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

I have for most of my life been fascinated by dinosaurs and other prehistoric beings. My father brought me books from the library where he worked even before I could read. I looked at the pictures and soon enough learnt to read the text. Many of those books were of prehistoric life. I distinctly remember the wonderful illustrations of Zdenek Burian - still to my mind the most real-looking pictures ever produced of ancient life. Now, almost every kid knows about dinosaurs, but there are many other things besides dinosaurs that lived long ago. One group that I was aware of very soon, was the mammal-like reptiles. I knew and understood much about them even when I was just six years old.

Now let me explain to you all what 'mammal-like reptiles' are. While that is the best ordinary English term for them, scientifically the matter is somewhat more complex. The mammal-like reptiles are a group of creatures that lived very long ago and included - as only a very small group within it - the actual ancestors of all mammals that live today. By strict cladistic standards, the mammals that live today, have to be included in the mammal-like reptiles - just as birds have to be included in the dinosaurs. A more scientific term for the mammal-like reptiles is the 'Synapsids' which refers to unique features of their skulls. The very earliest synapsids appeared in the Carboniferous period, over 300 million years ago. They looked very much like lizards - and other reptiles - back then. But they had the distinctive skulls already, with a single opening on each the side just behind the orbits of the eyes. This opening allowed for the development of strong jaw muscles, which played a major part in the subsequent success of the synapsids.

The early synapsids looked like lizards but they soon diversified to new and strange forms. A very early development was specialisation of the teeth. Most modern 'reptiles' have teeth of only one kind, while mammals usually have three different kinds of teeth - incisors, canines and molars. In the synapsids, the very early development was for the canines to be larger than the front and back teeth. Only relatively late in their history did they develop the typical incisor, canine and molar tooth pattern inherited by most of their descendants.

This tooth diversification helped the synapsids to eat better and 'process' more food and do it quicker than 'old-fashioned' reptiles. Some of them later also developed secondary palates closing their nostrils off from their mouth interiors, so they opened at the back of the palate and the synapsids could breathe while they were eating and chewing.

Other developments in the synapsids were of the legs. The very first species were crawling creatures with their legs sprawled sideways. Soon their descendents developed a semi-erect stance, with the body lifted off the ground. In many species the hind legs became fully erect, but the front legs in most cases still spread out sideways to a degree. The more upright stance allowed them greater speed and more sustained activity.

The more upright stance also allowed more efficient breathing. The synapsids' lungs were no longer compressed by the forward and backward motion of the legs as it happens in a crawling creature. Also, at some point the synapsids developed a diaphragm between the lungs and the abdominal organs, to make breathing even more efficient.

The better food processing, more upright locomotion, and better breathing, all together meant a higher metabolic rate. This means potential warm-bloodedness. Indeed warmbloodedness must have developed before the first 'true' mammals since all current true mammals have it. Along with warmbloodedness, is the development of hair and fur to cover the body and keep the warmth in. This too must have developed somewhere along the line in the synapsids.

The synapsids also must have developed milk glands, and suckled their young, at some point before the origin of 'true' mammals. However, the pre-mammalian synapsids most probably all layed eggs, rather than giving live birth, since two modern mammals, the platypus and the echidna or spiny anteater, still lay eggs but are considered 'true' mammals. Also, the pre-mammalian synapsids would not have had external ears, since the echidna and platypus don't have these either. If some pre-mammalian synapsids had ears and gave birth to live young, therefore, they would have had to acquire these characteristics separately and independently from the ancestors of today's mammals.

So ... the synapsids gradually developed all these mammalian characteristics and 'became' mammals ... but initially, they lacked all of them - except for the distinctive skull. And, apart from the developments that led to the ancestors of the mammals, synapsids acquired a great variety of other specialisations, leading to a diversity of different synapsid lines, of which only one eventually led to the mammals. Many of the synapsid lines became extinct soon after their origins - as a whole they seem to have had a high rate of extinction compared to other groups. In fact it seems that a great deal of luck must have been involved in actually preserving just those creatures who had the little bits of potential that made it possible for them to evolve into mammals like we know (and like ourselves!) today. But even after the first 'mammals' evolved (I put 'mammal' in inverted commas because there's still an issue over how 'mammalian' these things actually were at this point and some are now called 'mammaliaforms' rather than true mammals) there were still some other lines of non-mammalian synapsids that persisted - some almost up to the time of the extinction of the dinosaurs (or perhaps even longer). But eventually, all these other lines became extinct as well - along with a large percentage of the different lines of true mammals or mammaliaforms - to leave us only the mammals we know today.


To be continued ...

Dear Dr.. Willem Darwin,

Your magnificent exposition of synapsids has left me me here, feeling like a true nitwit.

Brilliant exxposition, ou kerel. Great minds always make their work sound easy and you are no exception.

Incidentally are we also busy evolving? I wonder. I have been suffering from having been given too much of my heart medication and I promise you that my skin is changing colour. !! My arms are now a warm beige colour - with just a white band where my watch is - with coco coloured spots. In fact I look like a leopard with measles. !!.

Perhpas I shall turn into a new species of a meisie from die Kaap.

Seriously though my friend, are you going to write a Introduction to the theory of Evolution for the young> It really is first class. Except that I cannot get my head around 300 million yearws ago. !!.

Well done.


christiane.
AlsoRan80!!.

Off to have something to eat. Might print it off to read. It looks like a promising series for to me.

Websailor

Hello Christiane and thanks for your comments! This 'exposition' is still very far from complete - it is only Part One! I'm going to write more. I'm currently working on a series of pictures of these creatures as well; I'm going to put them on my WebShots site (if all goes well). Pictures are very necessary for the complete story! I'll link to them from h2g2 when I'm finished.

Yes I believe we are still evolving! I have certain fairly unusual ideas about evolution, not quite 'orthodox'. If I said everything I believed about evolution I would be under attack from creationists on the one side and from orthodox evolutionists and dogmatic scientists on the other side! So I am a bit reluctant to write a book about evolution for 'the young'. Maybe if I had my own kids! But maybe other parents would object to their children being subjected to these strange notions!

I cannot imagine what kind of being *you* might be evolving into at the moment, though ... we'll have to wait and see!

I am going to write a little book about these particular mammal-like reptiles, at any rate. After all the bones are there, they've been dug out, and they are on display ... they must have belonged to *something*! No need to go into great theoretical depths ... I just wanna tell people about them! See the next journal posting (which I hopefully am going to complete a few minutes from now) ...

Oh hello Websailor! Another almost-simulpost! *off to write Part Two*

*waves* I am going this time

Websailor

Willem I told you what I looked like.

That is what I am evolving as


"a leopard with measles"!!

AR80

Incidentally very good that you read "Le Petit Prince" in French.I did not realise that you spoke French as well.