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Phytodinosauria and related topics



Hello to everyone

I'm back on this list after a long absence. I've been touring the world
(well some of it) looking at prosauropods (I managed to see first hand
Plateosaurs, "Sellosaurus", Riojasaurus, Coloradisaurus, Lessemsaurus,
Mussaurus, Casamiquela's "Plateosaurus", Massospondylus, Melanorosaurus,
?Euskelosaurus and a new melanorosaurid).

I learnt a great deal and can't wait to integrate my observations into a
new analysis.

Anyway being a topic that I'm now actively researching I'd like to add a
few comments to the "Phytodinosauria" thread.

Firstly I should say that all cladistic analyses strongly support the
Ornithischia-Saurischia dichotomy. George says he doesn't find the
characters that support the Saurischia very convincing, I have to say
that I do. I find the few phytodinosaurian characters (largely
adaptations related to including plant matter in the diet) are
outweighed by the number of saurischian apomorphies. Of course if you
don't want to play the character distribution-maximum parsimony game you
will not find this convincing. However if we go down that path each side
can pick the characters they like, call the other ones "unconvincing"
and that's that. 

An interesting aspect of my own and Max Langer's analyses is the support
for a Clade within the Saurischia, comprising Sauropodomorpha (sensu
lato) and Neotheropoda, exclusive of Eoraptor and Herrerasauridae.  This
clade was informally called 'Eusaurischia' in Padian Holtz and
Hutchinson.  The position of Eoraptor is flexible and we should really
wait for the upcoming monograph on its anatomy before discussing this
guy too much (but just for the record I'd like to place a bet that it
will eventually come out as the most basal sauropodomorph).

Guiabasaurus is even more unstable in its position, although it is
probably a 'Eusaurischian'or very near them.  Saturnalia is without
doubt a sauropodomorph (in the stem-group sense)  and this is supported
quite strongly in Max's Cladogram presented at SVPCA last year, and in
my cladogram (also presented at SVPCA) it is definitely basal to
Thecodontosaurus. 

 Jamie made some comment that he thought Herrerasaurid skulls had a
prosauropod look about them.  Although not talking in precise terms I'd
still have to disagree quite strongly. In my mind the skull of
Herrerasaurus most closely resembles (in phenetic terms) those of
rauisuchian crurotarsans.  Like them it has a tall, broad nasal process
of the premaxilla that forms a wide premax.-nasal suture behind the
naris, it has a premax.-max fenestra, it has a tall anterodorsal process
of the jugal that forms the base of the preorbital strut, it has a
blocky, rectangular shaped lacrimal and a very short tab-like
quadratojugal process of the squamosal. Some of these are plesiomorphies
and help to push Herrerasaurus outside the "Eusaurischia".

 Now a few words about the "fifth toe" of dinosaurs. The loss of this
digit has been frequently used to support the hypothesis that
Prosauropods are monophyletic with respect to sauropods or even that all
other dinosaurs are the monophyletic sister group of sauropods. However
this doesn't stand up to much scutiny. Most early saurischians retain
the fifth metatarsal and many have an associated phalange (eg. 
Herrerasaurus). This is exactly the same complement that is seen in
Sauropods, except that the fifth digit is a bit more robust and makes
contact with the ground. So the fifth didn't re-evolve, just the
proportions of pre-existing parts changed - no big deal in morphological
evolution. Why did the fifth digit re-establish contact with the ground?
Well this change is coincident with the dramatic shortening of all the
other digits of the foot, so it didn't grow longer the rest of the foot
shrank. Not that re-aquiring a digit was beyond the realms of
possibility just look at the fifth finger of Neoceratopsians. Indeed it
is possible that the basal condition for the dinosaur was to have no
phalange on the fifth metatarsal, and that it was re-aquired twice in
saurischians: once in Herrerasaurus and once in sauropodomorphs above 
Thecodontosaurus.

Whew that's enough for now. 


Adam Yates