Re: 2 questions

[chartiem@ERE.UMontreal.CA (Chartier Michel)]

> 
> here's 2 unrelated questions:
> 1) last week I asked about hadrosaur "hands" and if they were becoming 
> hooves. The answer was yes. This leads me to ask how the hadrosaurs 
> stacked up along the bipedal-quadrapedal line i.e., were some hadrosaurs 
> more quadrapedal than others? Were later forms more quadrapedal than 
> earlier ones? (alright so that's 2 questions right there ;)) I quess I'm 
> leading up to were hadrosaurs on the way to (re)turning to a quadrapedal 
> form?
> (I guess I'm thinking of this as a parallel of the evolution of horses' 
> hooves)
Since I'm a mammal guy, I'll let the dino guys answer this one.

> 2) Last night's PaleoWorld discussed dimetrodon and the synapsid, diapsid 
> split leading to mammals and dinos. Where in this batch are the 
> therapsids, which I thought were the forerunners of mammals? Were 
> therapsids a branch of synapsids? Am I just out of it?
> Blaise "it's Monday morning, and I'm burnt to a crisp" Considine 
> [bpc.apa@email.apa.org]
> 
> 
I don't have access to that program, but if it said that there was a split
between diapsids and synapsids (in other words, they are sister-groups),
then it is wrong. It is believed that the synapsids are the sister group
of an anapsid(turtles)-diapsid clade; thus the synapsid lineage would have
been the first (or one of the first) to branch off the amniote lineage (the
amniotes beeing composed of reptiles, archosaurs, birds and mammals). This
would mean that the state-of-the-art creatures that are mammals (being
ironic here) would be in effect a very ancient and primitive lineage.
To answer your question, therapsids are synapsids. They originated from one
of the pelycosaur groups, called the sphenacodontids, which include forms
such as Dimetrodon (but Dimetrodon is much too specialized to have been a 
direct ancestor). Mammals themselves originated from one of the therapsid
groups, called cynodonts. Right now there are some discussions as to where
to draw the therapsid-mammal boundary, the taxonomy is being revised, so
it's a very complex issue, and one that goes beyond the scope of this post.
I hope your question has been satisfactorily answered.

Michel Chartier