We’re chordates, despite having evolved beyond two of the three defining characteristics of Phylum Chordata – presence of a notochord (replaced by the backbone) and of pharyngeal gill slits (since we’re not aquatic, their function has been assumed by the lungs and they occur only briefly in the embryo, if at all). Nonetheless, we are derived from forms which possessed a notochord and pharyngeal gills, so we are validly part of Chordata.
Likewise an animal descended from mammals which had ceased nursing its young because of their highly precocial development at birth would remain a mammal, since it would be descended from mammals which exhibit the class characteristic of nursing young.
Because fossil forms can only be classified based on hard-parts classification, the jaw joint characteristic (along with the presence of the malleus and incus as auditory ossicles) is the key point. This site discusses the evolution of the jaw joint and other mammalian characteristics at some length. Another characteristic is the definition of a variegated dentition, with incisors, canines, biscuspids, and molars present in some form, though possibly modified or lost in development, in contradistinction to the reptilian homogeneity of dentition.
As I recall, the hypothetical transition from reptile to mammal recognizes seven characteristics, including (in no particular order) the jaw joint, variegated dentition, lactation, hair/fur, viviparity, and two others – and the jaw-joint transition falls square in the middle of the seven, and therefore, as a petrifiable character and the mark that a majority of the distinguishing characters have been achieved, makes an excellent break-point between “reptiles tending towards mammalian characteristics” and “mammals preserving reptilian characteristics.”