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Author Topic:   Evolution of the Mammalian Jaw
Dr Adequate
Member (Idle past 284 days)
Posts: 16113
Joined: 07-20-2006


(1)
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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 Message 1 by herebedragons, posted 01-08-2012 4:36 PM herebedragons has replied

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Dr Adequate
Member (Idle past 284 days)
Posts: 16113
Joined: 07-20-2006


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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 Message 3 by Wounded King, posted 01-09-2012 9:57 AM Wounded King has replied

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 Message 7 by Wounded King, posted 01-09-2012 7:16 PM Dr Adequate has replied

  
Dr Adequate
Member (Idle past 284 days)
Posts: 16113
Joined: 07-20-2006


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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 Message 7 by Wounded King, posted 01-09-2012 7:16 PM Wounded King has seen this message but not replied

  
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