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Re: dinosaurs did eat grass

Mike Keesey wrote:

Furthermore, skull material is not known for the European
rebbachisaurid _Histriasaurus_, is it?

Not yet. The skull of (or pieces of it) might be buried somewhere at the bottom of the Adriatic.

Thus it would be conceivable
that, even if rebbachisaurids first split off from other diplodocoids
in Europe, they may not have developed the unique jaw morphology
before they infiltrated Gondwana.

That could be. One thing that comes to mind is that rebbachisaurids weren't exactly a screaming success in the later Cretaceous. We only know of a handful of genera from the entire period, and rebbachisaurids are vastly outnumbered by titanosaurs and ornithischians. If the specialized craniodental morphology of rebbachisaurids was an adaptation to eating grasses, then it didn't really get them very far. Maybe Tom Holtz's "white rhino" analog is accurate in more ways than one: by the Late Cretaceous, rebbachisaurids were a lingering remnant of a once very diverse and successful lineage (diplodocoids).

Come to think of it, rebbachisaurids would have to have split off from
the dicraeosaurid/diplodocid line before the Morrison/Tendaguru,
probably during the Middle Jurassic or at least the Oxfordian, so
we've got a fairly decent ghost lineage here. The earliest known
diplodocimorphs are both Laurasian (Morrison) and Gondwanan
(Tendaguru), so it doesn't seem to me that we have any idea which part
of the globe diplodocimorphs originate from.

_Cetiosauriscus stewarti_, from the Middle Jurassic of England, is possibly a basal diplodocoid (sensu Wilson and Sereno, 1998). Strictly speaking, _Cetiosauriscus_ would not be a diplodocimorph (sensu Taylor and Naish, 2005) given that it probably lies outside the clade anchored in _Rebbachisaurus_ and _Diplodocus_.

(Although the presence of
a close relative, _Haplocanthosaurus_, in the Morrison might slightly
favor Laurasian for now, I suppose--but very slightly indeed.)

_Haplocanthosaurus_ is notoriously difficult to pin down in analyses. It usually hovers somewhere near the Macronaria-Diplodocoidea split, but as to exactly where it sits... _Haplocanthosaurus_ is keeping its cards close to its chest. If the braincase named _Morosaurus agilis_ belongs to _Halpocanthosaurus_, it might help the situation.

Cheers

Tim