Whether this statement is true or not I guess depends how pedantic you want to be, but for me the line of mammalian evolution is another perfect example of the gradual nature of evolution and the problems of wanting to put everything into neatly defined boxes.
It seems that the evolutionary line leading to mammals branched off just before reptiles had fully become reptiles and one of the main pieces of evidence for this is in the skin. Both reptiles and birds (which are well accepted to have evolved from within the reptiles) have skin make from beta-keratin, whereas mammals have alpha-keratin. Beta-keratin is much more rigid and gives reptiles their tough scales and birds their stiff feathers.
So, to take a quick trip through the history of early land dwelling vertebrates, we would start off with amphibians and the now extinct labyrinthodonts. In a relatively short amount of time (in geological terms anyway), some began to develop more reptilian features – in particular developing both waterproof skin and eggs to become fully terrestrial. There was a division in this group at a relatively early stage – one branch led to reptiles (with beta-keratin evolving after this branch point) and the other led to what are often called the mammal like reptiles or more correctly the synapsids, and the mammals eventually arose within this group. So technically the synapsids are not reptiles, though the differences from true reptiles are really very small – at this early stage anyway.
Synapsids first appeared around 310 million years ago, towards the end of the Carboniferous period, but the earliest mammal wasn’t to arise for another 130 million years. During that time they ever so gradually developed the various characteristic features of the mammals, and along the way became, for a while at least, a highly varied and dominant group. When they first appeared there were no dinosaurs – they were to come much later – so they radiated into many forms, large and small, carnivores and herbivores. You might say they were the dinosaurs of the day, and some were certainly very impressive beasts.
As I mentioned, it took 130 million years for synapsids to become fully mammalian, but of course the evolution of a group does not march forward in one steady progression, it can only act at single species level. There is a whole raft of characteristics that set mammals apart from reptiles and they didn’t all suddenly appear at the same time. So if, for example, one species evolves a particular feature, it will be the only synapsid with that attribute for a while, but given time it will certainly radiate out into a number of species. Amongst these, one of them may develop another characteristic and it will radiate in turn. So at any one time there will exist a number of species, some more mammal like than others. We eventually reach the point where we have one fully mammalian synapsid (mammals are also considered synapsids) and this will be the common ancestor to all mammals. Existing at the same time would have been a host of other synapsids with fewer mammalian characteristics, but these were all destined to become extinct, leaving mammals as the only survivors of the group.
I find it interesting to consider that through the 130 million years from the first synapsid to the first mammal, there was just one evolutionary line ultimately leading to mammals – all the other species that branched off this line were fated to eventually become extinct. So, at any one point in time there would have been just one species amongst the many synapsids that would have been the ancestor of all mammals, and, of course this would make it our own ancestor too.
I can’t really leave this subject without mentioning a few of the more important features that make a mammal a mammal.
Fur, warm blood and the production of milk to feed the young are obvious ones. The positioning of the legs is also very important. If you think of a typical reptile, the legs are splayed out to the sides. As mammals developed, the legs gradually moved directly underneath the body – a much better position for fast movement (this incidentally, was also an advantage that the dinosaurs evolved). The structure of the ear is also unique. Whereas reptiles have a single bone in the ear, mammals have three which act to magnify the movements from the eardrum and give much better hearing. Amazingly, the two extra bones were originally part of the structure of the jaws and their gradual migration to the ear can be traced through the synapsid evolution.
So, to come back to the original question, did mammals evolve from a reptile? Well, modern classification generally says no, it was something that was almost, but not quite fully, reptilian. It just goes to show how the gradual nature of evolution really doesn’t like things being put into sharply defined boxes, but if the difference is just down to the type of skin protein then my gut reaction goes with the reptiles.