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Zhongonris the juvenile confuciusornithid?



Tom Holtz wrote-

> CHUNLING GAO, LUIS M. CHIAPPE, QINJING MENG, JINGMAI K. O'CONNOR, XURI
> WANG, XIAODONG CHENG and JINYUAN LIU. 2008. A NEW BASAL LINEAGE OF EARLY
> CRETACEOUS BIRDS FROM CHINA AND ITS IMPLICATIONS ON THE EVOLUTION OF THE
> AVIAN TAIL. Palaeontology 51: 775-791. 10.1111/j.1475-4983.2008.00793.x

While coding Zhongornis I wondered if it could be a case analogous to 
Liaoxiornis- a juvenile misinterpreted as a basal taxon.  The specimen is 
obviously juvenile, based on the lack of carpometacarpal, synsacral, 
tibiotarsal and tarsometatarsal fusion, carpal and sternal ossification, and 
bone texture.  As Yates noted in his blog entry ( 
http://dracovenator.blogspot.com/2008/07/most-primitive-short-tailed-bird.html 
), juvenile enantiornithines also have well developed remiges and retrices 
(Chiappe et al., 2007) and thus these prove nothing about whether adult 
Zhongornis would retain its apparently primitive features as an adult.  Gao et 
al. find four characters place Zhongornis basal to Pygostylia-

70. Distal caudal vertebrae not fully fused into a pygostyle.  This is also 
true of juvenile enantiornithines such as Dalingheornis and IVPP V14238.

71. More than eight free caudal vertebrae.  This is due to the fact adult 
pygostylians fuse several vertebrae into their pygostyle, so is ontogenetic AND 
correlated with the previous character.

203. Posterior trochanter not hypertrophied. Bony processes are well known to 
enlarge with growth of the organism (e.g. sternal processes, pubic foot, 
cnemial crest in enantiornithines).

209. Tibiotarsus unfused. This is also seen in juvenile enantiornithines and 
confuciusornithids.

So there's no convincing evidence placing Zhongornis outside Pygostylia.  If it 
is a member of a known pygostylian group, confuciusornithids are the most 
obvious choice.  For one, they're the only pygostylians that retain the 
elongate third manual digit.  The only characters Gao et al. use to exclude 
Zhongornis are-

128. Deltopectoral crest not enlarged and quadrangular.  It's true that the 
deltopectoral crest is lower than in adult confuciusornithids, but this is also 
true of juvenile confuciusornithids like Eoconfuciusornis.  Juvenile 
enantiornithines also have lower deltopectoral crests than adults.  And while 
the crest of Zhongornis IS lower than adult confuciusornithids, it's higher and 
more quadrangular than most Mesozoic pygostylians.

172. Manual ungual II not reduced compared to manual ungual I.  This is true 
and while it could be ontogenetic, there's no reason to assume it is.

173. Phalanx III-1 not much shorter than III-2 or III-3.  Zhongornis is 
supposed to have a manual phalangeal formula of 2-3-3, which would be unlike 
confuciusornithids.  However, the base of digit III is hidden beneath phalanx 
II-1.  If there is a phalanx III-1, it would have to be short, as in 
Dalianraptor and confuciusornithids.  And even if it really is absent, a 
reduction to nothingness is still a reduction and could count as having the 
confuciusornithid condition.

231. Untwisted metatarsal I.  I'm not sure how certain this is in Zhongornis.  
Metatarsal I doesn't look very different from that in Confuciusornis (Chiappe 
et al., 1999 figure 44) to me.

The general morphology of Zhongornis otherwise resembles confuciusornithids.  
This includes proportions of elements as well as the complete toothlessness.  
It's possible the specimen really does represent a new near-pygostylian taxon, 
but at this point I feel an identity as a juvenile confuciusornithid is more 
likely.

Mickey Mortimer