Fossil fills a critical gap in avian evolution and offers evidence concerning the early evolution of birds
New York….January 10, 2001….Mark A. Norell, curator and chair of the Division of Paleontology at the American Museum of Natural History, and Julia Clarke, doctoral candidate in paleontology at Yale University, announced this week in Nature the discovery of a new dinosaur in the Gobi Desert of Mongolia. Named Apsaravis ukhaana, the fossil is the best-preserved specimen of a Mesozoic ornithurine (bird of modern aspects) bird found in over a century, and offers critical insight into a key stage of avian evolution, near the time when all living bird groups diversified. The fossil, which is about 80 million years old, was collected in 1998 by the American Museum of Natural History/Mongolian Academy of Sciences Paleontological Expedition at Ukhaa Tolgod, the site that has proved to be one of the world’s richest locations for vertebrate fossils from near the end of the age of dinosaurs.
Apsaravis ukhaana, the size of a small pigeon, was found buried in sandstone deposited from adjacent sand dunes, and yields a great deal of data important for assessing the early evolution of modern birds. Very few bird fossils have been found at Ukhaa Tolgod, which is virtually unparalleled in the extraordinary preservation of the hundreds of fossil mammals, lizards, and small dinosaur specimens.
Through phylogenetic analysis of the fossil, Dr. Norell and Ms. Clarke found strong evidence refuting the theory that early birds were divided into two groups — the Saururae, thought to be the dominant landbirds of the Cretaceous period, and the Ornithurae, thought to be transitional shorebirds restricted to near-shore environments and shorebird ecologies. The analysis by Dr. Norell and Ms. Clarke reveals, in fact, that early modern birds were not restricted to a near shore marine or wading bird habitat, but were, rather, small terrestrial flyers. Discovered in continental (not near-shore or marine) deposits, Apsaravis displays many characters placing it closely to Aves, or modern birds, yet it also retains characters common to earlier theropod species. It is the best representation yet of what an early modern bird may have been like: it is clearly from a terrestrial habitat and was probably a strong flyer.
Apsaravis offers insight into the assembly of the modern flight apparatus. While Archaeopteryx and most other primitive Mesozoic birds are considered to have been true flyers, many of the adaptations that are related to flight in modern birds were not present in them. Apsaravis has an important feature — the extensor process — which allows the wing to extend as in modern birds; an extensor process is not present in more primitive flyers. Characteristics of the fossil bird’s hand suggest that flight originated not as a single innovation, but was an accrual of evolutionary advances that occurred well after flight arose phylogenetically. While there is considerable morphological evidence that many aspects of the movements used in powered flight were present before the evolution of flight, there is no morphological evidence of avian coordinated extension — a key part of the avian flight stroke — phylogenetically before Apsaravis. The name of this type specimen is derived from the Sanskrit Apsara, or winged consorts prominent in Buddhist and Hindu art, and the Greek aves, or bird; Ukhaana refers to Ukhaa Tolgod. Apsaravis ukhaana is part of the collection of the Mongolian Academy of Sciences, to which it will be returned after it is studied in the United States.
Journal
Nature