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During the late Mesozoic era, from the Early Cretaceous to the latest Cretaceous (Maestrichtian), two distinct groups of birds co-existed as separate lineages1,2. One of these, the ornithurine birds, survived into the Cenozoic era to give rise to all modern birds3,4. The other lineage, the Enantiornithes, became extinct along with dinosaurs at the end of the Cretaceous3,5. The structure of Enantiornithine bird skulls is poorly understood. Until now, the best known was Gobipteryx, of which there are fragmentary adult and embryonic skulls6 showing the absence of teeth, a reduced antorbital fenestra and an Archaeopteryx -like quadrate.

Cathayornis from the Early Cretaceous of China has the typical elongated outer metacarpal and the characteristic shapes of the scapula, coracoid, and distal tibiotarsus found in later enantiornithine birds3,7. Photographs of the skull have been published8 and a drawing of it as it is preserved7, but there has been no detailed reconstruction of any enantiornithine skull published to date. Our reconstruction (Fig. 1) is based on the holotype of Cathayornis (specimen IVPP, V9769) and skulls from two skeletons referred to that genus (IVPP, V10896 and V10916). The latter two skulls provide information on the maxilla, lacrimal and a lateral view of the quadrate. The premaxilla bears four or five small teeth that are directed downwards or slightly forwards. It is covered externally with large nutrient foramina. The dorsal process of the premaxilla extends posteriorly slightly beyond the edge of the nares (nostrils). It is slightly shorter in Archaeopteryx (Fig. 1 c,d). The nares meet on the midline as in Archaeopteryx and are overlapped by the premaxillaries on their anterior process.

Figure 1: Reconstruction of skull structures of Cathayornis and Archaeopteryx.
figure 1

a, Lateral view of Cathayornis. b, Dorsal view of Cathayornis. c, Lateral view of Archaeopteryx. d, Dorsal view of Archaeopteryx. e, Quadrate of Archaeopteryx. f, Quadrate of Cathayornis. g, Labial view of tooth from the maxillary of Cathayornis. Abbreviations: an, angular; art, articular; dn, dentary; fr, frontal; j, jugal; la, lacrimal; ma, maxillary; na, nasal; oc, occipital; pa, parietal; pm, premaxillary; q, quadrate; qj, quadratojugal; sa, surangular.

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The premaxillaries are toothed and about one-third the length of the skull compared to about one-quarter in Archaeopteryx, and the nasals are shortened. The antorbital fenestra is large and triangular with two distinct anterior maxillary fossae. The maxillary is toothed. Teeth are set in sockets indicating that the individuals are mature. The teeth are not serrated and have the expanded base and waisted crown typical of all known toothed birds (Fig. 1g). The ‘T’-shaped lacrimal is inclined posteriorly. The braincase is expanded over that in Archaeopteryx (Fig. 1 b,d) and this may reflect an increased brain size coupled with an improved shoulder girdle (keeled sternum) and presumably more sophisticated powers of flight in Cathayornis. There is a bone in the posterior corner of the skull that might be a squamosal, but the quadrate articulation and basicranial region is not preserved in our material, nor is the quadratojugal and jugal.

There is a well-preserved quadrate (Fig. 1f) lying disarticulated and behind one skull (IVPP, V10916). It is a long slender bone with a single small proximal head and very little orbital process. It is similar to the quadrate (Fig. 1e) of the London, Eichstätt and seventh Archaeopteryx specimens9 and to that of Gobipteryx. In Archaeopteryx the dorsal process of the jugal is too far back to contact a postorbital even if one were present10. The reduction or loss of the postorbital frees the jugal bar for prokinesis and may have occurred more than once in avian evolution. Cathayornis lacks evidence for a postorbital but one is present in Confuciusornis. The flattened nature of the nasal and the nasal process of the premaxilla7 may also indicate the presence of prokinesis in Cathayornis.

If Archaeopteryx and the Enantiornithes are united into a monophyletic Sauriurae1, then the presence of a primitive Archaeopteryx -like skull in Cathayornis, after the derived postcranial differences between enantiornithine and ornithurine birds was established, indicates that the modern ornithurine skull and ‘typical avian kinesis’ was developed independently by ornithurine birds.