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Author Topic:   Evolution of the Mammalian Jaw
RAZD
Member (Idle past 1405 days)
Posts: 20714
From: the other end of the sidewalk
Joined: 03-14-2004


(1)
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:
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:
... 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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Rebel American Zen Deist
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This message is a reply to:
 Message 1 by herebedragons, posted 01-08-2012 4:36 PM herebedragons has replied

Replies to this message:
 Message 10 by herebedragons, posted 01-09-2012 10:17 PM RAZD has replied

  
RAZD
Member (Idle past 1405 days)
Posts: 20714
From: the other end of the sidewalk
Joined: 03-14-2004


(1)
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
Wayback 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?
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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This message is a reply to:
 Message 10 by herebedragons, posted 01-09-2012 10:17 PM herebedragons has seen this message but not replied

  
RAZD
Member (Idle past 1405 days)
Posts: 20714
From: the other end of the sidewalk
Joined: 03-14-2004


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
... to learn ... to think ... to live ... to laugh ...
to share.


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This message is a reply to:
 Message 10 by herebedragons, posted 01-09-2012 10:17 PM herebedragons has not replied

  
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