ACROCORACOIDS AND REPTILE PHYLOGENY

["Matthew Troutman" <m_troutman@hotmail.com>]

<<I believe there are Coelurosaurs outside of Avialae that possess an
acrocoracoid tuberosity, so I'm not sure it is necessarily a flight 
related
feature.  I'll certainly defer to more knowledgeable list members on 
that, however.>>

There sure are coelurosaurs outside of Aves that have an acrocoracoid, 
though it is known under a different name: biceps tubercle (Walker, 
1972).  Actually most maniraptoriforms seem to show this character 
(Ostrom, 1974, 1976a, 1985, 1994; Paul, 1988; among others) which is 
related to the elevation of the supracoracoideus abductor system (Paul, 
1988; Tarsitano, 1985; Walker, 1972).  Dromaeosaurids, oviraptorids, 
troodontids, ornithomimids, tyrannosaurs, and therizinosaurs all show 
some modification of the acrocoracoid process so it is undoubtedly a 
synapomorphy of maniraptoriforms+birds.  I don't know the distribution 
of this character among other avetheropods.

There are problems, of course, with making this a synapomorphy of 
birds+pterosaurs.  First of all, we have no idea about the polarity of 
this character.  Prolacertiforms may or may not show this, I am ignorant 
of the state of _Cosesaurus_, which does show a pterosaur-like pectoral 
girdle.  

As an aside, linking prolacertiforms with birds and theropods may cause 
some problems.  Though most recent works have considered the 
prolacertiforms as archosauromorphs, the sister of the Archosauriformes, 
some recent authors such as Rieppel and Wild have identified some 
lepidosauromorph characters in some of the advanced prolacertiforms.  
Protorosaurs, considered the sister to Prolacertiformes+Archosauriformes 
by most recent authors, also show some lepidosauromorph characters.  
_Drepanosaurus_ shares one particulary strong synapomorphy with 
lepidosauromorphs, astrogalus-calcaneum fusion, which other protorosaurs 
like _Megalancosaurus_ lack.  Birds lack most prolacertiform and 
lepidosauromoprh characters and are undoubtedly archosaurs (see also 
Gower and Weber, 1998 for an overview of cranial characters shared with 
crocodilians supporting birds inclusion in the archosaurs). Pterosaurs 
are particulary interesting in osteology, they share some 
lepidosauromorph characters like the Advanced Mesotarsal ankle joint.  
We need alot more fossils, especially of basal members of 
Lepidosauromorpha and Archosauromorpha to determine where exactly 
prolacertiforms fit.  The most recent molecular study by Hedges and 
Poling (1999) has shown that many of the taxon confidently assigned to 
the Lepidosauromorpha such as tuataras and turtles may actually be 
archosauromorphs.  Osteologically, tuataras (around 25 unamibiguous 
synapormorphies) and turtles are lepidosauromorphs, but on the molecular 
level they are archosauromorphs.  The classic conflict of molecules and 
morphology amplified.  Don't quote me on this, but perhaps based on this 
evidence the traditional line of Lepidosauromorpha and Archosauromorpha 
is blurred considerably.  Maybe the seperation between the two groups is 
less clear.  Certainly, somebody needs to look into this.

Anyway, back to the topic of acrocoracoids.  No matter which way you 
look at it (unless you are known for strange phylogenies), the 
acrocoracoid process evolved several times within Archosauria.  Derived 
crocodylomorphs like _Sphenosuchus_ and some basal crocodyliforms appear 
to have an acrocoracoid process on their coracoid (Walker, 1972).  No 
matter where birds nest, whether near crocodilians or near theropods, 
there is some obvious homoplasy with this character, rendering it 
useless as a character until other characters come and support a given 
phylogeny like the theropod-bird connection.  So like most of the 
features that pterosaurs and birds possess together, this one is 
doubtful.  This character also shows that the M. supracoracoideus wing 
abductor system evolved seperately a few times with Archosauria alone 
(once with crocodylomorphs, once within theropods).  It is also a 
doubtful symplesiomorphy since pseudosuchians leading up to crocs 
(ornithosuchids, parasuchians, aetosaurs, poposaurs) lack this character 
and ornithodirans leading up to birds (dinosauromorphs, dinosauriforms, 
basal theropods, tetanurans) seem to lack this character too.  This is 
how you find convergence.  

Matt Troutman 
m_troutman@hotmail.com

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