Serikornis: Pre-bird or flightless bird?

Several authors have wondered over the years
how we might be able to tell (or nest) a flightless post-Archaeopteryx  bird from a flightless pre-Archaeopteryx troodontid. Earlier we nested a very large flightless sapeornithid bird, Jianianhualong, distinct from its original nesting as a troodontid. So it can be done.

Lefèvre et al. 2017 bring us
a new Late Jurassic ground-dwelling theropod from China, Serikornis sungei (Figs. 1–3; PMOL-AB00200; 50 cm in length) that they nested with the derived troodontid, Eosinopteryx. They reported, “The plumage of this new specimen brings new information on the structure and function of the feathers in basal paravians and consequently on the early evolution of flight.”

By contrast
in the large reptile tree (LRT, 1050 taxa) Serikornis nests strongly with the Eichstätt specimen of Archaeopteryx, aka Jurapteryx recurva. That Solnhofen bird has large wing and tail feathers. The latest Jurassic, earliest Cretaceous formations from which Serikornis came are chronologically appropriate to this relationship. Apparently taxon exclusion by the Lefèvre team is the cause of the disparate nestings.

As earlier authors have noted
the most likely time for an early volant bird to go back to flightlessness is when they are still not very good at flight. And that seems to be the case here. Serikornis probably got too big to fly. And its teeth were larger, opposite the general trend for volant birds. And so its flight feathers, like those of any number of extant and extinct flightless birds, became less able to perform aerial duties.

What about that short coracoid?
It is not long and strap-shaped, a common shape in flapping tetrapods. The coracoids in the Eichstätt specimen are lost in a crack so coracoids could not be scored for that Solnhofen bird. That short coracoid of Serikornis must have been a reversal, an atavism. That happens. It’s only one trait out of 228.

Maybe a sternum was overlooked.
The two putative coracoids (Fig. 3) do not have the same outline. So I wonder if one of them was a sternum? Certainly part of the large furculum is buried.

At first I scored Serikornis
by copying the row for Eosinopteryx then renaming it. Soon distinct scores started appearing. The list became long. PAUP nested Serikornis apart from Eosinopteryx, among the very early birds and with Jurapteryx recurva, close to the base of the clade that includes all extant birds.

An abbreviated list of birdy traits in Serikornis include:

1. orbit in the posterior half of the skull
2. ascending process of premaxilla extends to frontals
3. tail longer than presacral spine
4. that long gracile pubis
5. fibula poorly ossified to absent at mid length
6. metatarsal 5 lacking phalanges

So the claim to fame for this taxon
should have been yet another one of the earliest flightless birds –- not a transitional troodontid documenting the advent of flight feathers. These flight feathers were on their way out, not on their way in.

References
Lefèvre U, Cau A, Cincotta A, Hu D-Y, Chinsamy A, Escuillié F and Godefroit P 2017. A new Jurassic theropod from China documents a transitional step in the macrostructure of feathers. Sci Nat 104:74. DOI 10.1007/s00114-017-1496-y

to soon yet for a Wikipedia article