With partial funding from the National Geographic Society, Irmis and his colleague Sterling Nesbitt of the American Museum of Natural History in New York excavated at the site last summer.
(National Geographic News is part of the National Geographic Society.)
Among their finds was a new species of dinosaur precursor that they named Dromomeron romeri.
The creature was very small, no more than three to four feet (about a meter) long, Irmis said. D. romeri's only known close relative was found in Argentina in rocks at least 15 million years older.
Another as yet unnamed dinosaur relative from the site is related to Silesaurus, a fossil found in Poland four years ago.
Paleontologists initially thought Silesaurus was a dinosaur, but it has since been demoted to dinosaurlike status.
Within the same 10-by-20-foot (3-by-6-meter) plot, the team also found bones of true dinosaurs.
One unnamed dino species is closely related to the well-known Ceolophysis, tiny dinosaurs that were at most 9 to 10 feet (3 meters) long, were bipedal, and were probably carnivorous, Irmis said.
Despite the dino diversity, most of the bones found at the site were from fish, amphibians, and a different group of reptiles.
This fits the picture of the known Triassic landscape, which was dominated by large carnivorous beasts called rauisuchians and the ancestors of crocodiles.
But researchers have long wondered why this branch of reptiles withered to just one line of early crocodiles by the beginning of the Jurassic, about 200 million years ago.
The scientists hope the fossils will finally shed light on why the dinosaurs of the Triassic and their relatives, which were small and rare by comparison, eventually took over.
"If you'd gone back to that time, you wouldn't have believed [dinosaurs] were going to rule the Earth," said Michael Benton, a paleontologist at Bristol University in England who was not part of the expedition.
The latest find will help make clear how dinosaurs and their close relatives fit into the Triassic world, Benton said.
"That gives us a more truthful picture of the origin of the dinosaurs."
Free Email News Updates
Sign up for our Inside National Geographic newsletter. Every two weeks we'll send you our top stories and pictures (see sample).
SOURCES AND RELATED WEB SITES