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Re: Re: A few more...



>Node-based names and stem-based names yield identical clades, because any
>node can be the base of only one clade. So, for example, your Neotheropoda as
>a stem-based name may be defined as "all taxa sharing a more recent common
>ancestor with birds than with herrerasaurians." Which type of definition you
>employ is strictly a matter of convenience, perhaps depending on whether you
>know just where the root node is or not.

There seems to be some confusion here.  Although it is indeed possible to
name a node rooted at the joining of two stem groups, there are not the same
taxa.

Here's an example:

Reptilia (node) = the most recent common ancestor of turtles, lepidosaurs,
and archosaurs, and all of that ancestor's descendants.  Includes living
taxa such as turtles, tuataras, squamates ("lizards" & snakes), crocs, and
birds, as well as nonavian dinos, other extinct archosaurs, and possibly the
ichthyosaurs and sauropterygians.

Sauopsids (stem) = reptiles and all taxa closer to reptiles than to
mammals.  Includes extinct non-reptilian sauropsids such as mesosaurs,
perhaps procolophonids (may be closer to turtles), and perhaps ichthyosaurs.

The node which joins Sauropsida with its sister-taxon, Synapsida, is called
Amniota.

Thomas R. Holtz, Jr.
Vertebrate Paleontologist
Dept. of Geology
University of Maryland
College Park, MD  20742
Email:Thomas_R_HOLTZ@umail.umd.edu (th81)
Fax: 301-314-9661
Phone:301-405-4084