Re: Triassic Sauropods

["Matthew Bonnan" <mbonnan@hotmail.com>]

Tim Williams said:

"On the subject of early sauropods, I've wondered if Madagascan sauropods
such as _Lapparentosaurus madagascariensis_ and the adult
"_Bothriospondylus_" material might be re-dated to Late Triassic (or at
least Early Jurassic) age.  Both come from the upper Isalo Group, and the 
_Science_ paper (1999) on the
prosauropods from the Isalo II site (estimated Middle Triassic) did suggest
that the *entire* Isalo Group was laid down during the Triassic.

Late Triassic titanosauriforms?"

Well, I can't speak for _Lapparentosaurus_, but I have seen the fore- and 
hindlimb material for _Bothriospondylus madagascariensis_ when I was in 
Paris this January.  I was in Paris for vacation, not for research purposes, 
so my time and access to the material was limited.

However, based on my brief notes, the humerus, radius, ulna, metacarpals, 
femur, tibia, and fibula are all of a Neosauropodan stage of morphology.  In 
fact, the femur has a well developed lateral bulge (which I would term the 
reduced lesser trochanter) and well-developed femoral condyles which is a 
condition we only observe in titanosaurian postcrania.  The metacarpals are 
robust and tall, and this would place _Bothriospondylus_ outside the 
diplodocids, which tend to have short metacarpals.  The metacarpals reminded 
me of some fragmentary but massive metacarpals from the Smithsonian that 
have been tentatively identified as "_Haplocanthosaurus_."

The humerus was rather camarasaurid-like, and the ulna and radius were of 
typical sauropod morphology, with the ulna being proximally triradiate and 
enclosing the radius in a deep radial fossa.  There is a nice picture 
(drawing) of this stuff in Glut (1997).

This mix of characters would make it difficult for me to say that this guy 
was a titanosaur, brachiosaur, or what-have-you.  However, the 
titanosaurid-like femoral morphology would make me lean towards a 
titanosaurid interpretation, or at least something in the brachiosaurid-type 
lineage.

As far as this material being an example of an early titanosaur from the 
Late Triassic or Early Jurassic, I would have my doubts.  As Mickey Mortimer 
pointed out, the locality of the _Isanosaurus_ is not from the Isalo 
Formation in Madagascar but in Thailand instead.  This means the date for 
the Isalo Formation is probably not Late Triassic.  Furthermore, no other 
basal sauropod we know of has this type of "advanced" neosauropod 
postcrania.  If you look at the position of the fourth trochanter on 
_Isanosaurus_ and compare it with later neosauropods, especially 
titanosaurs, you will notice that in _Isanosaurus_ and other basal sauropods 
the fourth trochanter is much more proximal.  In the titanosaur 
_Opisthocoelicaudia_, the fourth trochanter is very distal, terminating past 
the midline of the posterior face of the femur.

I would like to see more complete material before drawing an "early 
titanosaur" conclusion for _Bothriospondylus_ and _Lapparentosaurus_.

My two sauropod cents,

Matt Bonnan
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