The skull of Sapeornis: not so weird after all…

Sapeornis chaoyangensis 
(Zhou and Zhang 2002, Fig. 1) is known from many specimens including the holotype (IVPP V12698, postcrania only). When two skulls were discovered (Zhou and Zhang 2003) others reconstructed Sapeornis with a high anterior rostrum at odds with putative sister taxa. Here we review the skull of the holotype and find that it is not so odd as others have thought (Google “Sapeornis”). Instead, the newly reconstructed skull is more like Zhou and Zhang 2003 originally reconstructed it, with a low, bird-like rostrum.

Figure 1. The skull of the IVPP V13276 specimen of Sapeornis traced and reconstructed using DGS methods. This is not such an odd skull after all.

Figure 1. The skull of the IVPP V13276 specimen of Sapeornis traced and reconstructed using DGS methods. This is not such an odd skull after all. The mandible is from the 13275 specimen.

Those teeth are distinctive
They are shaped like short cones on slender roots. And if the maxilla has teeth, they are tiny.

Once again 
I encourage workers to start tracing bones in color, especially in crushed and broken fossils. Colors help the eye segregate one bone from another much better than a line drawing can both for identification, reconstruction and presentation.

Figure 1. Subset of the large reptile tree focusing on birds, not including bird-like taxa in more basal clades. Not the nesting of Jeholornis within the clade of Solnhofen birds, too often lumped under a single taxon, Archaeopteryx.

Figure 2. Subset of the large reptile tree focusing on birds, not including bird-like taxa in more basal clades. Sapeornis nests at the base of the Ornithurae, which includes extant birds.

Oddly, and distinct from sister taxa
the coracoids of Sapeornis are short and the sternum is absent (Fig. 3). Those big wings and short legs suggest we have a flying bird here, but the short coracoids and lack of a sternum argue against flapping.

Figure 3. Original reconstruction of Sapeornis with a repaired foot together with coracoids and clavicles from the two referred specimens.

Figure 3. Original reconstruction of Sapeornis with a repaired foot (putting digit 1 nest to digit 2) together with coracoids and clavicles from the two referred specimens. Note the coracoid convex articulation with the concave scapula, an enantiornithine trait by convergence here.

Zhou and Zhang 2003 reported,
“The skeleton of Sapeornis has several unique features, such as a distinctively elongated fenestra on the proximal end of the humerus, a robust furcula with a distinctive hypocleidum, and an elongated forelimb.”

Evolution goes its own way
and in this case, apparently, the coracoids and sternal area reverted to an ancestral state that was not successful beyond this taxon. We can guess that Sapeornis had a distinct niche and/or behavior regimen that did not select for flapping, despite the big wings. It’s proximal sister, Chiappeavis, had a sternum and elongate coracoids together with big forelimbs (Fig. 3). Despite the strong differences these two share more traits than any competing candidates.

Figure 2. Chiappeavis reconstructed. Is this specimen just another Pengornis? The large reptile tree does not nest them together.

Figure 3. Chiappeavis reconstructed. Is this specimen just another Pengornis? The large reptile tree does not nest them together.

The perforated humerus
is also found in the Solnhofen specimen of Archaeopteryx and its sister Confuciusornis. Chiappeavis may also have this trait. Hard to tell.

References
Zhou Z-H and Zhang F-C 2002. Largest bird from the Early Cretaceous and its implications for the earliest avian ecological diversification. Naturwissenschaften, 89: 34–38.
Zhou Z-H and Zhang F-C 2003. Anatomy of the primitive bird Sapeornis chaoyangensis from the Early Cretaceous of Liaoning, China. Canadian Journal of Earth Science 40:731-747.

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