| Xenoposeidon | |
|---|---|
 | |
| Holotype vertebra in multiple views | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Clade: | Dinosauria |
| Clade: | Saurischia |
| Clade: | †Sauropodomorpha |
| Clade: | †Sauropoda |
| Superfamily: | †Diplodocoidea |
| Family: | †Rebbachisauridae |
| Genus: | †Xenoposeidon Taylor &Naish, 2007 |
| Species: | †X. proneneukos |
| Binomial name | |
| †Xenoposeidon proneneukos | |
Xenoposeidon (meaning "strange or alienPoseidon", in allusion toSauroposeidon) is agenus ofrebbachisauridsauropoddinosaur from theEarly Cretaceous ofEngland, living about 140 million years ago. It is known from a single partialvertebra 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 byUniversity of Portsmouth studentMike Taylor, who formally described and named it in 2007 withDarren Naish.
Xenoposeidon isbased onBMNH R2095, a partialposterior back vertebra. The specimen lacks the anterior face of thecentrum and the upper portion of theneural arch. The centrum is estimated at 200 millimetres (7.9 inches) long, and the height of the preserved portion of the vertebra is 300 millimetres (12 inches). The average diameter of the posterior face of the centrum is 165 millimetres (6.5 inches), 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 severaldistinguishing 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. Theneural 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.[2]
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 aphylogeneticanalysis, they found that, although aneosauropod, it didn’t fit ‘comfortably’ into any of the established groups,Diplodocoidea,Camarasauridae,Brachiosauridae andTitanosauria.Xenoposeidon could be aderived member of one of the known groups, or may even represent a new group. The authors left it as a neosauropod ofuncertain affinities.[2] The titanosauriform phylogeny by D'Emic establishedXenoposeidon as anomen dubium due to its basis on nondiagnostic material.[3] However, Mike Taylor has disagreed with these findings online. According to a study published in 2018,X. proneneukos is a basal member ofRebbachisauridae, and 10 million years older than the next oldest member of the family,Histriasaurus.[4]
The below majority rules cladogram was found in the analysis ofXenoposeidon. Without the genus, the support for each group exceptFlagellicaudata was considerably higher.[2]
Fossil collectorPhilip James Rufford discovered the vertebra that would be later namedXenoposeidon in the early 1890s. It was found nearHastings inEast Sussex, England, in rocks of theHastings Bed Group. Thisformation dates to theLower Cretaceous, and is within theBerriasian andValanginianStages. The vertebra was probably from the Berriasian portion of theAshdown Beds within the Hastings, although precise information about the locality and stratigraphy have been lost if such data were ever recorded.[2]
The partialposterior back vertebra, cataloged as BMNH R2095, was briefly described by Englishnaturalist andpaleontologistRichard Lydekker in 1893. He thought that it might belong toCetiosaurus brevis, better known today asPelorosaurus conybearei.[2] The bone attracted little attention for decades, sitting on a shelf at the BritishNatural History Museum inLondon for 113 years. Mike Taylor, a sauropod vertebra specialist, stumbled upon it and became interested in the unusual specimen, entering into a description of it with Darren Naish.[5]
The new genus was first announced by the British Palaeontological Association on November 15, 2007. The genus consists of the single speciesXenoposeidon proneneukos. The generic name combinesGreekxenos, "strange", with a reference toPoseidon, the "earth-shaker". Thespecific name means ‘forward sloping’ in Greek, in recognition of the anterior sloping of theneural arch. This unusual characteristic, along with its other distinctive features, prompted the authors to erect the new genus.[2]
Like other sauropods,Xenoposeidon would have been a largequadrupedalherbivore.[6] It was relatively small for a sauropod. Extrapolating from the vertebra suggests that thetype individual ofXenoposeidon could have been about 15 metres (49 feet) long and weighed approximately 7.6 tonnes (8.4 short tons), if it resembledBrachiosaurus, to 20 m (66 ft) long and 2.8 tonnes (3.1 short tons), if it was built like the longer, lighterDiplodocus.[2]
