Chiappeavis – what is it?

There’s a wonderful new
Early Cretaceous bird out there, Chiappeavis (Figs 1, 2), named for a famous bird paleontologist, Luis Chiappe. The question is, what clade does it belong to?

Figure 1. Chiappeavis nests as an ornithurine bird in the large reptile tree, rather than as an enantiornithine. Click to enlarge. Image from O'Connor et al. 2015. 

Figure 1. Chiappeavis nests as an ornithurine bird in the large reptile tree, rather than as an enantiornithine. Click to enlarge. Image from O’Connor et al. 2015.

From the O’Connor et al. 2016 abstract: The most basal avians Archaeopteryx and Jeholornis have elongate reptilian tails. However, all other birds (Pygostylia) have an abbreviated tail that ends in a fused element called the pygostyle. In extant birds, this is typically associated with a fleshy structure called the rectricial bulb that secures the tail feathers (rectrices). The bulbi rectricium muscle controls the spread of the rectrices during flight. This ability to manipulate tail shape greatly increases flight function. The Jehol avifauna preserves the earliest known pygostylians and a diversity of rectrices. However, no fossil directly elucidates this important skeletal transition. Differences in plumage and pygostyle morphology between clades of Early Cretaceous birds led to the hypothesis that rectricial bulbs co-evolved with the plough-shaped pygostyle of the Ornithuromorpha. A newly discovered pengornithid, Chiappeavis magnapremaxillo gen. et sp. nov., preserves strong evidence that enantiornithines possessed aerodynamic rectricial fans. The consistent co-occurrence of short pygostyle morphology with clear aerodynamic tail fans in the Ornithuromorpha, the Sapeornithiformes, and now the Pengornithidae strongly supports inferences that these features co-evolved with the rectricial bulbs as a “rectricial complex.” Most parsimoniously, rectricial bulbs are plesiomorphic to Pygostylia and were lost in confuciusornithiforms and some enantiornithines, although morphological differences suggest three independent origins.”

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

Figure 2. Chiappeavis reconstructed. Is this specimen just another Pengornis? The large reptile tree does not nest them together. The wing size alone is enough to distinguish this taxon from Pelagornis. 

Elsewhere on the Internet, at
Theropoddatabase.blogspot.com, M. Mortimer presents arguments that Chiappeavis is just another Pengornis (Figs. 3, 4).

Figure 3. Pengornis reconstructed not from tracing, but from cutting out the bones and putting them back together. Color tracing is used only for the skull elements. This holotype specimen does not have the same morphology or proportions that Chiappeavis has and it nests within the Enantiornithes.

Figure 3. Pengornis reconstructed not from tracing, but from cutting out the bones and putting them back together. Color tracing is used only for the skull elements. This holotype specimen does not have the same morphology or proportions that Chiappeavis has and it nests within the Enantiornithes with Sulcavis.

Okay, this is going to ruffle a few feathers…
In the large reptile tree Chiappeavis nests firmly between the clade of Wellnhoferia (aka: the Solnhofen specimen of Archeopteryx + Confuciusornis and Archaeornithura, the now former basalmost ancestor of extant birds. I’m using different traits, but they seem to work. Unlike other studies we know of, there are no scores for absent traits, all derived taxa demonstrate a gradual accumulation of derived traits, and the tree remains completely resolved.

Figure 4. Pengornis in situ with tracing from O'Connor et al. identifying bones.

Figure 4. Pengornis in situ with tracing from O’Connor et al. identifying bones.

>If<  I’ve made enough mistakes
to shift Chiappeavis over to the enantiornithes, please let me know, but everything seems to check out from head to toe.

And yes,
I realize the shape of the scapula/coracoid articulation, the lateral shape of the coracoids, and the stem at the base of the clavicle are all obvious enantiornithine traits. Unfortunately, none of these traits are included in the large reptile tree. However, traits along the lines of a lack of a maxillary fossa, and the elongation of the premaxillary ascending process are included.

So two questions have been provisionally answered here.
Chiappeavis does not share enough traits with Pengornis to be considered conspecific in the large reptile tree. And, Chiappeavis does not share enough traits with enantiornithine birds to be nested with them. Rather Chiappeavis appears to be the new basalmost member of the Ornithurae, for which fan tails are standard equipment. And look at the size of those wings!!!

Perhaps the confusion might stem from
other studies that do not include the various specimens of Archaeopteryx as taxonomic units. Several are distinct and nests basal to one of several derived clades. Reconstructions also seem to help.

References
O’Connor JK, Wang X-L, Zheng X-T, Hu H, Zhang  X-M and  Zhou Z 2016.
An Enantiornithine with a Fan-Shaped Tail, and the Evolution of the Rectricial Complex in Early Birds.Current Biology (advance online publication) DOI: http://dx.doi.org/10.1016/j.cub.2015.11.036

Advertisements

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s