Friday, July 18, 2008

The most primitive short-tailed bird?

ResearchBlogging.orgThe latest issue of Palaeontology is choc-full of palaeo goodness. I might end up blogging quite a bit of it, then again time may elude me. For now I just want to say a few quick words about the paper by Gao Chunling et al. It describes yet another new bird from the Yixian Formation of Liaoning, China. Called Zhongornis haoae, Gao et al. bestow upon it a relatively high degree of importance because they believe it to be the sister group of all other short-tailed birds, a clade called Pygostylia. The image is from their paper (Gao et al. 2008). As the name would suggest pygostylians are characterised amongst other things by having a reduced number of tail vertebrae where the distal most ones are fused into a single element called the pygostyle. Although Zhongornis has various advanced pygostylian characters such as an elongate strut-like coracoid and a short tail (just 13 caudal vertebrae), it does not have a pygostyle. That is all 13 caudal vertebrae are unfused, separate bones. However, and this is a big however, it is clearly a juvenile specimen (based on small size, lack of fusion of wrist and manual elements and porous, grainy, bone texture). An therein lies the biggest problem with accepting Zhongornis to this privileged position. Other very juvenile early pygostylians also have unfused caudal vertebrae (e.g. ' Liaoxiornis'). Clearly like wrist fusion the formation of the pygostyle did not occur until quite late in ontogeny amongst early pygostylians. Gao et al try to side-step this problem by claiming the specimen was skeletally mature, or close to maturity because it had fledged, ie. it had grown long vaned feathers from its wings (remiges) and tail (retrices). This I think is the biggest fault with the authors' reasoning. Firstly fledging itself is a variable trait with variable timing of onset amongst modern birds (if I am not mistaken the modern mallee fowl, a kind of megapode, are hatched in a fully fledged state). Late fledging can be expected to be tied to altricial lifestyles. Altriciality is a derived trait in modern birds and should not be expected amongst the early pygostylians of the Cretaceous. Indeed the very same Liaxiornis I mentioned above is a very juvenile early pygostylian with well developed remiges. This more or less falsifies the argument of Gao et al.
So what is Zhongornis? Hard to say. It is toothless so there aren't that many contemporary taxa it could be a juvenile of (Bolouchia is a possibility). It has an unique configuration of phalanges in the third finger which could argue for its status as a valid taxon (although I would classify it as Pygostylia incertae sedis, rather than the pygostylian sister group). However I even wonder if it is possible for the digit three to be reduced as the bird matures, much in the same way that the juvenile claws of the hoatzin are lost as it matures.

GAO, C., CHIAPPE, L.M., MENG, Q., OCONNOR, J.K., WANG, X., CHENG, X., LIU, J. (2008). A NEW BASAL LINEAGE OF EARLY CRETACEOUS BIRDS FROM CHINA AND ITS IMPLICATIONS ON THE EVOLUTION OF THE AVIAN TAIL. Palaeontology, 51(4), 775-791. DOI: 10.1111/j.1475-4983.2008.00793.x

2 comments:

Darren Naish said...

You're right about mallee fowl: they (and some other megapodes) are so superprecocial that they emerge from the nest-mound with all their contour feathers and can fly within their first day.

"Sono una legge non scritta che cito testualmente" said...

I completely agree with Adam's arguments. Dalingheornis is another ornithothoracine based on a juvenile specimen: as in Zhongornis, it lacks a fused pygostyle. It would be interesting to include Zhongornis in a phylogenetic analysis without coding the age-related characters (I will do it soon...).