Distribution of the predentary bone in Mesozoic ornithurine birds

Abstract

In this paper we review the distribution of the predentary bone in Mesozoic ornithurine birds. The predentary bone, well known in ornithischian dinosaurs, has now been reported not only in hesperornithids and ichthyornithids but also in a number of Early Cretaceous basal ornithurines, such as Yanornis, Yixianornis, Hongshanornis and Jianchangornis. In many Early Cretaceous ornithurines the predentary bone is not preserved, but the anterior end of the dentary has a blunt, often inclined margin and usually shows a distinctive pit. These are characteristic features of a predentary attachment. The predentary bone is absent in extant birds, and examination of known enantiornithines and more basal avians now represented by hundreds of specimens show that none of them preserved a predentary bone, confirming that this bone was independently derived in the Ornithurae, and lost in the Neornithes. The predentary bone is found to be associated with both toothed and edentulous dentaries in basal ornithurines. In toothed birds the edentulous predentary bone always corresponds to an edentulous anterior portion of the premaxilla. Although it is possible that the presence of the predentary bone is related to a piscivorous diet in some birds, this may not be true for all the basal ornithurines with a predentary bone. It is almost certain that it was functionally very different from the predentary bone in ornithischian dinosaurs.