Encased in a plaster jacket, the bones had remained hidden in the American Museum of Natural History for decades. Recently, though, Norell and study coauthor Sterling Nesbitt, a graduate student, reexamined the long-warehoused fossils.
The pair soon spotted the distinctive "crocodile" ankle that first alerted them to the fossil's potential importance.
Ankle aside, Effigia had large eyes, a long tail, and a toothless beaknot unlike the ostrich dinosaurs.
Effigia also walked on two feet, unlike modern crocs.
These physical similarities suggest that Effigia and the ornithomimid dinosaurs evolved similarly during two different eras, the scientists say.
The fossil record shows that many different features have been reinvented time and again in different species. But the Effigia example is a bit surprising.
"We expect things like feeding adaptations to be convergentthings that we can assign a specific functionality to, like a duck bill," Norell said.
"But when a pelvis looks pretty much the same in two very different groups, that opens up a set of very fascinating questions."
The theory of convergence in evolution suggests that Effigia lived in a similar habitat to that occupied by the ornithomimid dinosaurs some 80 million years later.
The two also likely shared many of the same pressures.
Both Effigia and the ostrich dinosaurs would have evolved from a common ancestor that lived long before, the study says.
Croc Decline Key to Dino Rise?
Modern crocodiles are but one remnant of what was once a far more diverse croc family.
"Today we think of crocodiles as looking basically the same," said Nesbitt, of the American Museum of Natural History and Columbia University's Lamont-Doherty Earth Observatory. "But in their history they took on a wide variety of different body plans."
"Some looked like reptilian armadillos or cats, and others looked like little dinosaurs," Nesbitt said.
The crocodilian family may have been at its peak during the Triassic period.
"Toward the end of the Triassic period you have this crazy diversification of these crocodile relatives, including this animal," Nesbitt said.
"It was really the heyday of the crocodile-like animals, but the only lineage to really make it out of the Triassic was the lineage that led to modern crocodiles."
Nesbitt explains that dinosaur evolution was likely in its infancy during this period.
"We're not 100 percent sure what the story is," he cautioned.
"Now it seems that dinosaurs were there with these animals, they were part of the fauna. But they remained pretty small and not very diverse.
"We don't see the typical museum type dinosaurs until the Jurassic. Nothing is really happening [in dinosaur evolution] until after these crocodile relatives are gone," Nesbitt added.
It may be that Effigia-like animals were widespread but dinosaurs were comparatively rare during this era, Norell, the museum curator, explained.
If so, dinosaur evolution might have blossomed only after most crocodile-like animals became extinct. Those extinctions might have left an attractive environmental niche for the dinosaurs.
The theory seems to be backed by the abundance of Effigia-like animals found from North America's late Triassic period.
Many fossils once thought to be dinosaurs turn out, on closer inspection, to be crocodile-like animals that, like Effigia, only look like dinosaurs.
Many of these croc-like animals "are difficult to identify without nearly complete specimens, especially skulls," Nesbitt said. "If you just have a vertebra, for example, you can't tell them apart."
Free E-Mail 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