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Mammalia |
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Abbreviated Dendrogram
MAMMALIFORMES | MAMMALIA |--Australosphenida | |--Ausktribosphenidae | `--Monotremata `--+--Triconodonta `--+--Spalacotheroidea `--Cladotheria |--Dryolestoidea `--Theria | |--METATHERIA `--EUTHERIA |
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This section covers mainly the Mesozoic mammals, together with the monotremes. One of the major problems in this area is Dr. Thomas Rich. That is, everyone else seems fairly happy with the current arrangement, save that Dr. Rich keeps finding jaw bones that look for all the world like placental mammal (eutherian) bones, but are (a) older than any placental mammal ought to be and (b) located in Australia -- a placenta-free zone. Rich et al. (2001). The (now) conventional wisdom is that the characterstic "tribosphenic" molars of mammals evolved twice and that Dr. Rich's specimens are the results of convergent evolution in some monotreme ancestors, Australosphenida, and not placentals misplaced in time and space, Ausktribosphenidae. Luo et al. (2001a); Luo et al. (2002). With the unyeilding commitment to principled choice which characterizes our commentary, we have used both terms. This reflects the fact that, while we officially think Dr. Rich is wrong, we also have a sneaking suspiscion that he may be right. In either event, we are now completely prepared to claim that we supported the winner all along, no matter who may prevail. (ATW 020407)
The following is a condensed reminder of some of the many changes from basal synapsids to basal mammals. Beyond this level, there appears to be an unreasonable amount of disagreement on the phylogenetic branching order. I have recently seen two completely different published phylogenies of basal mammals, both in standard texts, and both of which purported to rely on exactly the same reference!
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