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Some Fossils


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#1 PhilC

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Posted 06 July 2010 - 05:37 AM

These are taken from the same book mentioned in the phsyiology thread.

Embryology shows that in fish the first pharyngeal arch becomes Meckel’s cartilage and eventually the lower jaw and the second pharyngeal arch becomes the hyomandibular bone.

Look at the hyomandiular (in red) which is supporting the jaw

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In amphibians, the jaw is fused with the cranium and, as emrbyology shows, the second pharyngeal arch becomes the stapes, which helps the amphibians hear. The stapes can perform its function because it is not being used to hold the jaw in place. As both the hyomandibular and the stapes both arise from the same place in the embryo they can be said to be homologous

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A process starts developing in the lower jaw that becomes the jaw joint. In amphibians reptiles and birds too the joint is made by the quadrate joining onto the articular )the quadrate and articular bones arise from ossification of Meckel’s cartilage which originates in the first pharyngeal arch. Early synapsids (mammal like reptiles) are like this, with the quadrate and articular moving back in the jaw, closer to the ear:

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Later ones have changed their jaw joint, the process developing above becoming the new one, freeing the quadrate and articular to be used elsewhere. And that then becomes much more mammal like, with the process described before now becoming the new joint, and the ossified parts of Meckel’s cartilage becoming two ear bones:

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Looking at the fossils a definite progression can be seen with bones moving from the jaw and into the ear. The swapping of jaw joints is the trick part, though. There must have existed an animal with two jaw joints, and one has been discovered known as Probainognathus.

This is embryological evidence of the pharyngeal pouches developing into different bones agreeing completely with the modern physiology showing (along with other evidence, such as the embryological circulation) that birds are closer to reptiles (because Meckel’s cartilage shows the same formation in both birds and reptiles, and the fossil evidence backs this up and shows a clear movement from fish -> reptiles (via amphibians) -> mammals.

One final point - look at the development of teeth through these fossils. That has an important bearing in another area. But that's another story for another day.

#2 PhilC

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Posted 09 July 2010 - 04:54 AM

Here we have a classic picture of transition from fish to mammals, with fossils backing up the embryological detail.

If we include the variation process mentioned here:

http://www.evolution...?showtopic=3491

We can see that few "beneficial" mutations would be required. It is the level of variation that is crucial, not the number of mutations, but this shows that they can occur:

http://www.evolution...?showtopic=3471




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