[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

New AVES definition refined (more testable?)

Dear All,
I would like to thank those who have given some constructive feedback on this matter. I fully expected my proposal to be controversial (but I was a little surprised just how strongly it is being dismissed before it is even tested). Mickey Mortimer's upcoming analysis will be an important first test, and *if* he can convince me that "enigmosauria" is holophyletic, I will certainly modify my proposal accordingly.
However, I am convinced that the case for "enigmosaur" paraphyly is at least as strong as it is for holophyly, and there may be some surprises for those who are drawing definitive conclusions before rigorous testing is done. Therefore, I cannot in good conscience back away from enigmosaur paraphyly just because that possibility is being so heavily criticized.
Now, it is certainly not easy coming up with a better and more precise redefinition of AVES until it becomes a cooperative effort, but I am proceeding as best as I can. What follows is a more refined definition (assuming enigmosauria will be shown to be paraphyletic after more rigorous testing is done). I will first give my proposal for the Primary Definition, and then follow it with a list of supportive characters I have come up with so far.
PRIMARY DEFINITION of Class Aves sensu lato (proposed): Theropods possessing both a "convex coracoid glenoid" AND at least one the two following carpal characteristics: (1) Distal carpals I and II enlarged and fused into a single element with a distinctive semilunate shape; and/or (2) this fused structure fully caps metacarpals I and II.
Notes: Protarchaeopterx and Caudipteryx meet this definition even though they apparently lack the distinctive semilunate "shape". Deinonychus also meets the definition, even though one of its metacarpals is not "fully" capped. Segnosaurians do NOT meet this definition, although Beipaiosaurus seems to approach it the best. The segnosaurians are most likely the sister group of Class Aves sensu lato, and therefore many of the supportive characters which are listed below will be found in some or all segnosaurians (as noted). Tyrannosaurs appear to be further out (contra Sereno's "Tyrannoraptora").
(1) Enlarged sternum articulating with more than 2 pairs of ribs. Also note that sternal anterior margin is apparently often slotted for coracoids (and said coracoids sharply reflexed).
(2) Possession of ornithoid eggs (microstructure modified or partially reversed in Troodontidae).
(3) Possession of vaned feathers (may occur in some outgroups including segnosaurians). A particular kind of vaned feathers or their bodily distribution may eventually distinguish Aves from the earlier forms (insufficient data at present).
(4) Laterosphenoid head small, and snovial joint small or absent (unknown if this also occurs in segnosaurians or not).
(5) Anterior trochantor splitting from greater trochantor closer to femoral head (occurs in tyrannosaurs as well).
(6) Preacetabular blade of ilium dorsoventrally elongated (also in segnosaurians, but it is distinctively expanded laterally).
(7) Bowed ulnar shaft (also present in some outgroup genera).
(8) Lacrimal-frontal contact present (also occurs in tyrannosaurs).
(9) Relatively large hypapophyses on anterior dorsals (also in Neimongosaurus and Ornitholestes).
(10) Often possess ossified uncinate processes.
(11) Dorsal carpals I and II have a pronounced trochlear groove on the proximal side (also occurs in Coelurus, segnosaurians and tyrannosaurs).
(12) Most Aves (but caenagnathiforms are primitive in this respect) have retroversion of pubes to varying degrees (retroversion in segnosaurians probably occurred in parallel--i.e. a mild case of convergence).
(13) The fused distal carpal element has often fused to the metacarpals (forming a carpometacarpus). Apparently arose at least twice in Class Aves sensu lato.
(14) Distal end of ulna more than 1.5 times as broad as tall (this character could probably use some refining).
(15) Anterior trochantor cylindrical (also in derived segnosaurians).
(16) Reduced supracetabular crest of ilium (also in some tyrannosaurs and segnosaurians).
(17) Several other characters I haven't had time to work on.
So there you have my viewpoint as it stands at present. If Mickey's analysis convinces me that "enigmosauria" is holophyletic, I will modify my proposal accordingly. However, I think even he may be surprised, and that the case for "enigmosaur" paraphyly is a very viable alternative. According to this alternative view, "enigmosauria" is based on plesiomorphies and parallelisms. We shall see. As always, *constructive* comments are welcome.
------- Cheers, Ken Kinman

Chat with friends online, try MSN Messenger: http://messenger.msn.com