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Re: you realize, of course, that this means war



In a message dated 97-07-31 20:18:55 EDT, longrich@phoenix.Princeton.EDU
writes:

<< Prosauropods are graviportal herbivores that spend at least some
 of their time on two legs. Segnosaurs are graviportal herbivores. Similar
 lifestyles, similar selective pressures, similar results. When the
 selctive pressures are the similar you will get similar results, that's
 why the great auk of the northern hemisphere and the penguins of the
 southern hemisphere and the plotopterids all look very similar in the
 construction of their wing bones, because they are all using them in
 similar ways. It's not fantastic to think that two big slow-moving
 herbivores should come to resemble each other in the feet. You'd expect
 it.  >>

I'm catching up on some of my back e-mail.

When I say segnosaurs and prosauropods look alike, I don't just mean that
they >look alike< through some kind of convergent evolution. I'm talking in
the >technical sense< here. Segnosaurs have numerous sauropodomorph features
in their cranial and skeletal anatomy; indeed, there are so many that it
seems as if wherever they differ from theropods, they do so by resembling
sauropodomorphs! And boy, do they differ from theropods. For example, in most
theropods the lacrimal bone rises above the skull table and squeezes the
prefrontal into the frontal. In segnosaurs, the lacrimal is below the skull
table, and the prefrontal is quite prominent: just like in prosauropods. In
most theropods, the dentary does not curve downward in front; in segnosaurs,
the dentary curves downward: just like in prosauropods. The teeth and
interdental plates of segnosaurs match those of prosauropods. The palate of
segnosaurs strongly matches that of prosauropods (why didn't Clark et al do a
detailed comparison for _Erlikosaurus_??). Let us not even mention the feet
of segnosaurs. While some segnosaur features show up in some theropods,
they're not known to show up >all in the same group of theropods<. But >whole
suites< of segnosaur features are found >together< in prosauropods. There is
no known group of theropods that has anything like the collection of
prosauropod and sauropodomorph features found in segnosaurs.

Look at the disposition and shapes of the skull and mandibular bones in
_Plateosaurus_ and in _Erlikosaurus_. Hold the figures side by side to
facilitate comparison. Do this for lateral views, dorsal views, and ventral
views. Do this for the braincases, too (more difficult because exactly the
same views aren't available). Read the verbal descriptions of the skulls. You
might want to read Greg Paul's article in JVP:

Paul, G. S., 1984b. "The segnosaurian dinosaurs: Relics of the
prosauropod-ornithischian transition?" Journal of Vertebrate Paleontology
4(4): 507515.