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The Clade Not Taken



-or- A Late Triassic Theropod Considers His Future

        Two clades diverged at a common node
        And, seeing divergence had occurred
        Among my descendants, long I stood
        And looked down one as far as I could
        To where they later were known as birds.

(apologies to Robert Frost)

In my continuing efforts to find ways to understand cladistics, I come
across the cladograms used by Fastovsky and Weishampel in _The Evolution
and Extinction of the Dinosaurs_ where they create a variety of cladograms
showing relationships of motor vehicles and time pieces. This helps
considerably to explain the concept in laypeoples' terms.

What I think has always been an obstacle in my perception of cladograms
is the way clades diverge in their graphical representation. In the
cladogram labelled Figure 3.3 on page 53 of the Fastovsky/Weishampel
text, trucks seem to be directly related to motor vehicles in a straight
line, whereas cars seem to diverge off at an obtuse angle. The truck
seems to be a more directly related to the clade "motor vehicles" while
the car seems to diverge more from the parent node than the truck does.

I realize that people with a professional background in the biological
sciences have no trouble seeing the true relationship, but if cladistics
is going to be taught in the public schools, allowance will have to be
made for counter-intuitive perceptions of what the cladogram is trying to
show.

For those who don't have the Fastovsky/Weishampel text, I'll illustrate
the problem using the theropod cladogram on page 61 of Lucas's
_Dinosaurs: The Textbook_. In that cladogram, coelurosaurs seem to
continue in a straight line from the Theropoda, whereas carnosaurs seem
to diverge at an obtuse angle. (If we think of the cladogram as a
roadmap, it looks like there is one road with a bunch of side streets.)

Is it too late to switch to Venn diagrams? ------Just kidding.

-----Amado Narvaez, Media Specialist; ex-German Language teacher;
Magician extraordinaire; Lyricist; brown belt in Origami; Silver Spring,
Maryland, 20904; e-mail: anarvaez@umd5.umd.edu