Register | Sign In


Understanding through Discussion


EvC Forum active members: 63 (9161 total)
4 online now:
Newest Member: popoi
Post Volume: Total: 915,580 Year: 2,837/9,624 Month: 682/1,588 Week: 88/229 Day: 60/28 Hour: 2/4


Thread  Details

Email This Thread
Newer Topic | Older Topic
  
Author Topic:   Evolution of the Mammalian Jaw
herebedragons
Member (Idle past 847 days)
Posts: 1517
From: Michigan
Joined: 11-22-2009


Message 1 of 2 (647211)
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

Admin
Director
Posts: 12993
From: EvC Forum
Joined: 06-14-2002
Member Rating: 2.1


Message 2 of 2 (647317)
01-09-2012 8:51 AM


Thread Copied to Biological Evolution Forum
Thread copied to the Evolution of the Mammalian Jaw thread in the Biological Evolution forum, this copy of the thread has been closed.

Newer Topic | Older Topic
Jump to:


Copyright 2001-2023 by EvC Forum, All Rights Reserved

™ Version 4.2
Innovative software from Qwixotic © 2024