Giant sauropod you've probably never heard of

the name? Xenoposeidon proneneukos

It's very much unknown, only described by  a partial posteriod back vertebra. The specimen lacks the anterior face of the Centrum and the upper portion of the neural arch. The centrum is estimated at 200 millimetres (7.9 in) long, and the height of the preserved portion of the vertebra is 300 millimetres (11.8 in) not huge by sauropod standards, but pretty darn big.

Fossil collector Phillip James Rufford discovered the vertebra  in the early 1890s, where it didn't get much attention. It sat collecting dusting in an England museum for more then a century, until Mike Taylor stumbled across it. Analyzing it, he discovered it to be rather different in overall structure then the bones in other sauropod families.

" I immediately realised the bone that would eventually become Xenoposeidon was something strange: it was unmistakable a dorsal vertebra from a sauropod, but didn't look like any dorsal I'd ever seen before. " stated Taylor

So unique is the vertebra that when Taylor attempted to classify it using  a phylogenetic analysis, they found that, although a neosauropod, it didn’t fit ‘comfortably’ into any of the established groups.

Size esimations from a back vertebrate are rather troubling in the field of Paleontology, but Extrapolating from the vertebra suggests that the type individual of Xenoposeidon could have been about 15 metres (50 ft) long and weighed approximately 7.6  tons, if it resembled  Brachiosaurus;  20 m (65 ft) long and 2.8 tonnes (3.1 short tons), if it was built like Diplodocus.

It very well could represent a new lineage of sauropods though, and only when we find more remains will we ever know.

I'd like to note i find the Brachiosaurid reconstructions of Xenoposeidon troubling, as the vertebrated do not resemble those of the brachiosaurs....I guess it's the best guess considering we only have a partial fossil