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Author Topic:   Theropods and Birds showing a change in kinds
arachnophilia
Member (Idle past 1344 days)
Posts: 9069
From: god's waiting room
Joined: 05-21-2004


Message 145 of 150 (598423)
12-30-2010 8:00 PM
Reply to: Message 142 by Taq
02-09-2010 12:23 PM


the reason archaeopteryx is a "bird"
taq writes:
cribbed from [talk origins]
oof, somebody needs to update t.o!
1. Feathers.
as noted in the other thread, this doesn't seem especially unique anymore. primitive feathers have even been found in ornithiscian dinosaurs, meaning that's almost certainly a non-avian dinosaur trait.
2. Opposable hallux (big toe).
as mentioned in the other thread, archaeopteryx does not have an opposed hallux. on most of the 7 specimens, the feet are a bit obscured, or twisted, and it was hard to tell. however, recent examination points out that the hallux is in precisely the same position and orientation as on other dromaeosaurid dinosaurs.
3. Furcula (wishbone) formed of two clavicles fused together in the midline.
furculae are about the first bones to disappear from dinosaur skeletons. but even coelophysis had one.
4. Pubis elongate and directed backward.
as in all dromaeosaurs. none of this stuff is particularly special. here are the avian characteristic of archaeopteryx from jacques gauthier's 1986 paper where he classifies avialae (his grouping of aves as not been generally well received) and comments by thomas holtz.
quote:
  1. Premaxillae elongate, narrow, and more pointed anteriorly, with longer nasal processes.
    [similar condition in bullatosaurs, and in the nondinosaurian Megalancosaurus, but unique to Archie and later birds in Maniraptora]
  2. Maxillary process of premaxilla reduced so that maxilla participates broadly in external naris (also in troodontids).
  3. Enlarged brain/basicranium (temporal musculature fails to extend origin onto frontal bones.
    [to a certain degree in bullatosaurs and therizinosauroids, and perhaps in some of the newer, "brainier" dromaeosaurids]
  4. Double-condyled quadrate displaced from distal position on opisthotic to more anteromedial position in contact with prootic.
    [Currie, pers. comm. and Walker, pers. comm., disagree with Whetstone's interpretation of the quadrate: Currie notes the anterior displacement of the quadrate in troodontids, and Walker does not consider the quadrate to be double-condyled in Archaeopteryx]
  5. Maxillary and dentary teeth reduced in size and number (or lost), with unserrated crowns and enlarged roots that completely enclose replacement teeth within them.
    [many of the smallest theropod teeth are unserrated, which may be a developmental constraint. The pinched roots of bird teeth are similar to those of troodontids, basal ornithomimosaurs, and (in at least a couple of cases) dromaeosaurids]
  6. Robust furcula for hypertrophied flight musculature .
    [true to a certain extent, although oviraptorids and (perhaps) some dromaeosaurids have substantial furculae as well, and even the new Morrison ?allosaurid seems to have a primitive furcula!]
  7. Scapula with more or less prominent acromion process for ligamentous connection to clavicle.
  8. Lenght/bredth ratio of scapula at midlength exceeds nine (not in penguins) and scapula tapers distally.
  9. Acrocoracoid tuberosity larger than in other coelurosaurs.
  10. Coracoid enlaged and inflected posteromedially more so than in other coelurosaurs.
  11. Very long forelimb and hands (e.g., in Archaeopteryx forelimb is 120-140% of hindlimb length, and more than twice as long as distance between glenoid and acetabulum), with forearm more than 87% of humerus length and mcII approaching or exceeding one-half of humerus length.
  12. Ischium compressed and dorsoventrally deep.
  13. Compared to other theropods, tibia, fibula, and metatarsals relatively more elongate with respect to femur, regardless of body size (mts short in penguins and some other birds).
    [Actually, as S. Gatsey and I have shown elsewhere, what actually is happening is a difference in the allometry of bird vs nonavian theropod hindlimbs. In nonavian theropods, the tibia/fibula and the metatarsus become relatively smaller as body size (and femur size) increases; in birds, the tibia/fibula and the metarsus become relatively longer as body size (and femur size) increases. Unfortunately, Archaeopteryx lies very close to the points at which the "bird" and "nonbird" allometric lines cross, so it is not possible to say at present to which of these curves the Urvogel belongs]
  14. Fibula attenuate distally, and may not extend to end of tibia.
  15. Proximal tarsals fused to tibia/fibula and to one another in adults.
    [also in some ceratosaurs]
  16. Distal tarsals and metatarsals fused at least distally in fully adult individuals (convergent in some ceratosaurs, elmisaurids, and Hulsanpes).
    [future work may show why this is no surprise for Hulsanpes...; also found in Avimimus]
  17. First pedal digit elongate and reversed (may be reversed in some extant birds).
    [the first "reversed" he uses is "reverted"; the second is "reversed" in the evolutinary sense]
  18. Metatarsal I attached on the distal quarter of metatarsal II.
  19. Tail reduced to no more than 23 free caudal vertebrae.
  20. Feathers cover limbs and tail, feathers on lateral margin of tail and posterior margin of arms enlarged, curved, and asymmetrically vaned, indicating aerodynamic function.
    [While it is true these features are currently only known in Archaeopteryx and later birds, there is only negative evidence with regards to this character in other theropods, as discussed in the paragraph immediately following his character list]
(source)
as you can see, it's pretty relative and arbitrary. and the more we learn about non-avian dinosaurs, the more similarities we have been finding -- which crosses a good number of these classification off the list.
Edited by arachnophilia, : No reason given.

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This message is a reply to:
 Message 142 by Taq, posted 02-09-2010 12:23 PM Taq has not replied

  
arachnophilia
Member (Idle past 1344 days)
Posts: 9069
From: god's waiting room
Joined: 05-21-2004


Message 149 of 150 (669850)
08-04-2012 12:05 AM
Reply to: Message 148 by xongsmith
08-03-2012 11:22 PM


not unless you believe this crackpot who makes david peters look positively academic.

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This message is a reply to:
 Message 148 by xongsmith, posted 08-03-2012 11:22 PM xongsmith has seen this message but not replied

  
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