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ENANTIORNITHINES AND LISSAMPHIBIANS



John Jackson wrote:  

<<When I said I wasn't happy about Paul Davis classing _Chaoyangia_ as 
an enant., (apart from the fact that I haven't studied - or indeed had 
the oportunity to study - his argument, which I'm sure is wonderful) I 
hadn't considered the obvious possibility that it was the first uncinant 
(since it has uncinate processes (little hooks) on its ribs - which no 
enantiornithiform...  type bird has, but some later dinos and all modern 
birds "have").  As such, I approve most heartily.>>

When I first heard of this I was very skeptical (and still am).  Hou et 
al. considered _Chaoyangia_ as an ornithurine by these characters:

1)  Laterally flexible furcula.
2)  Rounded concave scapular facet on the coracoid.
3)  Distinct straplike procoracoidal process.
4)  Keel almost reaching anterior margin of sternum.  
5)  Sternum elongated and reaching the abdominal region.
6)  Sternum with coracoidal sulci.
7)  Uncinate processes on ribs.  
8)  Distal-to-proximal metatarsal fusion.
9)  Fusion of the distal ends of the outer two metacarpals.
10)  Tarsal cap on the tarsometatarsus.

And it is placed above Liaoningornis in Ornithurae by the presense of: 

A)  Small pedal claws.
B)  Proximal fusion of the tarsometatarsus to the tarsal cap.

It seems that Science took the data matrix off the web so I am going to 
have to go by what is reconstructed: 

All of these characters except 9 (which is not preserved) appear to be 
present in the Chaoyangia skeleton.  All of these are shared by later 
ornithurines.  Even though the coding is relatively bad and some of the 
features can be easily thrown out (i.e. A: small pedal claws). 

The sternum can be distinguished from that of enantiornithines by:

"(i) the furcular arms are rounded and internally flexible; (ii) the 
furcular hypocleidium is undeveloped (enormously enlarged in 
enantiornithines); (iii) the sternum is elongated and not greatly 
emarginated posteriorly as in enantiornithines; (iv) the coracoidal 
facets are posterior to the anterior margin of the sternum (on that 
margin in enantiornithines); and (v) the sternal keel reaches almost to 
the anterior margin of the sternum (much posterior to the anterior 
margin in enantiornithines)."  (Hou et al., 1996; 1165). 

Character 1 is rather poorly coded at this time because the furcula of 
dromaeosaurs (with the rounded cross-section, but not the U-shaped form 
with a large hypocledium).  This character is still unique to 
ornithurines (as far as I am concerned so far) because dromaeosaurs 
still had a different furcula and the furcula of _Archaeopteryx_ and 
enantiornithines is grooved in cross-section.  This character (like most 
others) should be SOMETHING like this:

Furcula: a) Rounded cross-section; b) Flat cross-section
Furcula form: a) Flat angle; b) U-shape

As opposed to:

"(6) Laterally flexible furcula"
"(21) Broad furcular arms that are grooved posterodorsally (laterally 
incompressible)

But perhaps I am straying too far away now.  

Character 2 is an important part of the ornithurine triosseal canal.  
Enantiornithines lack the rounded scapular facet on the coracoid, rather 
they have a fundamentally different design with a scapula with a flat, 
squared coracoid articulation.  Hou et al. consider Chaoyangia to have a 
shoulder articulation like this.  Unless this is incorrectly interpreted 
(it may well be, I should not really be commenting on something I 
haven't read yet :-) ) Chaoyangia would be a very strange 
enantiornithine.  

Character 3 is a character that is not found in any enantiornithines 
(unless Chaoyangia is a strange enantiornithine).  

Character 4 is another fundamental difference between enantiornithines 
and ornithurines.  Enantiornithines (and _Iberomesornis_) have a sternum 
with a posterior sternal keel, making for a different design.

Character 5 is not found in any enantiornithines.  

Character 6 is a character that really (in my mind) doesn't mean much. 
This can be a reduction or they can be present in some forms.  

Chracter 7 is found in most ornithurines (except for the anhimids and 
diatrymids), but not in any or _Archaeopteryx_ enantiornithine as far as 
I know.  

Character 8 is not found in any ornithurines.  In ornithurine ontogeny, 
the first region of the tarsometatarsus to fuse together is the distal 
portion.  _Archaeopteryx_ and enantiornithines differ from that form in 
that the metatarsus is ossified in a different manner, 
proximo-to-distal.  The proximal portion of the _Chaoyangia_ published 
in Hou et al. lacks the proximal end, but the distal end is fused.  
Unless new specimens were found with extensive proximal fusion, this 
feature supports ornithurine placement of _Chaoyangia_. 

Character 9 is rather suspect because the distal metacarpals of some 
enantiornithines can be described as fused.  

Character 10 is not found in any enantiornithines, but is found in all 
ornithurines.  

What all of these features show (regardless of the bad coding) is that 
_Chaoyangia_ is very ornithurine-like, if not an ornithurine.  Of 
course, it is rather bad form to judge somebody's conclusions without 
actually seeing them.  But at the moment, until I am convinced 
otherwise, I find it more likely that _Chaoyangia_ is an ornithurine 
bird.  Dr. Davis can well be correct (I don't know the particulars, but 
if all the Yixian birds are enantiornithine, I have no problem with 
_Confuciusornis_ being enantiornithine), and more complete specimens may 
support his hypothesis.  Regardless, these are my views at the moment.  

LISSAMPHIBIAN ORIGINS

I have read recently on the web that Lauren and Reisz are coming out (or 
already have?) with an idea that the closest group to lissamphibians are 
the lepospondyls (micorosaurs, lysorophids, etc.; monophyly has been 
questioned for this group).  Their supporting evidence is pretty strong 
from what I have read.  Has this been published yet?  And if so, how 
could I have missed it?

Matt Troutman

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