An Unusual New Neosauropod Dinosaur From the Lower Cretaceous Hastings Beds Group of East Sussex, England

Abstract

Xenoposeidon proneneukos gen. et sp. nov. is a neosauropod represented by BMNH R2095, a well-preserved partial mid-to-posterior dorsal vertebra from the BerriasianValanginian Hastings Beds Group of Ecclesbourne Glen, East Sussex, England. It was briefly described by Lydekker in 1893, but it has subsequently been overlooked. This specimen's concave cotyle, large lateral pneumatic fossae, complex system of bony laminae and camerate internal structure show that it represents a neosauropod dinosaur. However, it differs from all other sauropods in the form of its neural arch, which is taller than the centrum, covers the entire dorsal surface of the centrum, has its posterior margin continuous with that of the cotyle, and slopes forward at 35 degrees relative to the vertical. Also unique is a broad, flat area of featureless bone on the lateral face of the arch; the accessory infraparapophyseal and postzygapophyseal laminae which meet in a V; and the asymmetric neural canal, small and round posteriorly but large and teardrop-shaped anteriorly, bounded by arched supporting laminae. The specimen cannot be referred to any known sauropod genus, and clearly represents a new genus and possibly a new 'family'. Other sauropod remains from the Hastings Beds Group represent basal Titanosauriformes, Titanosauria and Diplodocidae; X. proneneukos may bring to four the number of sauropod 'families' represented in this unit. Sauropods may in general have been much less morphologically conservative than is usually assumed. Since neurocentral fusion is complete in R2095, it is probably from a mature or nearly mature animal. Nevertheless, size comparisons of R2095 with corresponding vertebrae in the Brachiosaurus brancai holotype HMN SII and Diplodocus carnegii holotype CM 84 suggest a rather small sauropod: perhaps 15 m long and 7600 kg in mass if built like a brachiosaurid, or 20 m and 2800 kg if built like a diplodocid.

Xenoposeidon is the earliest known rebbachisaurid sauropod dinosaur

“…Taylor and Naish's (2007) history, geography, geology and description of the Xenoposeidon specimen requires no revision, and should continue to be considered definitive: this paper does not supersede it, but should be read in conjunction with it.…”

“…A similar pattern does appear in Rebbacdisaurus, as will be discussed below. D'Emic (2012:651) is probably correct that the "asymmetric neural canal" described by Taylor and Naish (2007:1553-1554) is a misreading of the tall centroprezygapophyseal fossae as being the anterior portion of the neural canal: as Taylor and Naish pointed out, "The vacuity is filled with matrix, so the extent of its penetration posteriorly into the neural arch cannot be assessed". Nevertheless, the shape and size of the fossa is unique among sauropods, and it is bounded by laminae which do not seem to be medial CPRLs.…”

“…In summary, Xenoposeidon proneneukos is a valid, diagnosable taxon, contra D'Emic (2012). Taylor and Naish (2007:1554-1557) compared the Xenoposeidon vertebra to those of the main neosauropod groups -Diplodocoidea, Camarasauridae, Brachiosauridae and Titanosauriaand concluded that it could not be convincingly referred to any of these groups. Their phylogenetic analysis (pp.…”

“…This element consists of the centrum and the base of a tall neural arch, broken off below the transverse processes and zygapophyses. Despite its fragmentary nature, it is recognisably different from all other sauropods, and Taylor and Naish (2007) diagnosed it on the basis of six characters that they considered unique among sauropods. D'Emic (2012:651) asserted that "the absence of diagnostic features renders Xenoposeidon a nomen dubium".…”

“…It is represented by a single partial mid-to-posterior dorsal vertebra, NHMUK R2095 (BMNH R2095 at the time of the original description by Taylor and Naish 2007). This element consists of the centrum and the base of a tall neural arch, broken off below the transverse processes and zygapophyses.…”