Re: No Cretaceous placental mammals?

[dinoboygraphics@aol.com]

Why, oh why does this keep coming up?  The fact that placentals 
divurged genetically from marsupials in the Jurassic is in no way 
contradictory to the fact that crown group placentals have a more 
recent divurgence time (possibly even Cenozoic), nor is it in 
opposition to the fact that we don't see fossils with "key" placental 
synapomorphies.  Genetic divurgence has to occur prior to phenotypic 
divurgence, and since we tend to pick "important" characters by 
reference to what we can observe, we should expect genetic divurgence 
to occur well before the evolution of the crown-group synapomorphies 
selected via post-hoc observation of extant groups as "important".

That is, the sibling populations that were (genetically speaking) never 
to mix again and give rise to groups that we would call placental and 
marsupial mammals did not utilize the derived form of reproduction seen 
in either group today.  That evolved later, most likely several 
millions of years after the speciation event that lead to the 
divurgence of these linneages.


Scott Hartman
Science Director
Wyoming Dinosaur Center
110 Carter Ranch Rd.
Thermopolis, WY 82443
(800) 455-3466 ext. 230
Cell: (307) 921-8333

www.skeletaldrawing.com



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