Published April 28, 2010
100 million years ago, a tremendous inland sea covered Texas and toothy, winged dinosaurs soared over what's now downtown Austin. A new fossil shows the range of such creatures -- and the end of the species.
Based on a fossil found in 2006, a new member of the winged lizard family commonly called pterodactyls has been identified. And while the dinosaur soared over the seas a stunning 95 million years ago, it's one of the youngest members of the family ever identified.
Pterosaurs ruled the skies from the late Triassic, more than 200 million years ago, to the end of the Cretaceous, about 65 million years ago, when they went extinct. They represent the earliest vertebrates capable of flying.
Evidence of these flying creatures has been rare in North America -- the newly identified Aetodactylus halli is only the second such dinosaur ever documented here, although toothed pterosaurs like it were common at the time elsewhere in the world.
Paleontologist Timothy S. Myers, who identified and named the toothy creature, describes it as the youngest members in the pterosaur family Ornithocheiridae. Myers, a postdoctoral fellow in the Huffington Department of Earth Sciences at Southern Methodist University, notes that it represents the end of the line for the species, at least those that flew over Texas.
"Discovery of another ornithocheirid species in Texas hints at a diversity of pterosaurs in the Cretaceous of North America that wasn't previously realized," Myers said. "Aetodactylus also represents one of the final occurrences of ornithocheirids," he explains, before the family evolved into toothless creatures.
The Texas specimen -- a nearly complete mandible with most of its 54 teeth missing -- is definitively younger than most other ornithocheirids found in Brazil, England and China, and five million years younger than the only other known North American relative.
Lance Hall, a member of the Dallas Paleontological Society, hunts fossils for a hobby and found the jaw bone in 2006 in North Texas. It was embedded in a soft, powdery shale exposed by excavation of a hillside next to a highway. The site was near the city of Mansfield, southwest of Dallas.
"I was scanning the exposure and noticed what at first I thought was a piece of oyster shell spanning across a small erosion valley," said Hall as he recalled the discovery. "Only about an inch or two was exposed. I almost passed it up thinking it was oyster, but realized it was more tan-colored, like bone. I started uncovering it and realized it was the jaw to something -- but I had no idea what. It was upside down and when I turned over the snout portion it was nothing but a long row of teeth sockets, which was very exciting."
Myers describes the new species in the latest issue of the Journal of Vertebrate Paleontology.
The 38.4-centimeter Aetodactylus jaw originally contained 54 slender, pointed teeth, but only two remain in their sockets, Myers said. The lower teeth were evenly spaced and extended far back along the jaw, covering nearly three quarters of the length of the mandible. The upper and lower teeth interlaced when the jaws were closed.
In Aetodactylus, changes in tooth size along the jaw follow a similar pattern to those of other ornithocheirids. However, Aetodactylus differs from all other ornithocheirids in that its jaws were thin and delicate, with a maximum thickness not much greater than 1 centimeter, Myers said. But the specimen does compare favorably with Boreopterus, a related pterosaur from the Early Cretaceous of China, in terms of the number of teeth present in the lower jaw, he said.
Myers has estimated the wingspan around roughly 3 meters, or about 9 feet, indicating Aetodactylus would have been a "medium-sized" pterosaur, he said. While it's not known how Aetodactylus died, at the time of death the reptile was flying over the sea and fell into the water, perhaps while fishing, Jacobs said.
Much of Texas was once submerged under the Western Interior Seaway. The massive sea split North America from the Gulf of Mexico to the Arctic Ocean.
"The ancient sea that covered Dallas provided the right conditions to preserve marine reptiles and other denizens of the deep, as well as the delicate bones of flying reptiles that fell from their flight to the water below," said Louis Jacobs, a professor in SMU's Huffington Department of Earth Sciences. "The rocks and fossils here record a time not well represented elsewhere in North America. That's why two species of ornithocheirids have been found here but nowhere else, and that's why discoveries of other new fossils are sure to be made by Lance Hall and other fossil lovers."