Updated March 16, 2016 with new images. The beak, if present, is ephemeral, questionable. Only two scores changed.
The spectacular plate and counter plate
of the basal ornithouromorph bird, Archaeornithura (Figs. 1-3, Early Cretaceous, Wang et al. 2015) appear to present every aspect of this specimen in full detail, but only the back of the skull (the occipital plate) appears to be readily preserved on the split surfaces.
Figure 2. That little patch by the shoulder could be the beak, eye and cranium of Archaeornithura.
Where is the rest of the skull?
It might be here (Fig. 2). At least part of it, the beak tip, scleral ring and cranial bones (frontal and parietal) give the impression of being there. I can’t be sure working from photos alone, but when you put the parts on a reconstruction of the rest of the body (Fig. 3), the parts fit both morphologically and phylogenetically.
Figure 3. Reconstruction of the basal ornithuromorph bird, Archaeornithura with skull added. Feathers and ribs omitted. The length of the tail is hard to determine.
Despite the rather short arms,
the long wing feathers (Fig. 1) made the wings large enough for flapping flight. The robust and long coracoids attest to the ability to flap with great vigor. The sternum is not flat, but more deeply keeled than in more primitive birds. The large pelvis anchors strong leg muscles. The fragile pubes framed larger air sacs. Despite robust sacral vertebrae that broadened the hips, the tail was reduced and without a robust parson’s nose-type pygostyle, which developed by convergence in other birds clades and in more derived ornithuromorphs. The perching toe was not so well developed and all pedal unguals were rather small, similar to those of wading pterosaurs like Ctenochasma.
Hedging paragraph:
I don’t think there is no way to tell how long the beak of Archaeornithura was given the present data. Currently I have the beak tip not very separated from the occiput giving it a rather short skull. Alternatively the length of the skull might be measured from the in situ beak tip to the in situ occiput. Then this bird would have had a longer rostrum, more like that of its beach combing analog among pterosaurs, Ctenochasma. Perhaps other specimens will help fill in the data gap here.
References
Wang M et al. (7 other authors) 2015. The oldest record of ornithuromorpha from the early cretaceous of China. 6:6987 DOI: 10.1038/ncomms7987
The Pterosaur Heresies 