Dec. 18, 2006 An American Museum of Natural History paleontologist and his colleagues have named a new order of mammals based on their description of a fossil of a bat- or squirrel-sized Mesozoic mammal, called Volaticotherium antiquus (meaning "ancient gliding beast"), which was capable of gliding flight.
The rock beds that yielded the fossil date to at least 125 million years ago, so the new fossil extends the earliest record for gliding flight in mammals by 70 million years or more and indicates that mammals experimented with gliding flight and aerial life at about the same time that birds first took to the skies, possibly even earlier. The team also completed an analysis of the evolutionary relationships among major groups of known Mesozoic mammals, which included the new data on V. antiquus.
The results, described in a new paper in the journal Nature, revealed that the gliding Mesozoic mammal represents a previously unknown and highly specialized group of mammals that the authors recognize as a new order of mammals. This is a previously unknown group and one of the most important discoveries or designations of a major mammalian group since Richard Owen's review of Mesozoic mammals in 1871.
The authors of the new paper are Jin Meng, Associate Curator in the Museum's Division of Paleontology; and his colleagues Yaoming Hu, Yuanqing Wang, Xiaolin Wang, and Chuankui Li, researchers at the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing.
The fossil of the gliding mammal, discovered in eastern Mongolia, preserves the animal's skeleton, as well as an impression of most of a large fold of skin membrane that stretched between the animal's fore and hind limbs, the direct evidence that the animal was adapted for gliding flight. The membrane, known as a patagium, served as an airfoil to support the small animal's weight and generated lift for it to travel horizontally through the air.
The fossil also preserves impressions of the fur that was found on the gliding membrane and on other parts of the body, one of the earliest records of the skin covering that is typical of mammals. The animal's limbs are elongated, which is typical for living gliding mammals. Longer bones allow for more skeletal and muscular support of a glider's airfoil.
The fossil vertebrae suggest that the animal had a long, stiff tail that served as a stabilizing rudder during gliding flight. The lengths of the animal's skull and skeleton suggest that V. antiquus weighed less than a pound, so it had a relatively light load in flight. Being lightweight and possessing a relatively large airfoil, V. antiquus was likely an agile glider, but perhaps not agile enough to chase insects as prey through the air.
Unlike living mammal gliders that are predominately herbivorous, the unique, highly specialized sharp teeth of V. antiquus provide evidence that this mammal was undoubtedly an insectivore, like most known Mesozoic mammals. Other features of the animal's limb and finger and toe bones indicate it was a climber, which allowed it to forage on trees and obtain height for gliding, as in the case of flying squirrels.
V. antiquus is the first known Mesozoic mammal capable of gliding flight, indicating that early mammals were more diverse in their early evolution than scientists had previously thought and also that unknown groups of mammals still remain for paleontologists to discover, buried in geologic time.
"This new evidence of gliding flight in early mammals is giving us a dramatically new picture of many of the animals that lived in the Age of Dinosaurs," Dr. Meng said. "Establishing a new order probably only happens once, if that, in the lifetime of a lucky paleomammalogist."
Prior to the description of V. antiquus, the earliest known gliding mammal was a rodent—known from a 30-million-year old fossil preserving the gliding membrane. The earliest confirmed record of bats, also aerial mammals but capable of powered flight rather than gliding flight, dates to about 51 million years ago. The newly discovered fossil shows that mammals experimented with aerial life at least 70 million years earlier than previously thought.
Although the new gliding mammal is comparable in size and shape to flying squirrels (which are members of Rodentia, an order of placental mammals), V. antiquus is not a direct ancestor of these or any other living mammals, including flying marsupials, flying lemurs, or bats. Instead, V. antiquus provides evidence for the independent origin of flight in this now-extinct lineage of mammals.
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