Chiappeavis is just another Pengornis

The last Mesozoic dinosaur named in 2015 is Chiappeavis magnapremaxillo (O'Connor et al., 2015b), electronically published on December 31st.  First of all, what a stupid species name.  Second, examination of the supposed diagnostic characters shows that like Parapengornis and IVPP V18632, it's just another example of Pengornis houi.  Which is a shame because Luis Chiappe really deserves a bird named after him.  So before anyone names a 'Chiappeornis', let's make sure it's a really distinctive specimen, okay?  It's also disappointing because O'Connor has an entire section of her thesis lamenting the fact so many enantiornithines are named without adequate diagnoses.  But here she's doing that herself (and yes, the contributions credit her with writing the manuscript).  I'll also note the paper lacks a measurement table and that Pengornithidae can be paraphyletic in their analysis depending on their assumptions (fig. S1E).  Anyway, let's look at the evidence...

Holotype of Chiappeavis magnapremaxillo STM 29-11 (after O'Connor et al., 2015).

Several characters were listed as supposedly diagnostic. The premaxilla was said to have a larger body and convex ventral margin, but the convex element has far too long of a ventral margin to be a premaxilla, so is more likely an incomplete maxilla with the base of the ascending process. The actual right premaxilla is then just the small anterodorsal portion right below the partially preserved left premaxilla, and has no visible ventral margin.

Left (top to bottom)- skull of Chiappeavis magnapremaxillo as seemingly interpreted by O'Connor et al. (2015b) (top), without interpretation (middle), and as interpreted by me (bottom) (after O'Connor et al., 2015b).  Right (top to bottom)- skulls of Parapengornis eurycaudatus holotype IVPP V18687 (top; after Hu et al., 2015), IVPP V18632 (middle; after Hu et al., 2014), and Pengornis houi holotype IVPP V15336 (after O'Connor and Chiappe, 2011).  Premaxilla colored red, maxilla colored blue, and nasal colored green.

The posterodorsal premaxillary process was claimed to be longer than other pengornithids, almost reaching the frontals. Yet this bone is highly abraded just posterior to figure 2A's 'l pm' label, so that this posterior portion could easily belong to the right nasal instead. This is bolstered by the fact it's the same width as the left nasal and that the right nasal is otherwise missing. These cranial reinterpretations also make far more sense as they match the morphology and preservation of other pengornithids, which would be expected as the postcrania are nearly identical.

The synsacrum has eight vertebrae (as in IVPP V18632; Parapengornis' is only partly preserved), while the authors claim Pengornis' holotype has seven. Yet the description of the latter says only that seven are visible, but that the anterior end is covered, and indeed there could easily be another one beneath the ilium. STM 29-15 (a Jiufotang pengornithid briefly described by O'Connor et al., 2015a) only has seven but is obviously younger based on its unfused sterna.

The "median trabeculae" of Chiappeavis are said to have concave lateral margins, but no such structure exists, so the authors clearly meant the posterolateral processes. Their slight lateral concavity is also seen in Parapengornis' holotype though not in STM 29-15 and perhaps IVPP V18632 (difficult to determine from the low resolution figure of the latter), while Pengornis' holotype doesn't preserve the sternum. [Edit] O'Connor cordially informed me she meant the posteromedian process, so incorrectly pluralized the term instead of my assumption that she used the wrong positional adjective ("median trabeculae is totally a valid term MORON - they fuse into xiphial region. check Baumel bitch" as per O'Connor pers. comm., 1-16-2016).  However, concave lateral edges on the posteromedian process are also present in IVPP V18632, absent in STM 29-15, and unknown in both Pengornis and Parapengornis. So this doesn't support Chiappeavis' validity either.

Sternum of Parapengornis eurycaudatus holotype IVPP V18687 showing concave lateral edge of posterolateral process (right edge of teal line) and [see discussion above] low posteromedian angle (purple lines) (after Hu et al., 2015).

The posteromedian angle of the sternum is said to be narrow, but while its 53 degree angle is a bit less than IVPP V18632's (at 68) or STM 29-15's (at 66), Parapengornis' holotype could have an identical angle if complete, and again Pengornis' holotype doesn't preserve the element.

Finally, the authors claim the proximal articular surface of the tibia is laterodistally inclined, but this is only true of the left tibia, with the right tibia having a right angle between the surface and the long axis of the bone. Furthermore, Pengornis' holotype and IVPP V18632 both have inclined surfaces, though Parapengornis' holotype lacks them. Given the variation in Chiappeavis' holotype, the variation is likely due to perspective or taphonomy.

Tibiotarsi of (left to right; flipped if necessary so that lateral is to the left) both of Pengornis houi holotype IVPP V15336 (after Zhou et al., 2008), right of IVPP V18632 (after Hu et al., 2014; left is in medial view), both of Parapengornis eurycaudatus holotype IVPP V18687 (after Hu et al., 2015), left of STM 29-15 (after O'Connor et al., 2015a; right is fragmented); and Chiappeavis magnapremaxillo holotype STM 29-11 (aftter O'Connor et al., 2015b).  Angle of proximal edge indicated in green; note that Chiappeavis' angles differ and that Pengornis' are both ventrolaterally angled.

Besides the characters listed in the diagnosis, O'Connor et al. note the short anterior cervicals are like the Parapengornis holotype but unlike Pengornis' holotype. As the latter is larger than the others, this may support ontogenetic cervical elongation.

They also correctly note the long pygostyle is like Pengornis' holotype, but unlike IVPP V18632 or Parapengornis' holotype.

Finally, the short metatarsal I is said to be like Pengornis' holotype, and is additionally like IVPP V18632 but unlike Parapengornis' holotype or STM 29-15. 

Thus the only real difference from Pengornis' holotype is the shorter anterior cervicals, and while there are a few differences from other pengornithid specimens (laterally concave posterolateral posteromedian sternal processes unlike STM 29-15 and (?)IVPP V18632; narrower posterior sternal angle than STM 29-15 and IVPP V18632; long pygostyle unlike STM 29-15, IVPP V18632 and Parapengornis' type; short metatarsal I unlike STM 29-15 and Parapengornis' type), there's no pattern of character distribution that would suggest separate pengornithid species (e.g. Parapengornis shares the cervical length, posterolateral process concavity and probably sternal angle, while IVPP V18632 is different in cervical length but shares metatarsal I length).  This is illustrated in the following table, showing the distribution of all real differences proposed to diagnose Jiufotang pengornithid taxa (known in more than two specimens) that aren't obviously ontogenetic- 

B- femoral length (mm) to indicate possible age.

C- robust base of maxillary ascending process.
D- low interclavicular angle.
E- metatarsal I short.
F- long anterior cervical vertebrae.
G- long pygostyle.
H- long posterodorsal lacrimal process.
I- laterally concave posterolateral posteromedian sternal process.
J- narrow posteromedian sternal angle.

Note the lack of a pattern unrelated to size.  Indeed, all but E, F and I correlate with size, and two of those conflict with each the other.  It's true that size is not fully correlated with osteological development in this series (e.g. STM 29-15 has unfused sternals unlike the slightly smaller IVPP V18632), but that's true of other taxa as well (e.g. Anchiornis, Archaeopteryx, Shenzhouraptor, Sapeornis, Archaeorhynchus).  So it's just like with Archaeopteryx and Microraptor- all specimens show uncorrelated differences, thus either every specimen is its own species or as far as we can tell they're a single species that shows variation.  Based on this I conclude all Jiufotang pengornithids should still be still retained in Pengornis houi.

References-  Zhou, Clarke and Zhang, 2008. Insight into diversity, body size and morphological evolution from the largest Early Cretaceous enantiornithine bird. Journal of Anatomy. 212, 565-577.
O'Connor and Chiappe, 2011. A revision of enantiornithine (Aves: Ornithothoraces) skull morphology. Journal of Systematic Palaeontology. 9(1), 135-157.
Hu, Zhou and O'Connor, 2014. A subadult specimen of Pengornis and character evolution in Enantiornithes. Vertebrata PalAsiatica. 52(1), 77-97.
Hu, O'Connor and Zhou, 2015a. A new species of Pengornithidae (Aves: Enantiornithes) from the Lower Cretaceous of China suggests a specialized scansorial habitat previously unknown in early birds. PLoS ONE. 10(6), e0126791.
O'Connor, Wang, Zheng, Hu, Zhang and Zhou, 2015b. An enantiornithine with a fan-shaped tail, and the evolution of the rectricial complex in early birds. Current Biology. http://dx.doi.org/10.1016/j.cub.2015.11.036
O'Connor, Zheng, Sullivan, Chuong, Wang, Li, Wang, Zhang and Zhou, 2015a. Evolution and functional significance of derived sternal ossification patterns in ornithothoracine birds. Journal of Evolutionary Biology. 28(8), 1550-1567.