroke was thought to be present before the origin of flight in other non-flighted theropod dinosaurs," Clarke says. "While other parts of the flight stroke were present in these other theropods, this part of the flight stroke is first clearly present only long after flight itself evolved."

Though earlier birds actively propelled themselves through the air, their flight was different from the flight of today's birds. "This particular muscle arrangement is present in all flighted living birds today," Clarke says, "but we have no evidence of it evolutionarily before the new fossil."

--- By Jacqueline Weaver


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