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Maybe the example can be used for further illustration.
Egg-laying is a TRAIT. Given its widespread occurrence in outgroups
to the mammalia (e.g. birds, turtles, some but not all squamates,
some but not all lissamphibians), it may be presumed primitive for
"mammals" (sensu lato: take your pick of mammaliaforms, Eucynodonts
(if you like Trevor Dykes's WWWebsite and Yahoo group),
synapsids...). Hence it wouldn't be thought good form to cite it in
diagnosing a subgroup of mammals.
NOT laying eggs is the derived state. In trying to reconstruct
family trees, we look for what derived states various critters share.
(That's what "cladistics" is about. It's something paleontologists
seem to have learned from Biblical scholars, who had been using the
same method-- with variant readings introduced by scribal error as
the derived traits-- to reconstruct the family tree of medieval
manuscripts of the Bible.) So... Suppose it is discovered (somehow)
that one of the Mesozoic mammalian groups Tom Holtz listed didn't lay
eggs. That would be an apomorphy (derived state) it shared with
Theria, and so evidence that it was more closely related (=had a more
recent common ancestor with) to Theria than Monotremes are. In fact,
Multituberculates seem to have had very narrow pelvic openin gs, and
this has been taken(*) as evidence that they did NOT lay eggs. So
NOT LAYING EGGS could be taken as one more entry in a data matrix
confirming that Multituberculates are more closely related to Therian
mammals than Monotremes are: a conclusion that seems to be widely
accepted on other grounds.
I don't THINK that not laying eggs is actually used as one of the
traits uniting Multituberculates and Therians to the exclusion of
Monotremes. The evidence that Multis bore live young is suggestive
rather than conclusive. And, of course, as Squamates show, it isn't
a terribly confidence-inducing apomorphy, since it has evolved many
University of Melbourne
(*) Not sure where this observation first made. Can be found in,
e.g., the multi chapter of Kielan-Jaworowska, Cifelli and Luo,
"Mammals of the Dinosaur Era." I suspect the observation-- and the
inference to live-birth-- were Kielan-Jaworowska's decades back.