In: Biology
How is Diarthrognathus related to Triconodonts, Docodonts, Multituberculates, Symmetrodonts, Pantotheres, Monotremes, Marsupials, and Placentals?
Diarthrognathus or 'two joint jaw' animals is an extinct genus of synapsids which were mammal like reptiles. It is considered closest to mammalian ancestory. The skull of diarthrognathus is only 4 to 5 cm long and its dentition is not as specialized as that of cerain other mammal like reptiles from the Upper Triassic.
Certain mammal like features of diarthrognathus included not only the different types of teeth but also features like presence of a secondary bony palate that separates the nasal passage from the mouth, legs that are more under the body rather than spreading out from the sides and change in the structure of the lower jaw and the jaw jaw. The lower jaw articulates with the skull at two points, rather than one.
Thus, it is believed that all the other groups evolved from diarthrognathus, showing improvement in dentary and other bones of the skull which became more evolved to hold a larger brain and accomodate the changes in the facial bones.