Early Mammal Evolution
For those among us who might be fascinated by evolutionary topics, here is an early mammal evolutionary biologist expert on the subject. Common knowledge on this is that mammals were only worth considering, perhaps only evolving near/after the K/T extinction event killed off the dinos. Actually the mammal line diverged from the reptile line WAY back in the late Carboniferous era, ~310 Ma, close to 100Ma before the rise of the dinosaurs in the Triassic.
http://www.fossilmall.com/Science/GeologicalTime.htm
The early pre-mammal line (usually called, confusingly, mammal-like reptiles) were the dominant large animal form on earth from that time until the end-Permian extinction, the greatest extinction event in Earth's history. Before the age of the dinosaurs, was the age of the mammal-like reptiles. And then after, they down-sized, specialized, and made a living all through the eras (Triassic/Jurassic/Cretaceous) dominated by the big lizards, sometimes making a living eating baby ones, usually nocturnal, usually insectivore, limited in size to that of a badger.
Marsupials appear in the lower Cretaceous (125Ma), I'm assuming eutherians/placentals in the upper Cretaceous, unless the metatherians & eutherians co-evolved (I don't know, seems unlikely). We know that marsupials persisted in Gondwanaland (incl. South America, Australia), while the early marsupial fossils were recovered in China (Laurasia), which implies there was no inherent geographical isolation between incipient metatherian/eutherian mammaliformes, which doesn't rule out some manner of derivation from metatherian to eutherian - this is above my understanding, at present, may be yet TBD.
[img]https://i.makeagif.com/media/10-13-2015/ADlQ85.gif [/img]
The monotremes (platypus/echidna) are a different story, as they apparently evolved in polar forests in Antarctica (Gondwanaland), and were indeed geographically isolated from the north, thereby do present a marvelously preserved early-mammal body plan, in the flesh.
One big remaining question is whether significant placental/eutherian mammalian radiation occurred well before, just before, or just after the K/T extinction event. The molecular clock data indicates well before, according to this expert, in the q/a
This expert tells much of that early story in some detail. Enjoy !
https://www.youtube.com/watch?v=ukJfEt0Jh6E