Figure 1. Phylogenetic relationships of the major groups of Mesozoic mammals and their minimal geological ages. Different possible definitions of Mammalia are indicated.

Figure 2. The origin of the mammalian ear and jaw articulation. (a) A generalized vertebrate. The dentary is one of many jaw bones; the articulation between the skull and jaw is formed by the quadrate (dark blue) and the articular (yellow). The tympanum, or eardrum (light blue), is behind the quadrate. (b) An early member of the mammalian lineage. The dentary is large, but the articulation between jaw and skull is still formed by the quadrate and articular. The tympanum is in the lower jaw, behind the dentary. (c) A mammal. The dentary is the only bone of the lower jaw. Bones of the old articulation between skull and jaw are now much smaller, free from the jaw and, with the exception of the stapes (green), have different names. The stapes, arranged at about 90° from the other ear ossicles, transmits the sound vibrations picked up by the eardrum and ossicles into the inner ear, where they are transduced into electrical impulses on their way to the brain.

Figure 3. Diversity of Mesozoic mammals and early relatives. (a) Pachygenelus, a member of the putative group from which mammals originated. (b) and (c) Sinoconodon and Morganucodon, respectively, members of the early radiation of mammals. (d), (e) and (f) Monotreme, Multituberculate and Vincelestes respectively, members of the middle radiation of Mesozoic mammals. Monotremes are the only survivors of these groups. (g) and (h) An early marsupial and placental relative, respectively. The roots of both, marsupials and placentals, seem to originate in the Mesozoic, but the living groups and their direct ancestors appeared later during the Tertiary. Figures (a–c) and (e) are adapted, with permission, from Hopson JA (1994) Synapsid evolution and the radiation of non-eutherian mammals. In: Prothero DR and Schoch RM (eds). Major Features of Vertebrate Evolution. Short Courses in Paleontology 7: 190–219. Chicago: The Paleontological Society.