h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

There's one more development that to me is the most fascinating of all. Unlike the other things that happened during the evolution of the synapsids, detailed in the previous posting, the usefulness of this is not immediately evident. What is this development? It is this: bones that used to be part of the lower jaw, ending up in the inner ear - so that, today, mammals use certain bones for *hearing*, that other animals use for *eating*! Surely you must agree that this is very weird... in fact it may be even harder to conceive *how* such a thing happened, than it would be to conceive *why*. Some people have used this as an argument against evolution. Unfortunately such arguments are rather old by now ... and there exist, at the moment, a very interesting series of fossils that vividly show, by clear steps, how this whole incredible process actually happened. The evolution really did happen.

The basic lower jaw of the ancient lizard-like ancestors of the mammals consists of four bones: the dentary, the angular, the articular and the quadrate. The 'hinge' of the jaw was formed by the quadrate and the articular. Over time in the synapsids, the configuration of the bones changed: first of all the dentary increased in size, and became the major tooth-bearing part of the lower jaw. The angular, very early on, developed a kind of 'recess' that may have supported some of the original ear-drum structure and helped conduct and amplify sounds. The quadrate and the articular gradually reduced in size. The angular later became a 'loose' bone at the side of the dentary, rather than its original form as a large bone at the rear. At some crucial point, the synapsids switched to having two 'hinges' in the jaw - one hinge, the 'old' type, using the quadrate and articular (which by this stage were very small bones) and another, the 'new' or mammalian type, between the dentary of the lower jaw and the squamosal of the upper jaw. There is a tiny little South African synapsid that exemplifies this condition - it is called Diarthrognathus, or 'two-hinge jaw'. This little creature is almost right on the dividing line between the mammalian and reptilian jaw.

And yet ... the evolution did not stop there! As the synapsids evolved even further towards the mammalian condition, the three bones, the angular, articular and quadrate, continued getting smaller and changing position. Once the dentary-squamosal jaw joint had been established, the quadrate and articular became unnecessary for the jaw hinge - in fact they became totally unnecessary as far as the structure of the lower jaw was concerned. But they still had some connection with the ear drum and the process of hearing, which started with that particular kind of 'recess' developed by the angular. The quadrate and dentary remained associated with the angular, and when they became 'loose' they could have helped with the transmission of vibrations to the inner ear. Shrinking in size, they became more suited to the transmission of fine vibrations, and indeed in mammals the quadrate and articular became the malleus (hammer) and incus (anvil) bones of the middle ear.

The whole progression - from ancient-reptilian to modern-mammalian - is visible in the fossil record, in clear stages. The South African mammal-like reptiles illustrate some of the middle to later stages of this process. Some of the 'gaps' are filled by fossils from other places ... the very early stages are shown in the pelycosaurs of America and Europe, while some of the middle and later stages are shown in advanced cynodonts from South America.

I will have to go and look for all the detailed info about this. I know a lot of it is in a book on Vertebrate Paleontology that I borrowed from the library of the University of Limpopo. There's another great book about mammal-like reptiles that I also think is to be found in the University library. I really want to put something about it into the mammal-like-reptile book I'm planning. I'll have to leave out the most contentious parts, though. The more contentious parts, I just might share with you folks here on h2g2! To just set it out in brief: the transition from the reptilian-style jaw, to the mammalian-style middle ear, is very well established in the fossil record, and - in my view - not only refutes creationism, but also 'strict' Darwinian evolution!

For, after all ... *HOW* does the gradual movement of bones from the jaw joint, to the middle ear, improve a creature's 'fitness'? In the myriads of small steps necessary to get the bones from the one place, to the other, hardly any one such step really made any difference to the creature it happened in. And yet ... in the end, it did make a difference: the current mammalian ear is certainly more sensitive to sounds than the 'old' ear used to be. The lower jaw, now being composed of just one, large bone, is simplified and potentially stronger, for better chewing. 'Science' has to explain how the various stages of this process were 'selected for' to make the eventual outcome possible. This is where I part company with Darwin. I simply postulate that it is not only the 'fittest' that survive. 'Fitness' is a term lacking a clear meaning (I will not go into that in detail here and now). In actuality, I say, changes can be 'selected for' simply because they do not drastically *disadvantage* the creature they happen to. There may be no discernible improvement in 'fitness' in a creature due to some kind of genetic change ... and yet that genetic change may be preserved and in some way spread to other creatures. Some such changes may end up opening up 'avenues' of change that may ultimately lead to improved 'fitness' ... which I will have to qualify in a changed understanding of the word 'fitness'. Basically, such changes allow a creature to embark on a different way of life ... and if there happens to be no serious local competition for such a 'different' way of life, a new species may become established.

More about synapsids in Part Three ...

Please teacher, can I read it tomorrow when my brain hopefully will feel less scrambled. !!

thank you.

AlsoRan80
Very tired face.

Likewise

Websailor

Hey, you should be grateful I'm not making you memorize a whole vertebrate paleontology textbook!

At any rate ... I will soon have pictures for you ... pictures make the whole thing much easier to understand!

Wandering jaw bones!

Adds additional meaning to the 'Jaw, jaw' phrase.
The intricacy that's evolved from a method that is based on hit or miss is amazing.