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Re: dinosaur synapomorphies? (especially postfrontals)




Mike, Jaime, and David,
Well I think Jaime hit the nail right on the head. The similarities between dinosaurs and basal Crurotarsi need to be very carefully reexamined. This especially true in view of the collapse of Ornithodira (sensu Sereno), which was purportedly sister group to Crurotarsi. In fact, I think Crurotarsi is even in more jeopardy than Dinosauria.
I'm beginning to think that any large clade based on morphology of ankles or toes is likely to be stuck clear up to its sternum or clavicles in a morass of homoplasy. What Dave did to ornithodiran synapomorphies is very likely to happen to Crurotarsi, and I'm obviously still having doubts about Dinosauria as well.
As for the first character (postfrontal absent), how good a character is that if it could have fused in ontogeny, especially in really scrappy primitive forms? And how many basal Crurotarsians might have postfrontals absent, but we just don't know (they are just assumed to be present based on circular reasoning and faulty assumptions)?
This is why I don't trust any of Sereno's dinosaur synapomorphies, and I don't have time to delve into all of them, so I want to find the strongest ones and concentrate on those. Postfrontal absent is definitely not going to be on the top of my list unless someone gives a really good defense of it. Its fusion or loss could occur independently in two or more related lines just as a perforated acetabulum can.
I still think the holophyly of Dinosauria is being taken for granted (as is that of Crurotarsi), and many of the conclusions in Sereno's 1991 paper must be reexamined in view of his placement of pterosaurs between crurotarsians and dinosaurs, which now appears to be erroneous. Reassignment of pterosaurs was perhaps just the beginning of the surprises.
-----Ken
******************************************
From: "T. Mike Keesey" <tmk@dinosauricon.com>
To: Ken Kinman <kinman@hotmail.com>
CC: <dinosaur@usc.edu>
Subject: Re: dinosaur synapomorphies?
Date: Mon, 27 Aug 2001 14:41:29 -0400 (EDT)

On Mon, 27 Aug 2001, Ken Kinman wrote:

>
> Dear All,
> It amazes me that so many people now take the holophyly of Dinosauria
> for granted. Many of the purported synapomorphies are very weak, and many
> remind me of the "kitchen list" of synapomorphies that was proposed for
> Ornithodira (sensu stricto).


_Ornithodira_ is being dismantled, or altered, or whatever, by some
because an alternative hypothesis was put forward as more likely, not
because the original hypothesis was weak.

Weakness is relative. To claim that support for _Dinosauria_ is weak, you
must show that something else is stronger.

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