An Alien Earth-Shaker, Xenoposeidon

In 2007 a new sauropod was named Xenoposeidon proneneukos, by paleontologist and sauropod specialist Mike Taylor joined by fellow colleague and paleontologist Darren Naish. The holotype was a single vertebra bone, reported from the Southeastern town of Hastings, East Sussex, England. Before this discovery in the collections of the Natural History Museum in London, it was initially overlooked by other paleontologists and fossil workers alike, furthermore because of this and its fragmentary nature, it was assigned to random genera prior to modern-day paleontologists examining it. On top of this, exact information is lackluster concerning where this singular indeterminate sauropod bone was found. This was likely due to methods not being very thorough in terms of documentation at the time of discovery from its dig site (called a “locality” amongst fossil workers to this day) during the Early 1890s, only scientifically described for the first time by Richard Lydekker three years later for it to be neglected in museum collections as mentioned prior for approximately 113 years.

Despite this background, what makes this bone so mysterious yet interesting isn’t the fragmentary nature, but the fascinating implications it has for its age, due to the rock unit it was discovered in: the Ashdown Beds Formation, which is dated to the Late Berriasian-Early Valangianian stages; thus discovered approximately in 139–140-million-year-old rock. Initially overlooked for a very long time prior to the description paper in 2007; the holotype specimen (NHMUK PV R2095) was revisited in 2018 by one of the authors, specifically the sauropod expert Mike Taylor, to classify it further into a better placement amongst sauropods. Albeit challenging due to the holotype’s at-first-glance indeterminate nature, hence why a revision was in order, it was not impossible if given enough clues from anatomical characters discovered under careful inspection. What Mike Taylor discovered was that Xenoposeidon is the oldest sauropod of its kind, confirming some of the first assumptions made in the 2007 description paper. This was accomplished from a revised diagnosis via a comparative anatomy approach resulting with five new unique anatomical characteristics concerning the X. proneneukos holotype. To clarify, a holotype is the first type specimen which the new animal (whether it be a new genus or new species) is described and based upon.

The significance of the location where Xenoposeidon was discovered, the Ashdown Formation, is its age which is the bottom-most formation under the Wealden Supergroup of South England. The older age as mentioned prior of the Ashdown Formation makes Xenoposeidon significant, however there are specific anatomical details linking Xenoposeidon to the holotype of another sauropod mentioned earlier, Rebbachisaurus garasbae, which was found in Middle Cretaceous aged rocks in Morocco (Cenomanian Stage, approximately 100-94 Million Years Ago) implying it lived after X. proneneukos. This data learned of was succeeded by a subsequent anatomical redescription of X. proneneukos with anatomical comparison to R. garasbae in the 2018 paper, ergo making Xenoposeidon diagnosable amongst other sauropods, ensuring it wasn’t a nomen dubium (a name which is dubious/not well described) or indeterminate in nature making it largely insignificant to paleontologists and fossil workers alike. Despite these possible outcomes with any fossil material, it’s always better to be thorough in observations and study.

Furthermore, Xenoposeidon was compared in 2007 to other groupings of sauropods such as Brachiosauridae, Camarasauridae, Diplodocoidea, and finally Titanosauria. X. proneneukos was not assignable to any, however this changed when the 2018 paper was published. The methods used to obtain this new data with the background information mentioned prior, later on elucidating its affinities with other sauropods, included access to the collections of the Natural History Museum of the United Kingdom (NHMUK) concerning where the holotype of X. proneneukos is stored and standard anatomical identifications specifically focusing on the holotype vertebra of X. proneneukos with a focus on comparing it to R. garasbae as mentioned prior; a procedure that Mike Taylor is well acquainted with. Other parts of the methodology provided by Mike Taylor himself were 3D modeling of the fossils. Major clues seen in anatomical observation include a “M-shaped” appearance on a side of the X. proneneukos holotype vertebra, which is a trait directly shared with Rebbachisaurus. Moreover, to define Rebbachisaurus and its namesake sauropod grouping (the Rebbachisauridae), they are sauropods which are closely related to the more famous whip-tailed diplodocids and the bizarre short-necked dicreaeosaurids. The rebbachisaurids furthermore differ from these other sauropods mentioned herein that they’re closely related to are clear differences in their vertebrae and primitive appearance relative to the diplodocids and rebbachisaurids.

The data then became significant when Mike Taylor recognized the anatomical traits of X. proneneukos not ever being seen in non-rebbachisaurid sauropods. This was a clue showing damning evidence of an actual true position amongst the sauropods and not just an indeterminate position like previously thought in the 2007 description paper. Other figures elucidating this were high-definition images of the X. proneneukos holotype featured in the 2018 paper which highlighted the five diagnosable traits mentioned earlier, recognized by sauropod expert Mike Taylor, coupled with a reconstruction of the fragmentary holotype as a rebbachisaurid vertebra as this 2018 paper suggests. Moreover, the author also offered some ideas concerning where this bone would be within the body of the animal, stating that it’s likely near the posterior area of the animal (the lower vertebrae closer to the pelvic region).

The ideas discussed within the paper show that Xenoposeidon is possibly the earliest recognizable rebbachisaurid dinosaur in the fossil record, with Histriasaurus (another rebbachisaurid from Croatia) appearing in the fossil record 10 million years later. This conclusion is reached based on the anatomical clues uncovered by Mike Taylor and the age of Xenoposeidon with respect to other rebbachisaurids. As potentially the most basal and oldest member of the sauropod grouping Rebbachisauridae, when not factoring in the enigmatic and now lost holotype of Maraapunisaurus fragillimus, Xenoposeidon is vital to understanding the evolutionary history and paleogeographic range of Rebbachisauridae. Furthermore, what makes Xenoposeidon interesting is that it’s a rebbachisaurid found in the northern hemisphere, a contrast against other rebbachisaurid sauropods which are found primarily in the southern hemisphere. Mike Taylor offers some interesting ideas to explain this realization with Xenoposeidon, stating how Xenoposeidon could be an artifact of an incomplete fossil record concerning these animals, or an example of a starting point concerning rebbachisaurid migration from the northern hemisphere to the southern hemisphere to places such as South America and Africa (where rebbachisaurids are usually found, an idea parsimonious with M. fragillimus’ identification as a basal rebbachisaurid). To further add, within the Ashdown Formation where Xenoposeidon was found, there are other sauropods present, such as another currently yet unnamed rebbachisaurid sauropod.

To add to the mystery concerning Xenoposeidon, Mike Taylor in the 2018 paper mentions that the exact location where Xenoposeidon was discovered may not even be in the Southeastern English town of Hastings, but possibly at Ecclesbourne Glen, 1-2 miles outside of Hastings (this was something also considered in the 2007 description paper with Darren Naish). Other things seen from the 2018 paper which require confirmation is where exactly Xenoposeidon fits in the high-spined family of dinosaurs it's now assigned to (Rebbachisauridae) and this most likely won’t be possible until more material of Xenoposeidon is uncovered.?