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THE AETOSAUR-ORNITHISCHIAN CONNECTION



Ken Kinman wrote:
<<It amazes me that so many people now take the holophyly of Dinosauria
for granted.  Many of the purported synapomorphies are very weak, and many
remind me of the "kitchen list" of synapomorphies that was proposed for
Ornithodira (sensu stricto).>>

I've been reading up on this and I am now convinced that Ken is absolutely
right about this.  Dinosauria almost certainly isn't monophyletic, and I
propose that ornithischians (with the same topology as Maryanska and Osmolska
1984) are really the sister group to Aetosauria:

-+--Aetosauria
 `--+--Ankylosauria
    `--+--Stegosauria
       `--+--Ornithopoda (incl Fabrosaurids)
          `--Marginocephalia


I think that this works for a number of reasons.  First off, ankylosaurs and
aetosaurs look a lot alike.  They're quadrupeds with big shoulder spikes and
body armor, and weak leaf-like teeth.  Additionally, this would demonstrate
that Ken's right, the imperforate acetabulum of ankylosaurs is indeed
primitive and not a reversal (even though basal ankylosaurs have perforate
acetabula, surely convergence), just like their aetosaurian "ancestors."

This would also demonstrate that basal ornithischians went through a big and
fat stage, and convergently evolved short forelimbs and bipedalism.

The only trouble with this theory is the real lack of synapomorphies I can
find aside from leaf-like dentition and armor and big and fat.  The
imperforate acetabula in aetosaurs and ankylosaurs is a PLESIOmorphy.  

Additionally, I am going to have to account for a mountain of convergences in
ornithischians and saurischians.  I mean aside from the perforate acetabulum
(which I just decided was convergent because I felt like it), there's the
peculiar condition of the dinosaurian hand, where digit one is twisted, and
digits four and five are short and bare no claw.  There's that advanced
mesotarsal ankle thing... And the complete lack of prefrontals and
interclavicles in any known dinosaur specimen, a very reduced pedal digit 5,
the s-curved neck etc.....

So wait one second... Nevermind.  Ken is completely wrong.  What IS your
competing hypothesis of saurischian and ornithischian origins Ken?  Even if
Dinosauria were to be found with only one synapomorphy, the fact that you
give no competing evidence still insists on Dinosauria being monophyletic to
the exclusion of other archosaurs.  So what gives?

Pete Buchholz
Tetanurae@aol.com