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new sauropod phylogeny
Mike Brett-Surman faxed to me the results of a new phylogenetic analysis of
the Sauropoda (he can provide the full reference: authors are Jorge O.
Calvo and Leonardo Salgado, paper is in Gaia but not in the sauropod volume).
The basis of the paper is the discovery of a great new specimen (and
species) of Rebbachisaurus from Albian-Cenomanian of Patagonia. R. tessonei
is more complete then previous specimens of Rebbachisaurus, lacks the
greatly enlarged neural spines of R. garasbae, and demonstrates that "R."
tamesnensis is not referable to this genus (it is, instead, a camarasaurid).
The structure of the phylogeny:
Prosauropoda (assumed monophyletic: still should be tested sometime!) as the
sister group to Sauropoda; Barapasaurus brances off first (no surprise
there); Omeisaurus (and, although not shown, probably the other
Euhelopodidae) second; Camarasaurus (and, although not shown, probably the
other Camarasauridae) next, then a dichotomy between (Brachiosaurus +
(Andesaurus + Titanosauridae)) on the one branch (also found in a
forthcoming analysis by Wilson and Sereno, prelim results at SVP a few years
ago) and a "haplocanthosaur"-Diplodocimorpha clade.
Within this latter clade, Haplocanthosaurus delfsi is the outgroup to a "H."
priscus-Diplodocimorpha clade. Diplodocimorpha contains Rebbachisaurus as
the first branch, and (Amargasaurus + Dicraeosaurus) + (Apatosaurus +
(Diplodocus + Barosaurus)) for the rest. Calvo and Salgado regard these
last five as forming "Diplodocidae", but I'd be just as happy with
Dicraeosauridae + Diplodocidae.
Salgado and others have another, more detailed analysis in press.
From the Conclusions:
"Rebbachisaurus tessonei sp. nov. exhibits many synapomorphies that justify
its position as the sister group of Diplodocidae, and it also confirms the
interpretation of this specimen as part of a large clade of
Diplodocimorpha. Eleven synapomorphies unite R. tessonei sp. nov to
diplodocids: pencil like teeth; anterior extension of the quadratojugal
placed beyond the anterior border of the orbit; anteriorly directed
basipterygoid processes; infratemporal fenestra oval or slit shaped; tall
neural arch in caudals (at least 1.5 times higher that [sic] that of the
centra); whip-lash tail; wing-like transverse processes in anterior caudals;
humerus/femur ratio less than 0.70. These characters allow us to recognize
the presence of a large clade that we call Diplodocimorpha.
" Rebbachisaurus tessonei sp. nov. does not present typical characters that
support the monophyly of the family Diplodocidae such as bifurcated cervical
and anterior dorsal neural spine, hyposphene-hypantrum in anterior dorsals,
prominent ambiens process on anterior part of the pubis, slightly procoelous
anterior caudal vertebrae, closed hemal canal in anterior caudals, midcaudal
chevrons with fore and aft directed processes, wide dorsoventral contact of
both distal ends of ischia, and expansion of the distal end of the ischia.
These characters are regarded as synapormorphies of Diplodocidae."
Incidentally, these authors consider Nemetosauridae and Quaesitosaurus to be
titanosaurs, so Rebbachisaurus is the youngest diplodocimorph in the fossil
record (according to them).
Thomas R. Holtz, Jr.
Vertebrate Paleontologist Webpage: http://www.geol.umd.edu
Dept. of Geology Email:firstname.lastname@example.org
University of Maryland Phone:301-405-4084
College Park, MD 20742 Fax: 301-314-9661
"There are some who call me... Tim."
-- Tim the Enchanter, "Monty Python and Quest for the Holy Grail"
"Tim?!? They called me TIM?!?!"
-- me, on seeing the credits to "The Ultimate Guide to T. rex" :-)