Shorebirds from the peak of the dinosaur era shuffled and foraged just like today's birds, fossils show.
Bird tracks from the height of the Age of Dinosaurs show behaviors identical to modern birds.
One explanation is that the ancient shorebirds are the ancestors of today's shorebirds.
Another possibility is that the early Cretaceous and modern shorebirds underwent convergent evolution.
Very early shorebirds that pecked the ground alongside dinosaurs were already behaving exactly like their modern counterparts, according to 110 million-year-old rock fossils.
Today's shorebirds shuffle along muddy flats pecking and probing for invertebrates to eat. The marks they leave behind are the same as those found in ancient rocks in South Korea.
"These tell us what animals were doing," said paleontologist Amanda Falk of the University of Kansas in Lawrence. "The behaviors are pretty much identical to modern plovers and sandpipers."
Falk and her advisor Stephen T. Hasiotis will be presenting the discovery on Oct. 31 at the meeting of the Geological Society of America in Denver.
The marks and tracks found in the rocks of the Haman formation are from two different species of birds, or perhaps a single species working at two different speeds.
Other bird "trace" fossils have been found in Utah and Alaska, but none from so far back as the early Cretaceous -- at the height of the age of dinosaurs.
Even the tracks alone say a lot about the feeding behavior of the ancient birds, commented paleontologist Martin Lockley of the University of Colorado in Denver.
"Herons do a shuffling behavior to stir up the substrate" and find food, said Lockley. These are just the sort of tracks seen in the rocks. "Then again, there are no herons known from the Cretaceous."
The tracks plus the behavioral marks suggest two interesting possibilities, said Lockley. One is that some modern lineages of birds are very, very old and that some birds today are virtually living fossils.
"The tracks are also a dead ringer for modern spoonbills," Lockley told Discovery News. Only, there are no spoonbill fossils from the early Cretaceous, so there is no way to say for sure.
The other possibility is that the birds of the early Cretaceous were not closely related to modern birds and only evolved the same kind of feet, beaks and feeding strategies because they occupied a very similar ecological niches.
This would be a lot like how modern emus, ostriches and rheas evolved separately into very similar-looking, but not closely related, large flightless birds. It's what's called convergent evolution.
The only difference in the shorebird case is that they were separated by time, whereas the large flightless birds were separated by oceans.
"All these wader and water birds have very similar feet," Lockley. And so there's no reason to think they could not have evolved 110 million years ago as well.
The Haman formation, where the tracks were found, has been the focus of research on ancient birds for decades, said Falk. But until now most of the work was on classifying the tracks.
"It's not just the classification that's important but the behavior that was important," said Falk.