A new study reveals how some fossil birds evolved teeth for a specialized diet.

A team of researchers studied the fossil of an early bird - Sulcavis geeorum - a new species of enantiornithine bird (photo) discovered in the Liaoning Province of China.

Sulcavis geeorum lived 125 million years ago, during the early Cretaceous period. When the research team analyzed the bird species, they found that the remnants of the food consumed by the bird were still present in its stomach.

The bird had ornamented tooth enamel and the teeth had grooves on the inside surface that supported them when the bird consumed harder food items.

This suggests that Sulcavis geeorum had a durophagous diet, meaning that the bird fed on crustaceans like insects and crabs which have hard exoskeletons.

The research team revealed that no previous bird species had dental ornamentation such as preserved ridges and serrated edges like the Sulcavis geeorum.

Most of the enantiornithes birds retained teeth, unlike the modern birds that have evolved beaks. Enantiornithes were a unique species, which had different dental patterns. This sheds light on the uneven diversity of birds during the Cretaceous period.

"While other birds were losing their teeth, enantiornithines were evolving new morphologies and dental specializations. We still don't understand why enantiornithines were so successful in the Cretaceous but then died out -- maybe differences in diet played a part," Jingmai O'Connor, lead author of the new study, said in a statement.

The findings of the study are published in the journal of Vertebrate Paleontology.