New AVES definition refined
kinman at HOTMAIL.COM
Thu Feb 7 15:17:12 CST 2002
Some major developments have occurred since my first posting on an
expanded Class AVES sensu lato. I placed Segnosauriformes as the basalmost
clade, but more detailed analysis shows that they do not possess a true
semilunate (and they also lack a convex coracoid glenoid---another very key
character). Their lack ornithoid eggs and other characters also show that
this gap is more significant that previously thought. Segnosaurians are
therefore removed from Class Aves (which is thus not expanded as much as was
originally proposed). Segnosaurians are apparently the first major sister
group to Class Aves sensu lato, and the significant gap between them and
Aves is pretty clearly the one I have been seeking.
What follows is a more refined definition of Class Aves sensu lato. 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 of 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 element fully caps metacarpals I and II.
Notes: Protarchaeopteryx and Caudipteryx meet both criteria of this
definition even though they apparently lack the distinctive semilunate
"shape". Deinonychus also meets both criteria, even though one of its
metacarpals is not quite "fully" capped. Segnosaurians do NOT meet either
criterion of this definition. Since they are most likely the sister group
of Class Aves sensu lato, 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 controversial "Tyrannoraptora").
However, Sereno's controversial 1999 analysis does support my exclusion of
(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
(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
(17) Several other characters I haven't had time to work on yet.
So there you have my viewpoint as it stands at present. More rigorous
cladistic analyses are in progress. As always, *constructive* comments are
------- Cheers, Ken Kinman
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