Fossil Evidence

In looking for fossil evidence of evolution we are looking for transitionals. Here is a sequence of fossils which show a transition between fish and mammals. These fossils exist, so they are not hypothetical. These are simplistic images, but actual diagrams of some of these fossils can be found here, also a much more detailed discussion of the topic can be found at the Talk Origins page.

Notice that the top jaw is not connected to the cranium. Also notice the bone that holds the jaws in place, the hyomandibular. The jaw hinge is based around the quadrate and articular bones. This is the state of fish today and is seen in fish fossils.

Now the jaw is fused to the cranium. The hyomandibular is now used to transmit sound. The quadrate and articular bones are still used as the jaw hinge. Once again, this is seen in fossils and modern amphibians.

This is the most interesting part. The process that is growing up from the lower jaw becomes the new jaw/skull joint and the process that is growing behind the jaw becomes the timpanic bone. There are examples of animals with two jaw joints, such as Probainognathus where the usual quadrate and articular bones still operate as a jaw joint and also the squamosal bone being used as a hinge, like modern mammals.

Here the full development of the mammalian ear can be seen and also the full differentiation of the teeth which is an important part of efficient eating necessary for survival as a warm blooded creature.

As taken from Biology A functional approach M B V Roberts 1986 Nelson