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Re: Sauropod Reference



In a message dated 95-10-22 23:38:53 EDT, zooamy@zoo.latrobe.edu.au (Adam
Yates) writes:

>You won't like it. It's largely cladistic. However you will have to agree 
>that the opisthocoelous caudals of Opisthocoelocaudia is an unusual 
>autapomorphy for this genus and tells us nothing about what it is related 
>to. As for bifid nueral spines they seem to pop up time and again in 
>sauropod evolution (Euhelopodids, Camarasaurids, Diplodicoids and 
>Opisthocoelocaudia). This is one of those characters I was reffering to when
>I talked about rampant convergence within the Sauropoda, in an earlier 
>posting. I don't know what it would be but it would seem that having bifid
> nueral spines was a structural advantage to a gigantic long necked
creature.
>
>

I have nothing against cladistics; I've been using a species of cladistics
myself that avoids number-crunching and brute-force manipulation of character
matrices, for which there is little solid justification. But I'm against
cladistic taxonomies when they get dogmatically out of hand, and I detest
cladists who persist in being arrogant and evangelistic about handing out
"the word." The methodology is only a step or two above trivial, really.

I think a fairly consistent phylogeny of the sauropods can be worked out by
concentrating on the evolution of bifid neural spines, the movement of the
narial openings caudally along the skull, the lowering of the infratemporal
fenestra below the orbit, the development of elongate cervical ribs, the
increased elevation of the cervical neural spines, the pneumatization of the
vertebral centra and arches, the number and appearance of the sacral
vertebrae, the shapes of the pelvic bones, and the shapes of the midcaudal
chevrons. The other features--including the teeth--sort themselves out fairly
well once the main branches are defined. The limb elements are pretty useless
except for their proportions; they had very few features one could hang one's
hat on.

I once saw a relationship between _Opisthocoelicaudia_ and titanosaurids
myself, until I realized it had largely to do with the forelimb anatomy of
_Argyrosaurus_ and the placement of the caudal neural spines on their centra.
I no longer think _Argyrosaurus_ is a titanosaurid; it may be a derived
camarasaurid close to _Opisthocoelicaudia_, but the jury is still out on that
form. My latest take on _Op---_ is that it was a derived euhelopodid, but we
need a complete neck at least to be certain.

G.O.