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Re: mammal evolution



>Since I've had pet ferrets, I've occasionally wondered how far back they'd
>have a common ancestor with humans. Also, since *so* many people seem to
>think ferrets are rodents or are closely related (despite being carnivores),

As someone who works on some REAL carnivores (Tyrannosaurus rex,
Deinonychus antirrhopus, etc.), I'd like to remind people that the
preferred common name for members of the mammalian order Carnivora is
"carnivoran", not "carnivore".  Many animals in Carnivora are not
carnivorous, and most carnivorous animals are not carnivorans.

>I've wanted to find out how far removed they actually are from rodents. Not
>that I have anything against rodents, but if someone called a dog or a cat a
>rodent, I'd feel inclined to correct them.
>
>I consulted the book 'The Rise of the Mammals', by Michael J. Benton. In
>it there's a diagram of mammal evolution which indicates that the paths
>leading to rodentia and carnivora diverged roughly 85 million years ago,
>whereas those leading to primates and carnivores diverged only 70 million
>years ago or so. I thus felt ready to tell all the 'rat-sayers' out there
>that *they* are as closely related to rodents as ferrets are.

This seems to be closer to what mammalian specialists believe than the
source quoted below.  Rodentia, Carnivora, and Primates are members of
three of the largest mammalian clades: Glires, Ferae, and Archonta,
respectively (although Ferae [=Carnivora + the ungulates may not be
monophyletic]).  True archontans are known by at least the Maastrichtian.

However, our understanding of placental mammalian phylogeny is in even
worse shape than our understanding of dinosaur phylogeny (probably due to
the rapid diversification of the Eutheria).

>I was looking through another book though, 'The MacMillan Illustrated
>Encyclopedia of Dinosaurs and Prehistoric Animals', and a diagram there
>of mammal evolution indicates that the line leading to primates branched
>off in the early Cretaceous, maybe 130 million years ago.

I know of no good fossil evidence for this.

>So which is right? Or is the history of mammal evolution still that iffy?
>I thought I had read of a genetic test that allows you to determine how long
>ago two life forms had a common ancestor. Wouldn't it be simple just to run
>comparative tests on members of all the existing orders of mammals, in order
>to get a more accurate picture?
>
>One more question, 'The Rise of the Mammals' mentions a now extinct group
>of mammals I had not previously heard of, the multituberculates. The book
>doesn't go into great detail about them, though it mentions they probably
>had pouches like marsupials. What are some of the unique characteristics of
>this group that separates them from marsupials?

Really, really, REALLY bizzare teeth!  Also, at least some multis (as they
are called for short) appear to have been arboreal critters.

                                
Thomas R. Holtz, Jr.                                   
tholtz@geochange.er.usgs.gov
Vertebrate Paleontologist in Exile                  Phone:      703-648-5280
U.S. Geological Survey                                FAX:      703-648-5420
Branch of Paleontology & Stratigraphy
MS 970 National Center
Reston, VA  22092
U.S.A.