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Author Topic:   Evolution of the Mammalian Jaw
herebedragons
Member (Idle past 858 days)
Posts: 1517
From: Michigan
Joined: 11-22-2009


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Message 1 of 13 (647316)
01-08-2012 4:36 PM


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.
Note: My original source for this image was my Futuyma, evolution textbook, but I could not figure how to add a scanned photo (not to mention the legality). This image is almost identical.
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 = dentary
Q = 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

Replies to this message:
 Message 3 by Wounded King, posted 01-09-2012 9:57 AM herebedragons has replied
 Message 4 by Dr Adequate, posted 01-09-2012 3:20 PM herebedragons has replied
 Message 6 by RAZD, posted 01-09-2012 5:34 PM herebedragons has replied

  
herebedragons
Member (Idle past 858 days)
Posts: 1517
From: Michigan
Joined: 11-22-2009


Message 8 of 13 (647484)
01-09-2012 8:05 PM
Reply to: Message 3 by Wounded King
01-09-2012 9:57 AM


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

This message is a reply to:
 Message 3 by Wounded King, posted 01-09-2012 9:57 AM Wounded King has seen this message but not replied

  
herebedragons
Member (Idle past 858 days)
Posts: 1517
From: Michigan
Joined: 11-22-2009


Message 9 of 13 (647490)
01-09-2012 9:16 PM
Reply to: Message 4 by Dr Adequate
01-09-2012 3:20 PM


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

This message is a reply to:
 Message 4 by Dr Adequate, posted 01-09-2012 3:20 PM Dr Adequate has not replied

  
herebedragons
Member (Idle past 858 days)
Posts: 1517
From: Michigan
Joined: 11-22-2009


Message 10 of 13 (647492)
01-09-2012 10:17 PM
Reply to: Message 6 by RAZD
01-09-2012 5:34 PM


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:
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.
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:
For reasons we don't fully understand, several lineages of synapsids including the one that would eventually give rise to the mammals began to evolve changes in the jaw joint.
This happened not just once but several times.
quote:
Some evidence suggests that the change in the quadrate-articular complex improved hearing. Other evidence suggests that these changes were a byproduct of early mammals' increasing brain size. These ideas are not mutually exclusive, of course, and more research is needed.
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

This message is a reply to:
 Message 6 by RAZD, posted 01-09-2012 5:34 PM RAZD has replied

Replies to this message:
 Message 11 by RAZD, posted 01-10-2012 3:11 PM herebedragons has seen this message but not replied
 Message 13 by RAZD, posted 06-20-2017 9:05 AM herebedragons has not replied

  
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