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Member (Idle past 5084 days) Posts: 651 From: Jareth's labyrinth Joined: |
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Author | Topic: Dinosaur-bird link in trouble? | |||||||||||||||||||||||||||
LudoRephaim Member (Idle past 5084 days) Posts: 651 From: Jareth's labyrinth Joined: |
A recent find of how the leg structure of birds and their lungs have seemingly led to a doubt among a few scientists about the dinosaur origin of birds. The story is in Science Daily at:
http://www.sciencedaily.com/...ases/2009/06/090609092055.htm comments? "The Nephilim where in the Earth in those days..." Genesis 6:4
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Dr Jack Member Posts: 3514 From: Immigrant in the land of Deutsch Joined: Member Rating: 8.7 |
I'm not following their reasoning. From a quick read through of the paper, they appear to be saying Theropods (for which they bizarrely pick Tyrannosaurus as an example rather than any of the group more closely associated with bird evolution, such as microraptor) don't have features present in modern birds and central to their respiration. But they also say these features are not present in Archaeopteryx, and didn't emerge until "the appearance of Late Cretaceous ornithurines"...
So... doesn't that rather imply it's a feature that evolved within the avians and wasn't present in whatever group they evolved from, anyway?
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Percy Member Posts: 22392 From: New Hampshire Joined: Member Rating: 5.3 |
Mr Jack writes: So... doesn't that rather imply it's a feature that evolved within the avians and wasn't present in whatever group they evolved from, anyway? After reading the Science Daily article I went and read the Wikipedia article on birds, and it says you're right, that the current most popular theory is that birds evolved their hip structure independently. I read a couple of the related articles, and one was about Alan Feduccia, who is another scientist who demurs from the out-of-dinosaurs theory. He claims to have shown that bird hands are actually digits 2, 3 and 4, while therapod hands were digits 1, 2 and 3, and if true it reduces the likelihood that birds descended from the therapods. --Percy
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Dr Adequate Member (Idle past 285 days) Posts: 16113 Joined: |
But they also say these features are not present in Archaeopteryx, and didn't emerge until "the appearance of Late Cretaceous ornithurines"... Can you give me a quote about Archaeopteryx please? Thanks.
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Blue Jay Member (Idle past 2698 days) Posts: 2843 From: You couldn't pronounce it with your mouthparts Joined: |
Hi, Dr A.
Dr Adequate writes: Can you give me a quote about Archaeopteryx please? They basically argue that birds have restricted movement in the femur, which is contained inside the body of the bird, rather than as an external segment of the limb, as in the theropods. This arrangement allows the femur to support the airsac and prevent it from collapsing when negative pressures are generated during inhalation. But, in the same paper, as Mr Jack pointed out, they mention that these adaptations are not found in birds until the Late Cretaceous. From page 10 in the article (Quick and Ruben, 2009):
quote: ----- They did not formally present this as an argument against the theropod-to-bird theory in the paper, but rather, as an argument against the idea that theropods had an avian respiratory mechanism. This was a wise choice on their part, because, by their own admission, the fossil evidence is still fluidly consistent with theropod-to-birds. However, to the popular media, they unscrupulously present the work as definitive evidence against the idea that birds evolved from theropods, despite the fact that they refuted this argument themselves in their own paper! The ScienceDaily article is just a ploy to get attention for their unpopular theory. Edited by Bluejay, : Rewording. Edited by Bluejay, : No reason given. -Bluejay (a.k.a. Mantis, Thylacosmilus) Darwin loves you.
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Dr Adequate Member (Idle past 285 days) Posts: 16113 Joined: |
Thank you.
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Dr Jack Member Posts: 3514 From: Immigrant in the land of Deutsch Joined: Member Rating: 8.7 |
I read a couple of the related articles, and one was about Alan Feduccia, who is another scientist who demurs from the out-of-dinosaurs theory. He claims to have shown that bird hands are actually digits 2, 3 and 4, while therapod hands were digits 1, 2 and 3, and if true it reduces the likelihood that birds descended from the therapods. Pharyngula has just done a piece on this. In summary: it's true that birds develop from digits 2,3 and 4; and that dinosaur digits are usually numbered 1,2 and 3 on morphological grounds. However, birds were numbered 2, 3 and 4 by morphologists until embryological data overturned that; we don't have embryological data for dinosaurs so the comparison can't stand. The link has some more detailed discussion.
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Dr Jack Member Posts: 3514 From: Immigrant in the land of Deutsch Joined: Member Rating: 8.7 |
Ooo, and more: Limusaurus inextricabilis appears to a be a transitional on this very feature.
To quote the summary:
quote: So: Feduccia argues that birds can't have evolved from dinosaurs (I'm presuming he's actually talking Theropods here?) since the dinosaurs they evolved from have the wrong three fingers (I, II and III instead of II, III and IV) but we couldn't tell from a fossil bird that this was the case, making it an invalid comparison and we've found a transitional form showing the matching loss in a suitable basal dinosaur. Case closed?
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Hyroglyphx Inactive Member |
This information is not new. What may be new is that more scientists may be following Feduccia's lead and agreeing with him.
I've never found the avian-therapod connection too strong for multiple reasons. There are other morphological similarities that do make it appealing, but there seem to be some internal inconsistencies that make it less plausible. The older theory that they are more closely related to modern-day reptiles makes more sense to me, in my opinion. But honestly I don't really give a crap either way so... "Fix reason firmly in her seat, and call to her tribunal every fact, every opinion. Question with boldness even the existence of a God; because, if there be one, he must more approve of the homage of reason, than that of blindfolded fear." Thomas Jefferson
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Dr Jack Member Posts: 3514 From: Immigrant in the land of Deutsch Joined: Member Rating: 8.7 |
What may be new is that more scientists may be following Feduccia's lead and agreeing with him. I don't see any evidence at all of that happening. If anything, things are moving the other way.
I've never found the avian-therapod connection too strong for multiple reasons. There are other morphological similarities that do make it appealing, but there seem to be some internal inconsistencies that make it less plausible. Could you elaborate? What internal inconsistencies are these?
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Hyroglyphx Inactive Member |
I don't see any evidence at all of that happening. If anything, things are moving the other way. The article seemed to imply otherwise.
Could you elaborate? What internal inconsistencies are these? Well, one could look at some of the more superficial morphology like backward hinged legs like avian and dinosaurs have and make an argument. But there are a lot of differences between avian and therapods. For instance avian are ectothermal and therapods are exothermal. If they were directly related, shouldn't we see some exothermic avian that survived? Those are some of the internal inconsistencies. I suppose I should have clarified. When I said "internal inconsistencies" I was referring to the organs. Then again, we know next to nothing about therapod organs since they didn't survive decay. So I suppose we should assume more about what similarities that we do know about versus the proposed dissimilarities we only know of with avian. "Fix reason firmly in her seat, and call to her tribunal every fact, every opinion. Question with boldness even the existence of a God; because, if there be one, he must more approve of the homage of reason, than that of blindfolded fear." Thomas Jefferson
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Perdition Member (Idle past 3238 days) Posts: 1593 From: Wisconsin Joined: |
For instance avian are ectothermal and therapods are exothermal. Do you have some evidence for this? I've only heard that people are beginning to think that therapods were not cold blooded.
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Meddle Member (Idle past 1271 days) Posts: 179 From: Scotland Joined: |
For instance avian are ectothermal and therapods are exothermal. If they were directly related, shouldn't we see some exothermic avian that survived? Those are some of the internal inconsistencies. I suppose I should have clarified. When I said "internal inconsistencies" I was referring to the organs. However, the bones of therapod dinosaurs exhibit haversian canals, which today are generally only found in endotherms, although this may not be a direct correlation.Also in Message 9 you suggest birds are more closely related to modern reptiles, and Feduccia's argument is they share an early common ancestor with therapods i.e. a basal archosaur. Yet is there any evidence that either of these groups were endothermic? As for organs, it has been shown that some therapod bones were pneumatised, along deep infiltration by air sacs in a similar distribution to that seen in birds. You can read more here Edited by Malcolm, : No reason given.
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Hyroglyphx Inactive Member |
The website I found agrees that that there were avian-related and non-avian-related dinosaurs, which I think we distinguished with therapods versus diapsids? Here are some of his theories concerning endothermy as they relate to our current duscussion.
"Fix reason firmly in her seat, and call to her tribunal every fact, every opinion. Question with boldness even the existence of a God; because, if there be one, he must more approve of the homage of reason, than that of blindfolded fear." Thomas Jefferson
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Meddle Member (Idle past 1271 days) Posts: 179 From: Scotland Joined: |
The website I found agrees that that there were avian-related and non-avian-related dinosaurs, which I think we distinguished with therapods versus diapsids? No because all dinosaurs are classed as diapsids, whereas therapods are a suborder of dinosaurs. The term diapsid relates to the ancestral structure of the skull, and includes crocodiles, snakes and lizards as well as birds, unlike mammals which are synapsids (like dimetrodon). I'm not saying dinosaurs were definitely endothermic, it was just you were suggesting this was a significant obstacle to a theropod/bird relationship, yet you argue for ancestral birds being related to some other group showning no evidence of endothermy.
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