Dinosaur Discovery Chronicles

From I09:  Newly discovered mega-tooth could belong to one of the biggest dinosaurs ever

A tooth recently discovered in Argentina is an incredible 75 millimeters, or nearly 3 inches long. What's more, the tooth belonged to a member of the titanosaurs, a group of gigantic sauropods similar to brachiosaurus and apatosaurus. And it might just be the biggest of the bunch.
The tooth was discovered by Rodolfo García and his paleontological team at Argentina's National University of Río Negro. It's not even close to the biggest teeth ever discovered, but that's only because plant-eating titanosaurs didn't need chompers the size of their giant predator counterparts. The tooth is 32% longer than the length of any previously known titanosaur tooth, and it's twice the size of those belonging to most other sauropod taxa. The dinosaur that once owned this tooth might well be the biggest animal to ever walk the Earth, or at least very much in conversation for the title.
The problem is that it isn't really possible to extrapolate the size of an entire dinosaur from just a single tooth. We can make reasonable guesses about the size of the skull based on the size of the tooth, and the size of the skull can in turn help us guess the size of the entire dinosaur, but there's way too many leaps of logic involved here to say for certain how big the dinosaur that possessed this tooth really was.
Writing in Cretaceous Research, Dr. Garcia runs down the mysteries surrounding the tooth:

The unusual dimensions of the tooth described here, suggest different hypotheses about the specimen. The MML-Pv 1030 tooth could have belonged to a specimen with disproportionately large teeth, "a big-toothed titanosaur". However, it is also possible that this tooth belonged to an individual with an enormous skull, probably to a short-necked titanosaur or to a taxon of unusual dimensions for a titanosaur. Whatever the option mentioned above, this taxon has characteristics never previously recorded for the Upper Cretaceous (middle Campanian–lower Maastrichtian).
Another interesting aspect with regard to the studied specimen is its total absence of pits on its wear facet or enamel surface, a condition that differs from that of numerous titanosaurs. The absence of pits suggests a diet with scarce grit or an absence of hard vegetable material able to mark the teeth. On the other hand, the tooth scratches on the labial face suggest a feeding mechanism type where the collection of food was performed with the anteriormost teeth, cropping or, stripping of leaves from the branches but in an oblique way.

A photo of the tooth in comparison to other titanosaur teeth is available with the original paper.