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Re: No Cretaceous placental mammals?

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


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