Geomagnetism and the rate of Sea-floor Spreading

Hey Percy,I get the exact same thing Joe does occasionally. I learned quickly that the server error message was bogus - the post I tried to make was actually posted. I just have to crawl back through EvC's front door (just like starting a new session), and my message is nicely posted (singly). Hitting "back" then resubmit gets the double post.

Right - only at EvC. I'll be happy to forward the error message next time it happens.

It's a bit more complicated, but you have the gist. The earliest major radiation of marsupials post-dates the breakup of Gondwanaland when what later became Australia, Antarctica and South America were joined. Interestingly, the earliest metatherian fossils (Late Cretaceous) are from North America AND South America. All N. American marsupials went extinct during the Miocene. It looks now that marsupials went from S. America to Australia via Antarctica, and possibly from N. America to Asia. Weird, hunh? Mesosaur fossils found in South America and Africa also show these two continents were once joined, but not since about 125 Mya. Correct. The Great American Interchange took place approx. 3 mya. The only living N. American marsupial, Didelphus virginiensis, entered at that time from S. America. It also marks the end of the great metatherian radiation in S. America, possibly due to competition from placentals from the north.Obviously the Antarctic populations went extinct when the continent arrived in the polar regions, leaving only Australia to carry on with the impressive evolution of marsupials.

I have no idea who you quoted here, but it certainly wasn't me. Questions of biogeography don't have any obvious application (to my mind) on the issue of magnetic field reversal. If you'll refer back to the post to which I was replying, I was confirming Jazz's memory of marsupial phylogeny and biogeography. The latter DOES have relevance as an additional line of evidence that continental landmasses which are now halfway around the world from each other were once joined, which was the point. If they weren't, then there's no other explanation for the distribution of these organisms.The "weird" comment was in relation to N. America being the original home of most marsupials. They then disappeared completely from the continent, then were re-established by immigration from continents to which they had originally dispersed. I just think it's interesting (hence, "weird") that they had to recolonize their "birthplace" as it were.As far as this bit goes: We "know" this because we have a record of primitive marsupials at point A, then they disappear and other marsupials appear at point B and C, then different ones later at point D, and finally much later even more derived marsupials reappear at point A again - with these latter related fairly closely to intermediates found at point B. Infer what you will, this is the observed evidence from the rocks.