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An early ancestor of today's birds had teeth -- and not just any teeth, but ones evolved for a special diet, U.S. paleontologists say.

Writing in the Journal of Vertebrate Paleontology, researchers say a study of a species of early bird, Sulcavis geeorum, suggests it had a durophagous diet, meaning the bird's teeth were capable of eating prey with hard exoskeletons like insects or crabs.

The new specimen, a fossil from the the Early Cretaceous period of 121 million to 125 million years ago, greatly increases the known diversity of tooth shape in early birds and hints at previously unrecognized ecological diversity, they said.

The fossil from an early group of birds known as enantiornithines was found in China and has robust teeth with grooves on the inside surface that likely made them stronger to deal with harder food items, researchers said.

No previous bird species have been found with such grooves, ridges, striations, serrated edges or any other form of dental ornamentation, researchers said.

"While other birds were losing their teeth, enantiornithines were evolving new morphologies and dental specializations," lead study author Jingmai O'Connor said.

"We still don't understand why enantiornithines were so successful in the Cretaceous but then died out -- maybe differences in diet played a part."