528 FOSSIL VERTEBRATES/Mesozoic Mammals Figure Mammaliaforms (skulls in ventral view) of the Late Triassic and Early Jurassic, showing the derived (diagnostic) mammalian characteristics that evolved with these transitional mammaliaforms Sinoconodon, from the Lower Jurassic, is one of the most primitive mammaliaforms known; it has a jaw hinge of dentary condyle and squamosal glenoid, a petrosal promontorium for the cochlear canal, and a braincase floor for the trigeminal cranial nerve ganglion Morganucodon, from the Late Triassic and Early Jurassic, is the earliest known mammaliaform with precise occlusion of the upper and lower molars and a diphyodont dental replacement pattern Hadrocodium from the Early Jurassic, is the most derived among mammaliaforms in having a more enlarged braincase, an enlarged promontorium with curved cochlea, and the absence of primitive mandibular structures for attaching the middle ear (Morganucodon modified from KA Kermack et al (1981) Zoological Journal of Linnean Society London 71: 158.) from premammalian cynodonts is one of the best documented cases for the origin of a major vertebrate group Jaw Hinge and Middle Ear The earliest mammaliaforms are characterized by a ‘double joint’ in their jaw hinge (Figure 2: Morganucodon) They developed the derived (mammalian) dentary condyle to squamosal glenoid in the jaw joint, but also retained the primitive (cynodont) features of an articular-quadrate jaw joint The earliest mammaliaforms are similar to premammaliaform cynodonts in that the middle bones (also known as the postdentary bones) and the ossified Meckel’s cartilage are attached anteriorly to the mandible, in contrast to extant mammals, in which the embryonic Meckel’s cartilage disappeared and the middle ear bones are separated from the mandible in adults The quadrate (homologue to the incus of the mammalian middle ear) has developed contact with the petrosal bone in some mammaliaforms, as in modern mammals In other words, the earliest mammaliaforms have achieved the diagnostic mammalian features (the dentary/squamosal jaw joint and the way in which the middle ear is suspended by the cranium via the quadrate) (Figure 2) Other features of the jaw hinge and the middle ear in these Jurassic mammaliaforms are primitive features shared by more distant premammaliaform cynodonts In the more derived eutriconodont mammals, the middle ear elements, such as the angular (homologue to the tympanic in mammals), achieved some Figure Evolutionary relationships of Mesozoic mammals, mammaliaforms, and cynodonts The phylogenetic ranks of Mesozoic mammals and mammaliaforms correspond approximately to the temporal sequence of their diversifications Taxic macroevolution of Mesozoic mammals is characterized by several successive episodes of diversifications and emergence of new clades with new dental features Numbers represent nodes for successive ranks or hiearchies of Mesozoic mammalian phylogenetic relationships: 1, Mam maliaformes; 2, Mammalia; 3, Australosphenida; 4, Trechnotheria; 5, Cladotheria; 6, Zatheria; 7, Boreosphenida; 8, Eutheria; 9, Placentalia; 10, Metatheria; 11, Marsupialia (Cladogram modified from Z X Luo et al (2003) Science 302: 1934 1940.)