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Author | Topic: Evolution of the Mammalian Jaw | |||||||||||||||||||||||||||||||||||||||||||
herebedragons Member (Idle past 885 days) Posts: 1517 From: Michigan Joined:
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The origin of mammals from their amniote ancestors is considered to be one of the most fully documented examples of the evolution of a major taxa (ie. macroevolution). The changes occurred gradually over about a 130 Myr period from the Synapsids of the Late Carboniferous / Early Permian to Morganucodon of the Late Jurassic. Below is a illustration that depicts this sequence.
I want to focus this discussion primarily on the changes in the lower jaw bone. The illustration below highlights this progression.
Key to the bone names: D = dentaryQ = quadrate Ar = articulate An = angular I = incus (anvil) Ma = malleus (hammer) Ty = tympanic annulus (I do not have names for several of the bones, but I think it is irrelevant to this discussion) The key thing to note here is that the primitive Synapsid’s jaw is composed of several pieces whereas the early mammal’s jaw is made up of almost entirely the dentary bone (D). The angular bone (An) is greatly reduced in size and becomes the tympanic annulus (Ty). The articulate (Ar) becomes the malleus (Ma). The quadrate bone (Q) is also greatly reduced in size to become the incus (I). These latter three bones become the bones of the middle ear in modern mammals. The mammalian jaw has far fewer bones (Dimetrodon has 10 and early mammal has 8) and a less complicated structure than the Synapsid ancestor (jaw made up almost entirely of dentary). There is also a major change in the jaw joint with a transition from a quadrate / articulate joint in the Synapsids to a double articulation in Morganucodon and finally a dentary / squamosal articulation in mammals. I found this series very convincing until my professor made the comment that a one piece dentary bone was being selected for because it was stronger. This made no sense to me. Dimetrodon was the most fearsome predator of its time. It had a powerful bite and was specialized for killing other large land vertebrates. Morganocodon on the other hand, was a small, mouse-sized animal that feed on insects (source: Prehistoric Life DK Publishing). Why the need for a stronger jaw? It seems that in order for natural selection to drive the change to a one piece dentary bone, Dimetrodon would have to be snapping jaw bones in half and thus and being unable to produce offspring. Those that had larger (thus stronger) dentary bones would have been more reproductively successful and passed on the trait. Ok, that may be an exaggerated situation, but it does not appear that Dimetrodon had a problem with its jaw bone. I also do not see how genetic drift could account for this change. It would seem that this is a clear case of directional selection. Perhaps individuals with larger dentary bones were more attractive to potential mates? I am just at a loss as to how to explain this progression using the ToE. What are your thoughts? HBD
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Admin Director Posts: 13038 From: EvC Forum Joined: Member Rating: 2.1 |
Thread copied here from the Evolution of the Mammalian Jaw thread in the Proposed New Topics forum.
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Wounded King Member Posts: 4149 From: Cincinnati, Ohio, USA Joined:
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I don't know anything as to the merits of your lecturer's comments that this was due to selection for increased strength, but looking for some relevant references I came upon an interesting paper on the overall trend for the reduction in bone number in Synapsid skulls (Sidor, 2001 PDF).
The author suggests that rather than being driven by selection this trend, which includes the bones of the lower jaw, may be driven by an underlying bias in morphological changes which favours a reduction in the number of centers of ossification during development as opposed to their gain. This argument doesn't really address the very acute size reduction of many of the remaining elements, but there at least their co-option to the auditory system provides a clear selective pressure at work. TTFN, WK
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Dr Adequate Member (Idle past 312 days) Posts: 16113 Joined:
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I found this series very convincing until my professor made the comment that a one piece dentary bone was being selected for because it was stronger. This made no sense to me. Dimetrodon was the most fearsome predator of its time. It had a powerful bite and was specialized for killing other large land vertebrates. Morganocodon on the other hand, was a small, mouse-sized animal that feed on insects (source: Prehistoric Life DK Publishing). Why the need for a stronger jaw? Remember that Morganucodon is only a representative of a stage. There's no reason to suppose either that the selection pressures that got it so far were still acting on it, nor that it was ancestral to more derived forms.
It seems that in order for natural selection to drive the change to a one piece dentary bone, Dimetrodon would have to be snapping jaw bones in half and thus and being unable to produce offspring. Those that had larger (thus stronger) dentary bones would have been more reproductively successful and passed on the trait. Ok, that may be an exaggerated situation, but it does not appear that Dimetrodon had a problem with its jaw bone. Well, there's more to strength than just not snapping. There's structural stability too. One of the developments in mammalian dentition was the development of molars and chewing, which contributes to getting nutritional value out of food; this would be particularly selected for as mammals became warm-blooded and needed a higher turnover of calories. It seems to me that a single solid jaw would be better adapted to this purpose.
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Dr Adequate Member (Idle past 312 days) Posts: 16113 Joined: |
The author suggests that rather than being driven by selection this trend, which includes the bones of the lower jaw, may be driven by an underlying bias in morphological changes which favours a reduction in the number of centers of ossification during development as opposed to their gain. Oh, we're going to be orthogenesists now? Should we also paint ourselves with ochre and worship trees? Edited by Dr Adequate, : No reason given.
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RAZD Member (Idle past 1433 days) Posts: 20714 From: the other end of the sidewalk Joined:
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Hi herebedragons.
Nice post. A couple of great sources of information I have used about the stages of development of mammals in general and therapsids in particular are: Palaeos: Page not found(unfortunately this wonderful site is under reconstruction - and I will likely need to rebuild my bookmarks when the reopen - you could spend hours going up and down the tree of descent, visiting many different branches) and http://www.geocities.com/...naveral/Hangar/2437/therapsd.htm(and it appears that this is another broken link) Fortunately archived copies were made by Wayback Machine, and I found this version Wayback Machine0/http://www.geocities.com/CapeCanaveral/Hangar/2437/therapsd.htm
quote: This is an important point, because there was not a sudden jump from one hinge to the other. The animals in this transitional period would have a double hinge jaw like a snake has, and this could have significant advantages in holding prey. This alone is an excellent example of transition through many stages to develop "large scale" change in morphology. Another aspect to focus on would be the development of the different (and new in mammals, not existing in reptiles) kinds of teeth. These may tell you more about how the jaw developed. Enjoy. Edited by Zen Deist, : english mucho?by our ability to understand Rebel American Zen Deist ... to learn ... to think ... to live ... to laugh ... to share. Join the effort to solve medical problems, AIDS/HIV, Cancer and more with Team EvC! (click)
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Wounded King Member Posts: 4149 From: Cincinnati, Ohio, USA Joined: |
Oh, we're going to be orthogenesists now? I believe the preferred current term is 'Process Structuralist' But if you read the paper you will see that it isn't really an argument along orthogenetic lines but rather a probabilistic one based on the comparative frequencies of bone loss to bone gain in the fossil record.
Sidor writes: Evolutionary trends in morphological characters pose a problem similar to that of transition and/or transversion substitution probabilities in molecular systematics; not all character state changes are equally likely to occur [...]. The difference resides in the fact that whereas there are biophysical explanations for transition and/or transversion bias [...], the mechanisms underlying morphological biases are far less clear. In addition, not only can active evolutionary trends make some types of characters more probable to change than others (i.e., presence/absence characters), but they set up a systematic bias in the direction of character state change (i.e., changes from presence to absence). Perhaps because of the intractibility of assigning an objective differential weighting scheme to these types of characters, paleontologists have, on the whole, been content to consider all transitions to be equally likely [...]. Recognizing instances of biased morphological change is imperative for appropriately modeling character evolution in phylogeny reconstruction. The fact that not all genotype changes are equiprobable should surely render wholly uncontroversial the suggestion that all phenotype changes are similarly not equiprobable. It follows naturally that the distribution of likelihood of possible phenotypic changes (and their associated genotypes) is going to affect the evolutionary trajectory of the population.
Dr. A writes:
You totally should. Should we also paint ourselves with ochre and worship trees? TTFN, WK
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herebedragons Member (Idle past 885 days) Posts: 1517 From: Michigan Joined: |
an interesting paper on the overall trend for the reduction in bone number in Synapsid skulls Thanks, it does seem to provide at least part of the answer. I will read it more thoroughly in the next couple days.
This argument doesn't really address the very acute size reduction of many of the remaining elements, but there at least their co-option to the auditory system provides a clear selective pressure at work. I thought of this too. But it doesn't seem it would play a factor until the advanced cynodont stage as the bones aren't co-opted to the auditory system until then (Thrinaxodon had 2 ear bones - Procynosuchus still had reptilian auditory system). HBD
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herebedragons Member (Idle past 885 days) Posts: 1517 From: Michigan Joined: |
Remember that Morganucodon is only a representative of a stage. Yes, I realize that. In fact it is considered a "cousin".
There's no reason to suppose either that the selection pressures that got it so far were still acting on it Never-the-less, this is a clear trend that would at least appear to be driven in a specific direction. Of course all intermediates did not face the same selective pressures, but the pattern of change in this particular feature suggests directional selection. Plot the ratio of the size of the angular to the dentary and there is a clear directional trend. But I do see your point. It could involve different selective pressures at different stages.
nor that it was ancestral to more derived forms. It may not be a direct ancestor, sure. But it is placed in a series to show the development of a major taxon. It is more than reasonable to evaluate its features and assume it represents a stage in the development. To say that almost sounds like "Its in the series but it doesn't necessarily represent what actually happened."
There's structural stability too. One of the developments in mammalian dentition was the development of molars and chewing, which contributes to getting nutritional value out of food; True, I don't believe reptiles can move their jaw from side to side in a grinding motion. That would be an advantage once molars developed and diet began to become more varied. I don't see the solid jaw being better suited. Again Dimetrodon's bite was powerful as were many other reptiles and their jaws were plenty strong enough. The jaw joint would be a definite advantage, though. The jaw joint could explain the quadrate reduction and the expansion of the dentary bone in the area marked "Cp"(That is where the jaw muscle is attached in mammals, correct?). But I am still at a loss about the rest, particularly the angular. In order for natural selection to affect the direction of a characteristic, that characteristic needs to confer a reproductive advantage. The characteristic needs to improve the ability of the organism to leave offspring. I don't see how a reduction in bone size and a transition to a one piece dentary bone does that. That is my main question here. We have what appears to be directional selection with no reason to believe it confers a reproductive advantage. HBD
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herebedragons Member (Idle past 885 days) Posts: 1517 From: Michigan Joined: |
Hi Zen, (formerly known as RAZD correct?) glad to see your still around. How goes the battle?
Thanks for the links, unfortunately none of them worked. I was able to get on palaeos.com by going to the main page, but there was not many active links. copy of what I wrote to Dr.A:
quote: I am just trying to apply what I learn in my evolution course to a real scenario and hopefully learn something new as well!
The animals in this transitional period would have a double hinge jaw like a snake has, and this could have significant advantages in holding prey Really? I didn't take it to be like a snake jaw at all. A snake does have a double hinge, but it has a hinge between the quadrate and the articulate as do all (or most) reptiles. Then it has a second hinge at the top of the quadrate that allows the quadrate to rotate.
The double jaw joint of this intermediate group is quite different (not sure what genus this image represents, but it shows the mammalian and reptilian joints.)
I actually saw it as a disadvantage as the jaw would have to come off of one joint to rotate on the other. And indeed, it did not last long and the reptilian joint quickly gave way to the mammalian joint. While looking for that last image, I ran across this site. A couple interesting quotes
quote: This happened not just once but several times.
quote: I think it shows something extraordinary is going on here. Maybe I bit off more than I can chew trying to solve this problem.
Another aspect to focus on would be the development of the different (and new in mammals, not existing in reptiles) kinds of teeth. These may tell you more about how the jaw developed. Yea, the teeth are an interesting part of this too. The intermediates have permanent molars like mammals, but the rest of their teeth are replaced like reptiles. There is even a transition in how the teeth are fixed in the jaw. It may be easier to explain changes in teeth with natural selection and if the changes in teeth effect jaw development ... Thanks HBD
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RAZD Member (Idle past 1433 days) Posts: 20714 From: the other end of the sidewalk Joined:
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Hi herebedragons,
(formerly known as RAZD correct?) Yes, RAZD is the initials for Rebel American Zen Deist (see signature).
Thanks for the links, unfortunately none of them worked. Because this oneWayback Machine0/http://www.geocities.com/CapeCanaveral/Hangar/2437/therapsd.htm Is archived, clicking on the link doesn't take you to the archived site, you need to copy and paste the whole link. Let me see if this works:
archived therapsid site Yep.
I was able to get on palaeos.com by going to the main page, but there was not many active links. Yep. I wish they had kept the old site until the new one was up and running. It was awesome.
Really? I didn't take it to be like a snake jaw at all. A snake does have a double hinge, but it has a hinge between the quadrate and the articulate as do all (or most) reptiles. Then it has a second hinge at the top of the quadrate that allows the quadrate to rotate. Sorry to confuse: I didn't mean they had the same joints, just that both are double jointed (which obviates any creationist objection to them not being useful).
I actually saw it as a disadvantage as the jaw would have to come off of one joint to rotate on the other. And indeed, it did not last long and the reptilian joint quickly gave way to the mammalian joint. In the therapsids this could enable opening the back of the mouth for swallowing. Especially as it had to start with the intermediate bone being as large as seen on the reptile ancestors, yes? It may also have facilitated chewing and evolution of a jaw that can chew.
Yea, the teeth are an interesting part of this too. The intermediates have permanent molars like mammals, but the rest of their teeth are replaced like reptiles. There is even a transition in how the teeth are fixed in the jaw. It may be easier to explain changes in teeth with natural selection and if the changes in teeth effect jaw development ... Tie it together, the jaw and the teeth, perhaps it was co-evolution? Enjoyby our ability to understand Rebel American Zen Deist ... to learn ... to think ... to live ... to laugh ... to share. Join the effort to solve medical problems, AIDS/HIV, Cancer and more with Team EvC! (click)
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Dr Adequate Member (Idle past 312 days) Posts: 16113 Joined: |
But if you read the paper you will see that it isn't really an argument along orthogenetic lines but rather a probabilistic one based on the comparative frequencies of bone loss to bone gain in the fossil record. But a statistical tendency isn't a causal factor. Q: Why does John shop at WalMart?A: Lots of people shop at WalMart. It's not really an answer. You can't sensibly put the word "because" at the start of it.
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RAZD Member (Idle past 1433 days) Posts: 20714 From: the other end of the sidewalk Joined: |
Thanks for the links, unfortunately none of them worked. I was able to get on palaeos.com by going to the main page, but there was not many active links. Here's another site that talks about the transition from reptile-like to mammal-like critters:
quote: So changes to the jaw, the teeth and the skeletal orientation of the legs ... all by gradual microevolution steps with traits favoring faster hunting, better chewing, and better hearing being selected over time. But they all were synapsids, therapsids, they didn't evolve in one or two generations to be something entirely different ... Enjoyby our ability to understand Rebel☮American☆Zen☯Deist ... to learn ... to think ... to live ... to laugh ... to share. Join the effort to solve medical problems, AIDS/HIV, Cancer and more with Team EvC! (click)
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