The Lord is my banner said:
Jet Black, I disagree - the changes you imply have not been proven. Changes within species are minimal compared to those claimed for the progression of simple creatures to more complex. Where does the new information for that come from? You can only modify what is aready present, either overtly or hidden as in recessive genes. We only need to look at all the breeds of dog with their great variety in size, shape etc. to see that varity within a kind has taken place, and still does. But they're still dogs, and they will never become anything in future but more dogs.
this is why characters are derived... they are modified forms of their ancestral characteristics. continual modification basically creates something completely novel when one compares the start point to the end point. No-one is saying that all canines will ever produce anything other than an organism with derived canine characteristics, and this is what we expect. The problem is, when you turn that round and work backwards, where do you stop? There is already a line of transitional species stretching back to the most primitive carnivores, the mesonyx and so on.
To go over the therapsid in more detail:
First I will start off with a little overview of reptile mammal comparisons: Reptile skulls consist of a larger number of individual bones than mammals. Reptile teeth are all conical, and have multiple sets within a lifetime, compared to Mammals who have a number of different tooth types, from incisors to molars, and many of these are permanently set within the jaw. The Lower jaw of reptiles consists of a number of bones, whereas the mammal jaw consists of only one (the dentary, which articulates on the squamosal bone). Reptiles only have one bone in the ear - the stapes, whereas mammals have three. Reptiles can also hear through their jaws, since the jaw transmits sound to the ears. This is not the case in mammals. Reptiles have a small hole in the skull where the pineal gland extends through, in mammals this is absent.
When we look at the triassic therapsids we see a more or less reptillian type of jaw, with the articular bone attached to the quadrate bone in the skull (reptile jaw joint), but in later fossils, these bones are much smaller, and the dentary and squamosal bones have grown and got much closer together (more mammalian).
By time we get to the much later therapsids, the cynodonts, we see in Probainognathus that it actually has two jaw joints - both reptilian and mammalian! - this is about as intermediate as one can get; having one feature of each. Now the earlier quadrate-samosal joint which made the reptillian jaw was still jointed as a jaw, however now it also has a connection to the stapes in the ear, and begins to resemble the incus bone found in the mammalian ear. Later on in the ictidosaurians, the mammalian joint continues to become stronger, and the reptillian joint becomes weaker, but is still present, and the squamosal and dentary bone joint is now completely self functional and no longer reliant on the assistance of the older reptilian joint. A particular fossil, Diarthrognathus, is about as finely tuned between reptile and mammal in this respect as one can get.
By time we get to the late triassic, we are finding examples of Morganucodonts, where the double joint is still present, but with the mammalian joint taking effectively all of the stresses of jaw operation and the articular-quadrate hinge functioning more or less only for sound transmission.
bear in mind that this is not a literal transitional line, but a chart of transitional species. The species that eventually became the mammals is probably not included in our fossils, but this is not what is being demonstrated. Some of the sister species will have lived along side the species that eventually branched off into our line (and species that also branced into the monotreme and marsupial lines as well) so we expect to find fossils of therapsids around at the same time as their "more advanced" cousins.
so we can see the evolutionary path from reptile to mammal jaw, in which the bones slowly migrated together to form a second functional jaw joint, for a while there were two jaw joints and one took on more stresses while the other became more responsible for sound transmission with the reptilian jaw's quadrate bone becoming the mammalian incus, and the reptilian articular bone becoming the malleus. So this is in opposition to your earlier comment that the therapsids are just a species that exist in the middle, they are whole groups of species, which over time developed, and no doubt competed with other therapsid species just like we compete with other mammal species and one of those species eventually became the mammals.