And the 'scientists' are REAL sure about this!
Yep! So sure that they PREDICTED many of the specifics of how this transition took place BEFORE the fossils were found. For instance, since the mammalian ear bones are involved in the reptilian jaw joint, and an animal obviously can't chew without a jaw joint, it was predicted that some critters among the "mammal-like reptiles" must have had a double jointed jaw, with both the reptilian and the mammalian articulations operative in the same creature.
There's not reason on earth to predict such a thing apart from the assumption that the transition really took place, must have done so by gradual steps, and basically that evolution is true (mammals did evolve from reptiles). But indeed exactly such critters were subsequently found as fossils.