Two new sauropod dinosaurs were named today. Nigersaurus and Xenopsoeidon. ON Tetrapod Zoology it says that Xenoposeidon would beat  Nigersaurus in a fight? Is this really true?

I think Darren (Naish, who runs Tet Zoo) is just having a bit of fun. I doubt sauropods would ever fight each other seriously, and  these two did not live in the same time and place.

The reason for his comment is that both dinosaurs were formally described on the same day and Darren is trying to claim that his (Xenoposeidon) was 'better'.

David, I can assure you that Xenoposeidon could kick Nigersaurus's arse without breaking a sweat.

(And what a terrible day for the Nigersaurus paper and skeletal reconstruction to come out!  Couldn't they have just hung on a couple of days?  Oh well.)

By the way, we do mention Nigersaurus in the Xeno paper, on p. 1560.  Here is the relevant chunk, with all the citations snipped out.

Taylor and Naish 2007 wrote:

Historically, Sauropoda has been considered a morphologically
conservative group, showing less diversity in body shape than the
other major dinosaurian groups, Theropoda and Ornithischia. For many
decades, the basic division of sauropods into cetiosaurs,
mamenchisaurs, diplodocoids, camarasaurs, brachiosaurs and titanosaurs
seemed established, and as recently as 30 years ago, Coombs (1975,
p. 1) could write that `little information in the form of startling
new specimens has been forthcoming for sauropods over the last 40
years'. Recent finds are changing this perception, with the discovery
of previously unknown morphology in the square-jawed rebbachisaurid
Nigersaurus, the long-legged titanosaur Isisaurus (originally
`Titanosaurus' colberti), the short-necked dicraeosaurid
Brachytrachelopan, and the truly massive titanosaurs Argentinosaurus,
Paralititan and Puertasaurus. During the same period, Rebbachisauridae
has emerged as an important group.

Perhaps most interesting of all is the recent erection of two sauropod
genera that arguably do not fit into any established group: Agustinia
and Tendaguria. Both of these genera are represented by specimens so
different from other sauropods that they have been placed by their
authors into new monogeneric `families', Agustiniidae and
Tendaguriidae. Together with Xenoposeidon, these taxa emphasize just
how much remains to be discovered about the Sauropoda and how little
of the full sauropod diversity we presently understand. It is hoped
that the discovery of new specimens will allow the anatomy and
relationships of these enigmatic new sauropods to be elucidated.

That, really is the take-home message of Xenoposeidon: that sauropods were much stranger, cooler and more awesome than we have yet appreciated, and that a lot remains to be discovered about them.