Xenoposeidon

Xenoposeidon (meaning "strange or alien Poseidon", in allusion to Sauroposeidon) is a possibly dubious genus of sauropod dinosaur from the Early Cretaceous of England, living about 140 million years ago. It is known from a single partial vertebra with unusual features, unlike those of other sauropods. This bone was first discovered in the early 1890s but received little attention until it was found by University of Portsmouth student Mike Taylor, who formally described and named it in 2007 with Darren Naish.

Description
Xenoposeidon is based on BMNH R2095, a partial posterior 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). The average diameter of the posterior face of the centrum is 165 millimetres (6.50 in), with a concave surface. This concavity is deep enough to suggest that the anterior faces of vertebrae from this part of the dinosaur's spine would have been convex to articulate with such a shape.[2]

The specimen displays several distinguishing characteristics. The base of the neural arch covers the length of the centrum and is continuous with the centrum's posterior face. The neural arch leans anteriorly at 35° and there are broad areas of featureless bone on the lateral surfaces of the arch. The neural canal is large and teardrop-shaped anteriorly but small and circular at its posterior opening. The various bony struts and sheets that make up the arch have a distinctive configuration.

Classification
Xenoposeidon's distinct suite of vertebral characteristics is unlike those found in other groups of sauropods, which differ in various proportional and structural features. So unique is the vertebra that when Taylor and Naish 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, Diplodocoidea, Camarasauridae, Brachiosauridae and Titanosauria. Xenoposeidon could be a derived member of one of the known groups, or may even represent a new group. The authors left it as a neosauropod of uncertain affinities.[2] The titanosauriform phylogeny by D'Emic established Xenoposeidon as a nomen dubium due to its basis on nondiagnostic material.[3] However, Mike Taylor has disagreed with these findings online.

The below majority rules cladogram was found in the analysis of Xenoposeidon. Without the genus, the support for each group except Flagellicaudata was considerably higher.