Sulcavis – an enantiornithes bird without a sternum

Figure 1. Click to enlarge. Pre-bird and bird sternae. Note the replacement of the sternum with gastralia in Sulcavis.

Ever since the advent
of the dual sternae in Velociraptor and kin, and of the single sternum in Archaeopteryx (Fig. 1), most birds had/have an ossified sternum. One exception is the enantiornithine bird, Sulcavis (Fig. 1-4).

Sulcavis geeorum (O’Connor et al. 2013, Early Cretaceous, BMNH Ph-000805) is a robin-sized enantiornithes with a relatively small skull and, remarkably, no sternum. Teeth with grooved enamel radiating from the tips gave it its name (sulcus = groove). That was seen as the most distinctive feature. A sternum replaced by gastralia was not considered an issue (see below).

Soft tissue
Although the specimen includes some soft tissue, O’Connor et al. report one pubis missing and another present only proximally. The ischium was reported missing. My examination identifies areas were both pubes (green) and ischia (magenta) used to be (Fig. 2).

Figure 2. Sulcavis in situ with GIF animation original tracing from O’Connor et al. in black and white. Colors identify elements originally reported as missing. Pubis (green), ischium (magenta), ilium (cyan). Reconstruction in figure 2. A proximal ischium was mislabeled a sacral rib.

Enantiornithes are like basal birds
except for the following traditional traits listed by O’Connor et al. 2013 :

1. Pygostyle proximally forked and distally constructed with ventrolateral processes
2. Furcula Y-shaped and dorsolaterally excavated
3. Coracoid with convex lateral margin
4. Proximal humerus rises dorsally and ventrally to centrally on the concave head
5. Metacarpal 3 longer than mc2
6. Distal tarsals fused to metatarsals, but metatarsals unfused distally

Figure 3. Sulcavis reconstruction. PILs on foot. Note the lack of a sternum. The pedal ungual length and curvature indicate an arboreal lifestyle.

Unfortunately, none of theses traits are listed as characters in the large reptile tree, yet Sulcavis nests with Cathayornis sharing the following traits distinct from other birds:

1. Skull not shorter than cervicals
2. Posterior quadrate straight
3. More than 4 premaxillary teeth
4. Posterior mandible deeper anteriorly
5. Retroarticular descends
6. Metatarsals 2-3 aligned with 1
7. Pedal 2.2 > p2.1

More pertinent taxa would reduce this list.

Figure 4. Sulcavis skull as originally interpreted (above) and traced using the DGS method (middle) to create a reconstruction (below). Note, several bones here were not originally identified. It looks possible that a substantial mandibular fenestra might have been present.

Due to the contrived problem
of digit identification in birds and bird-like theropods described and falsified here, O’Connor et al. describe the three manual digits as the

1. alular digit
2. major digit
3. minor digit

Such renaming of digits 1-3 is totally unnecessary.

Re: The sternum
O’Connor et al. report, “No direct information regarding the morphology of the sternum is preserved.” That’s because there is no sternum in this taxon (Figs, 1, 2). The gastralia run right up to the coracoids. So, does this taxon appear to demonstrate how the sternum in enatiornithine birds is formed? Yes, by enlarging and fusing the gastralia, not as a new single, complete bone.

Sternae also appear in dromaeosaurs and oviraptors by convergence. Twin sternae in these taxa do not appear to be homologous with the single sternum of birds. A single sternum originates as a small bone, wider than long followed by a long set of gastralia extending to the pubis, distinct from large twin sternae.

References
O’Connor JK, Zhang Y, Chiappe LM, Meng Q, Quanguo L, Di L 2013. A new enantiornithine from the Yixian Formation with the first recognized avian enamel specialization. Journal of Vertebrate Paleontology 33(1):1-12.