New Haven, Conn. – 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.
The nearly complete specimen of the small 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 journal Nature, was analyzed by Julia Clarke, a doctoral candidate in vertebrate paleontology atYale University, 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," said Clarke, who is in Yale’s Department of Geology. "This fossil is just outside the group or ‘clade’ that includes the decendants 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."
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. These specimens of Ichthyornis are part of the permanent collection at Yale’s Peabody Museum of Natural History.
Clarke said 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 Mesozoic, but went 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 said. "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 not restricted to a near shore or marine habitat and wading bird ecology , but that they were already occupying diverse environments and ecologies as different as those of seagulls and pigeons today.
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 down stroke.
"This automated part of the flight stroke was thought to be present before the origin of flight in other non-flighted theropod dinosaurs," Clarke said. "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 flight as we know it today in living birds. "This particular muscle arrangement is present in all flighted living birds today," Clarke said, "but we have no evidence of it evolutionarily before the new fossil."
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