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Re: No Cretaceous placental mammals?
On 6/22/07, Jeff Hecht <firstname.lastname@example.org> wrote:
The most recent combination of molecular and fossil data I've seen pushes early
modern bird divergence back to almost 90 million years (the origin of
Neornithes). See Kerryn E. Slack et al, Mol. Biol Evol 23 (6) 1144-1155, 2006.
It is curious that living birds seem to have originated well before the K/T, in contrast to mammals.
Mammals (i.e., the crown group) originated long before the avian crown
group, back in the Triassic. Even if you exclude monotremes, therians
still originated before (crown-)avians (although not by much, if the
90Mya estimate is correct). I think you mean placentals?
And, even then, when you consider that the criteria for surviving the
extinction and for prospering in its aftermath are likely different,
it's not that surprising. After all, several mammalian and avian (or
nearly avian) lineages that survived the extinction died out later:
multituberculates, several stem-placental lineages (according to the
recent study), lithornithids, etc. (I also have to wonder how much
luck is involved with the survival of certain, none-too-speciose
clades, such as monotremes and [to a lesser extent] ratites.)