the large reptile tree (subset Fig. 1) divides mammals into pre-therians (monotremes and kin), and therians (marsupials + placentals). Note: there are no allotherians here. They all nest elsewhere.
are multi-rooted teeth that erupt only once in mammals as they approach adulthood. Some molars are not multi-rooted, but ever-growing and lack enamel (xenarthrans). Most molars have several cusps, but a few (i.e. Stylinodon, Vintana, Glyptotherium) do not.
holds that marsupials have four molars while placentals have three (see below). I tested that tradition in the LRT and found that you can’t always count on this rule. Turns out there is a mix of molar numbers in marsupials and placentals (Fig. 1).
According to Wikipedia: “The early marsupials have… three premolars and four molars. In other groups [derived marsupials] the number of teeth is reduced. Marsupials in many cases have 40 to 50 teeth, significantly more than placental mammals … and they have more molars than premolars.”
In the LRT
the situation is a little different. Four molars is the basal number for all mammals. You’ll find four molars in Sinoconodon. By contrast, you’ll find that Kuehneotherium, Amphitherium and Akidolestes increase that number to six while another sister, highly derived Ornithorynchus sheds all molars as an adult. Juramaia had three molars. In tiny, but adult, Hadrocodium only two molars were present.
Among marsupials, the creodonts beginning with Hyaenodon had only three molars. The odd wombats Vintana and Zalambdalestes also had three molars. In Vintana a tooth anterior to the molar row is vestigial and non-working.
In placentals, four molars remain the basal number. The fourth molar is lost by convergence in several clades (Fig. 1), but retained in Asioryctes, flying lemurs, Henkelotherium + Nambaroo, tenrecs + whales and Onychodectes at the base of the Condylarthra. Glyptotherium appears to have had six molars. Bradypus appears to have had four. Only the anterior one of five teeth in Bradypus is distinct from the others and they are all ventral to the jugal, hence the molar designation. By contrast, in the related Peltephilus, the molars are vestiges and three premolars appear to be present.
With maturation, the anterior premolars of Morganucodon are shed and not replaced, and a diastema forms behind the upper canine that is elongated over time as premolars are lost.
All this is very interesting
and points to the importance of establishing a cladogram of relationships before establishing phylogenetic ‘rules.’ for various traits.