MESA, ARIZONA--There's no shortage of anatomical evidence that birds evolved from dinosaurs. But paleontologists have still had to contend with a slightly embarrassing gap in the fossil record: The oldest known bird fossil, Archaeopteryx, is much older than fossils of its closest dinosaurian relatives. Now, that gap has been plugged.
Archaeopteryx is known from several 150-million-year old specimens found in Upper Jurassic limestone in Germany. Aspects of its skeleton--such as its bony tail, claws, and teeth--are similar to those of a group of predatory dinosaurs called maniraptorans. However, the oldest fossils of those agile dinos were discovered in rocks only 125 million years old, leaving paleontologists to infer that some older relatives of these maniraptorans gave rise to Archaeopteryx and all other birds.
One of those older dinosaur relatives, a contemporary of Archaeopteryx, was described here yesterday at the annual meeting of the Society of Vertebrate Paleontology. Collected in 2000 from Upper Jurassic rock layers in eastern Wyoming, the fossil's bones were so delicate that William Wahl, of the Wyoming Dinosaur Center in Thermopolis, needed several years to prepare them.
About 60% of the skeleton is preserved, including the skull. A groove along the jaw was one of several features that allowed Scott Hartman, also of the Wyoming center, to identify the skeleton as that of a kind of maniraptoran called a troodontid, but the researchers have not yet named the new species. They estimate that the creature was about 1.5 meters long. Like the later troodontids, the proportions of the feet and limbs suggest it was terrestrial rather than tree-dwelling, further supporting the idea that flight evolved from the ground up, Hartman says.
Paleontologists had expected such fossils would eventually be discovered, and they're pleased that this one begins to fill the time gap between bird fossils and their closest dinosaur relatives. "This is seriously cool," says Jim Kirkland of the Utah Geological Survey, who has studied birdlike dinosaurs.