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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
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
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
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
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.
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?
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