plesiomorphies

Not that Dimetrodon and squamates don't share certain plesiomorphies sprawling posture, probably scales -- although those of lizards are fairly derived but Dimetrodon is actually closer to us than it is to any lizard.

Life's Time Capsule: The Synapsid Gallery ART Evolved 2009

I don't know if he still supports it, but no one else seems to: the proposed synapomorphies are either more likely to be plesiomorphies (J. van der Made, pers. comm.) or aren't convincing in view of the fact that basal hippos lack characters that unite suids and peccaries (Boisserie 2005).

More on what I saw at the zoo Darren Naish 2006

Old World and New World peccaries also lack many characters present in all suids, but these carry less weight as, theoretically, they could be retained plesiomorphies if Old World peccaries are suids and not peccaries proper.

Why putting your hand in a peccary’s mouth is a really bad idea Darren Naish 2006

As for whether peradectines are really opossums (as in, true members of Didelphidae), you are right that this classification is probably based on plesiomorphies.

More on phorusrhacids: the biggest, the fastest, the mostest out-of-placest Darren Naish 2006

I don't know if they share anything but plesiomorphies with Didelphidae.

More on phorusrhacids: the biggest, the fastest, the mostest out-of-placest Darren Naish 2006

Most of the features which distinguish P.a. parkelj from P.a. anguinus are plesiomorphies [= features not unique to olms, but present also in related salamanders] and hence P.a. parkelj may be the ancestor of the White olm.

Archive 2006-03-01 Darren Naish 2006

Old World and New World peccaries also lack many characters present in all suids, but these carry less weight as, theoretically, they could be retained plesiomorphies if Old World peccaries are suids and not peccaries proper.

Archive 2006-08-01 Darren Naish 2006