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Author Topic:   Evolution of the Mammalian Jaw
herebedragons
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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


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Message 2 of 13 (647318)
01-09-2012 8:51 AM


Thread Copied from Proposed New Topics Forum
Thread copied here from the Evolution of the Mammalian Jaw thread in the Proposed New Topics forum.
    
Wounded King
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Message 3 of 13 (647331)
01-09-2012 9:57 AM
Reply to: Message 1 by herebedragons
01-08-2012 4:36 PM


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
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Message 4 of 13 (647395)
01-09-2012 3:20 PM
Reply to: Message 1 by herebedragons
01-08-2012 4:36 PM


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
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Message 5 of 13 (647396)
01-09-2012 3:22 PM
Reply to: Message 3 by Wounded King
01-09-2012 9:57 AM


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
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Message 6 of 13 (647455)
01-09-2012 5:34 PM
Reply to: Message 1 by herebedragons
01-08-2012 4:36 PM


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:

http://www.palaeos.com/...s/Units/400Therapsida/400.000.html
(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 http://web.archive.org/, and I found this version

http://web.archive.org/web/20090104191910/http://www.geocities.com/CapeCanaveral/Hangar/2437/therapsd.htm

quote:
... The earliest therapsids show the typical reptilian type of jaw joint, with the articular bone in the jaw firmly attached to the quadrate bone in the skull. In later fossils from the same group, however, the quadrate-articular bones have become smaller, and the dentary and squamosal bones have become larger and moved closer together. This trend reaches its apex in a group of therapsids known as cynodonts, of which the genus Probainognathus is a representative. Probainognathus possessed characteristics of both reptile and mammal, and this transitional aspect was shown most clearly by the fact that it had TWO jaw joints--one reptilian, one mammalian: ...

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?


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This message is a reply to:
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Wounded King
Member (Idle past 1566 days)
Posts: 4149
From: Edinburgh, Scotland
Joined: 04-09-2003


Message 7 of 13 (647476)
01-09-2012 7:16 PM
Reply to: Message 5 by Dr Adequate
01-09-2012 3:22 PM


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:

Should we also paint ourselves with ochre and worship trees?


You totally should.

TTFN,

WK


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herebedragons
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Posts: 1334
From: Michigan
Joined: 11-22-2009
Member Rating: 6.4


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


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herebedragons
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Posts: 1334
From: Michigan
Joined: 11-22-2009
Member Rating: 6.4


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


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herebedragons
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Posts: 1334
From: Michigan
Joined: 11-22-2009
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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


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RAZD
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From: the other end of the sidewalk
Joined: 03-14-2004
Member Rating: 3.8


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Message 11 of 13 (647637)
01-10-2012 3:11 PM
Reply to: Message 10 by herebedragons
01-09-2012 10:17 PM


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 one
http://web.archive.org/web/20090104191910/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?

Enjoy


we are limited in our ability to understand
by our ability to understand
Rebel American Zen Deist
... to learn ... to think ... to live ... to laugh ...
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Dr Adequate
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Message 12 of 13 (647649)
01-10-2012 4:25 PM
Reply to: Message 7 by Wounded King
01-09-2012 7:16 PM


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
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Joined: 03-14-2004
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Message 13 of 13 (812790)
06-20-2017 9:05 AM
Reply to: Message 10 by herebedragons
01-09-2012 10:17 PM


Another therapsid mammal transition site
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:
Science Olympiad - Synapsida

In the Early Carboniferous amniotes split into two lineages the synapsids and the reptiles or sauropsids (anapsids & diapsids). Traditionally, synapsids have been referred to as mammal-like reptiles. Synapsids did not evolve from reptiles, but both groups share a common ancestry with basal amniotes. The non-taxonomic term protomammals is preferred over mammal-like reptiles (Prothero, 1998, p. 379). Synapsids underwent an adaptive radiation during the Permian to become the dominant land animals. Synapsids (Class Synapsida) are traditionally divided into the pelycosaurs (a paraphyletic group unless it includes all synapsids) and the Therapsids (a parahyletic group unless it includes higher synapsids and mammals) (Prothero, 1998. pp 382-383).

Therapsids

Pelycosaurs became extinct by the Late Permian, but a second radiation of synapsids, the therapsids (Order Therapsida) would come to dominate the landscape. Therapsids are more advanced synapsids from which all mammals evolved. Evolutionary trends included a more upright posture, with legs tucked more directly beneath the body, enlarged temporal fenestrae to accommodate larger, more powerful jaw muscles, and increased heterodonty with teeth differentiated into incisors, canines, and molars. Therapsids evolved into a variety of carnivorous and herbivorous forms. We will focus on several groups.

Becoming a Mammal

A variety of synapsid fossils document the evolution of early amniotes to mammals as reflected in changes to skeletal structure. Many of these changes in skeletal structure may reflect the development of a new method for controlling body temperature (Dixon, 1988, pp. 184-185). Reptiles and early protomammals were cold-blooded or ectotherms. Ectotherms rely on external sources for body heat. Ectotherms may seek the sun, shade, or different water temperatures to control body temperature. Endotherms also exhibit behaviors to adjust body temperature but also generate heat from within the body. Endotherms like birds, mammals, and the later therapsids rely on a fast metabolism fueled by the quick and frequent processing of food for their internal source of heat energy.

Teeth became increasingly differentiated a condition called heterodont dentition. ... The quadrate and articular bones not only formed the jaw hinge they also made contact with the stirrup or stapes of the inner ear.

... The enlargement of the dentary continued until it came into contact with the squamosal bone, forming a new joint. The dentary bone also developed the coronoid process, which became an extra attachment point for jaw muscles. The dentary/squamosal jaw joint is a characteristic of mammals. The transition from the articular/quadrate joint to the dentary/squamosal joint is recorded in both the fossil record and embryological development of mammals.

Diarthrognathus (“two jaw joint”) is a synapsid that had both jaw joints functioning side by side, the articular/quadrate and the dentary/squamosal. The articular and quadrate eventually became detached from the jaw to form the bones of the middle ear. The quadrate came into contact with the stapes or stirrup, which had been in the ear since the early tetrapods. The quadrate became the incus or anvil and the articular became the malleus or hammer. The malleus, incus, and stapes make up the auditory ossicles that transmit vibrations from the tympanic membrane (eardrum) to the oval window of the inner ear. A recent fossil find provides further evidence for this sequence of events. Yanoconodon, from the lower Cretaceous of China, had the middle ear bones still connected to the lower jaw. So, this organism transmitted sound with its jaw/middle ear bones (Prothero, 2007, p. 280). Early in our embryonic development these bones start out as part of the jaw, but are transferred to the ear later in ontogeny (Prothero, 1998, p. 381). Interestingly, the layout of bones in the ear of Yanoconodon is the same as cartilage precursors in mammalian embryos before the ear and jawbones separate. The ontogeny (development of an organism) can sometimes reflect the phylogeny (evolutionary history) of a species.

The idea that ontogeny recapitulates phylogeny was first formulated as the Biogenetic Law by German zoologist Enrst Haeckel (1834-1919) in 1866. Haeckel’s law stated that an organism’s development followed the same path as its evolutionary history. Although the original form of his law is now rejected, embryological development is used to help build evolutionary histories of species.

Therapsid limbs and joints were modified to change from a semi-sprawling gait to an erect gait. The pelvic and pectoral girdles along with the limbs became structured to tuck the limbs under the body. The backbone came to support an up and down flexure instead of a side-to-side bending. These limb improvements had a greater potential for fast movement. ...


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 ...

Enjoy


we are limited in our ability to understand
by our ability to understand
RebelAmerican☆Zen☯Deist
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