Over and over again the same dinosaurs show up in the news: Tyrannosaurus,
Way back in elementary school zoology class my classmates and I learned that one way to tell a mammal apart from a reptile is to look at their teeth. While most mammals have several different kinds of teeth in their jaws most reptiles have only one kind. This general trend still holds true for many living members of these groups, but Incisivosaurus is a startling exception to the rule. A pair of large, incisor-like teeth stuck out of the front of its upper jaw, and further back in its mouth were rows of small, peg-like teeth which showed a heavy amount of wear. Together these traits appeared to indicate a herbivorous or omnivorous diet, and this was especially interesting because Incisivosaurus evolved from carnivorous ancestors.
But the significance of Incisivosaurus goes beyond its peculiar dentition. As paleontologists began working out the relationships of bird-like dinosaurs, some authorities suggested that at least one group, the oviraptorsaurs, were actually birds that lost the ability to fly. If this were true it would account for many of the specialized bird characteristics seen in dinosaurs like Oviraptor and Citipati, but Incisivosaurus supports a different hypothesis. As an early form of oviraptorsaur Incisivosaurus illustrated that many of the avian traits were not present in early members of the group, and this means that the bird-like characteristics among later oviraptorsaurs evolved independently.
Xu, X., Cheng, Y., Wang, X., & Chang, C. (2002). An unusual oviraptorosaurian dinosaur from China Nature, 419 (6904), 291-293 DOI: 10.1038/nature00966