AllanCrossman comments on The Featherless Biped - Less Wrong

1 Post author: Annoyance 02 September 2009 05:47PM

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Comment author: AllanCrossman 31 October 2009 07:45:38PM *  2 points [-]

to avoid circularity, it is sufficient to take the MRCA of a few indisputable mammalian groups such as primates, rodents, carnivores, ungulates, etc. to include all mammals

But the MRCA of "indisputable" groups won't be an ancestor of basal groups like the monotremes or marsupials.

However, there's no dispute about including monotremes. The clade that excludes them is called the Theria. Likewise with the marsupials: the clade that excludes both them and the monotremes is the Eutheria. Every clade potentially has a name; Mammalia is just a particularly well known one.

Things get dicey if the evolutionary relationships are unclear, of course, or if some conventional group is recognised as not being a true clade.

Comment author: DanArmak 31 October 2009 08:14:16PM 2 points [-]

You're right, of course. I was just pointing out that clades nest nicely. Whether you talk about Theria or Eutheria, the species included or excluded by the differences will be the most distantly related ones such as monotremes; but no clade anywhere similar in scope to Eutheria would be able to exclude dolphins. In that sense, it doesn't matter much which "indisputably" mammalian groups you take, their MRCA will be an ancestors of dolphins as well. For instance, the MRCA of humans and of cats is also an ancestor of dolphins.

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