The Tail of Shanweiniao

Fossil tail feathers of S. cooperorum, from O'Connor et al., 2009.

As a follow-up to last month's post on the smallest Mesozoic theropods, here are a few additional observations on the small longipterygid Shanweiniao. Like other longirostravisines*, Shanweiniao cooperorum had reduced "hands" entirely lacking claws. This reduction of wing claws seems to have occurred independently of modern birds within this uniquely specialized group of enantiornitheans. (Euornitheans seem to have lost the bulk of their wing claws around the level of Carinatae, though many modern birds still retain at least keratinous claws on their wings, and longirostravisines may have as well).

S. cooperorum itself is most well-known for its elaborate tail made up of six ribbon-like feathers. Those feathers overlapped at the base, and may have acted as an air brake for precise landings with the feet on small branches. It's possible that most other enantiornitheans, which lacked long feathery tails and also retained wing claws, landed by simply smacking clumsily into tree trunks or brush and grabbing on with all four limbs.

Given the more advanced pectoral anatomy of enantiornitheans (often with mysterious adaptations like little antler-shaped processes on the sternum), it's difficult to envision a purely gliding mode of flight for them, but the lack of braking mechanisms and retention of wing claws in many would make an odd sort of sense if these things relied heavily on simply making prolonged jumps between trees. Having independently evolved an arboreal ecology separately from modern birds, and being derived from relatively basal avialans with rather weak flapping abilities, some odd mode of flight or near-flight specifically tailored to living in tree canopies should be investigated in future studies, at least.

Type specimen of L. hani, from Hou et al. 2004. Note the poor preservation of the tail feathers.

But, as mentioned above, longirostravisines were quite a bit different from your typical enantiornithean. An increased ability to engage in controlled, precise perch-landings may have led to the reduction and ultimate loss of the wing claws. Of all longipterygids, Longirostravis and Shanweiniao are the only ones that preserve evidence of tail feathers. The paper describing Longirostravis stated that two ribbon-like feathers were present, though they are poorly preserved and appear to be rather broad (possibly indicating the presence of more than two?). Interestingly, there's a note about this in the comments for this amazing painting by Bian Choo, which depicts Longirostravis picking lice off of Yutyrannus (a novel interpretation of their feeding ecology that, given the foot anatomy, almost makes more sense than the original mud-probing hypothesis). Choo states that he wanted to add Shanweiniao-like rectrices... until Jingmai O'Connor (one of the co-authors of the paper describing R. pani) discouraged him from doing so! Maybe the original interpretation of the L. hani specimen was in error? Very hard to tell what's going on in the published photos, though the authors do explicitly note the presence of long rectrices in this species.

Hind limbs and tail from the type specimen of R. pani, from Morschhauser et al. 2009. Am I seeing things here?

No feathers at all are reported in the Rapaxavis pani holotype (save a few potential traces of the wing), but looking at the figures in the paper, I could swear there are four reasonably distinct rectrices similar to those of Shanweiniao present here as well. Of course I could just be crazy or have caught Dave Peters' pareidolia syndrome, so definitely don't take my word for it (photos are notoriously unreliable for finding faint soft tissue impressions due to quirks of lighting, variation in the rock matrix, etc.). Also, O'Connor et al., in their re-description of Rapaxavis, noted that the original description had been done before most preparation was complete, and that the first round of prep was botched in a way that was somewhat damaging to the specimen, so it could be that the traces on the slab are the result of that prep work, or simply quirks of the lighting. Either way, it would be nice to get additional specimens (or more detailed descriptions of the existing specimens, preferably under UV) of these weird little birds.

*Longirostravisinae was implicitly named by Zhou and Zhang 2006 when they named Longirostravisidae, though most recent analyses find this to be a sub-group of Longipterygidae. It hasn't yet been defined phlyogenetically, but it's roughly everything closer to Longirostravis than to Longipteryx, though some Longipteryx-like fossils such as "Camptodontus" yangi and Boluochia haven't yet been included in longipterygid analyses as far as I'm aware. (Image at left: restoration of Shanweiniao cooperorum by Matt Martyniuk, all rights reserved).

References

• Hou, Chiappe, Zhang and Chuong, 2004. "New Early Cretaceous fossil from China documents a novel trophic specialization for Mesozoic birds." Naturwissenschaften, 91(1): 22-25.
• Morschhauser, Varricchio, Gao, Liu, Wang, Cheng and Meng, 2009. "Anatomy of the Early Cretaceous bird Rapaxavis pani, a new species from Liaoning Province, China. Journal of Vertebrate Paleontology, 29(2): 545-554.
• O'Connor, Wang, Chiappe, Gao, Meng, Cheng and Liu, 2009. "Phylogenetic support for a specialized clade of Cretaceous enantiornithine birds with information from a new species." Journal of Vertebrate Paleontology, 29(1): 188-204.