I found this series very convincing until my professor made the comment that a one piece dentary bone was being selected for because it was stronger. This made no sense to me. Dimetrodon was the most fearsome predator of its time. It had a powerful bite and was specialized for killing other large land vertebrates. Morganocodon on the other hand, was a small, mouse-sized animal that feed on insects (source: Prehistoric Life DK Publishing). Why the need for a stronger jaw?
Remember that
Morganucodon is only a representative of a stage. There's no reason to suppose either that the selection pressures that got it so far were still acting on it, nor that it was ancestral to more derived forms.
It seems that in order for natural selection to drive the change to a one piece dentary bone, Dimetrodon would have to be snapping jaw bones in half and thus and being unable to produce offspring. Those that had larger (thus stronger) dentary bones would have been more reproductively successful and passed on the trait. Ok, that may be an exaggerated situation, but it does not appear that Dimetrodon had a problem with its jaw bone.
Well, there's more to strength than just not snapping. There's structural stability too. One of the developments in mammalian dentition was the development of molars and
chewing, which contributes to getting nutritional value out of food; this would be particularly selected for as mammals became warm-blooded and needed a higher turnover of calories. It seems to me that a single solid jaw would be better adapted to this purpose.