A revision of enantiornithine (Aves: Ornithothoraces) skull morphology

Abstract

Enantiornithines are the most speciose avian clade in the Mesozoic, with a fossil record that nearly spans the Cretaceous; however, with less than half of known taxa preserving skull material, our understanding of their cranial morphology remains incomplete. Here we present a comprehensive overview of the current knowledge of enantiornithine skull anatomy and discuss the range of morphologies known for each of the main cranial elements. The typical enantiornithine skull retains numerous ancestral features such as the absence of fusion among bones, the presence of a postorbital bone, a primitive quadrate with a single headed otic process, an unforked dentary, and teeth. The postorbital in at least one taxon is unreduced, suggesting the existence of a complete infratemporal fenestra and thus an unmodified diapsid skull as in confuciusornithids. The rostrum is well known and shows considerable variation, typical of theropods; however, in terms of rostral proportions, enantiornithines are extremely limited within the modern avian spectrum. Although Late Cretaceous skull material is extremely fragmentary, when compared to Early Cretaceous material it reveals a trend towards more specialized morphologies in younger taxa. The foramen magnum in all taxa points caudally, indicating that the ‘flexed’ type skull morphology may not have evolved in this group. Enantiornithine teeth show considerable diversity in numbers, size, morphology and placement, ranging from taxa with large teeth found throughout the jaws to taxa with small, rostrally restricted teeth, to the fully edentulous. Despite limited preservation of skull material, a number of trophic specializations can be deduced from the range of preserved morphologies, further hinting at the morphological and ecological diversity of the Cretaceous Enantiornithes.

Keywords:

• avian
• skull
• variation
• Cretaceous

Acknowledgements

We are grateful to Stephanie Abramowicz for producing several of the illustrations for this paper. We thank Jesus Marugán-Lobón for his assistance with measuring and plotting the skull units and use of his dataset, Zhang Zihui for data provided through personal communication, and two reviewers for their useful comments on this manuscript. We also thank Zhou Zhonghe and Zhang Fucheng of the Institute of Vertebrate Paleontology and Paleoanthropology, and Gao Chunling of the Dalian Natural History Museum for access to specimens. Special thanks to Judy and Ron Perlstein and Lynn and Carl Cooper for additional funding and support. We thank David Bottjer of the University of Southern California for advising this project.