The “X” in Xenotarsosaurus is certainly apt, because much about it is shrouded in mystery. Well, more so than usual for an extinct carnivore no one’s ever seen!

What we do know comes from some vertebrae and leg bones found “floating” in the strange green sandstone of Argentina’s Bajo Barreal Formation, likely a fun combination of volcanic activity and flash-flooding. Weirder still, its heel and ankle bones were fused to each other as well as the shin, hence the name “strange tarsal reptile”. Further fossil-poking ruled out any teenage awkwardness or some kind of club-foot, and based on its thigh bone, it probably weighed as much as a polar bear (460 kg).

Given the lack of skull and other body parts, it’s been difficult placing it in a specific family, and so far it’s veered between that of a dinosaur repulsed by dung-covered prey, and a Disney villain. At the moment, it’s considered part of the cooler one, and yes, I mean the latter.

Even without the “meat-eating bull” poster child just shown, any group that takes over from “shark-toothed reptiles” earns an impressive amount of badass points. The abelisaurids gradually yanked the southern continent, or Gondwana, from the carcharodontosaurids during the Cretaceous period, and were the only large predators to terrorise the terrain until the dreaded comet crashed the party.  Most known abelisaurids, and therefore probably Xeno too, had funky head gear above their eyes. It was likely only for display, but you’d expect nothing less than outrageous colours and crazy keratin from a group of dinosaurs found in the 80s. A good thing too, because it would have distracted from their puny little arms.

Abelisaurid forelimbs were even tinier than T-Rex‘s and were on the way out evolutionary speaking. But we shouldn’t make fun of them too much, because they were quite robust and not completely useless. And their owners were bitey.

Luckily for me they’ll never get to see this drawing, 89 million swear words in the making.

Their skulls were usually broad, tall and short and could withstand the shock of multiple chomps and “extended prey interaction”. Xeno’s preferred victims were probably the duck-billed and aptly named “severed lizard” (Secernosaurus), and Malawisaurus’ cousin, the scaled titanosaur Drusilasaura. 

Until we find a skull we won’t know how “metal” its headgear was, but the aforementioned “X” in its name is cool at least, and exceedingly helpful to anyone compiling an alphabetic animal fact file.

TLDR

Meaning: “Bonaparte’s strange tarsus reptile” (Xenotarsosaurus bonapartei)

What: South-American carnivorous dinosaur

Where: Argentina, 99-89 million years ago during the Late Cretaceous

How big: Estimated at 5.4 m / 18 ft long

Endangered? Nope, flat out extinct

Probable motto: [crickets]

Just to prove I’m not fibbing

Atkinson, L. “XENOTARSOSAURUS: from DinoChecker’s dinosaur archive“. Web access: 24th Aug 2018.

Bradford, Alina. 2014. “Polar bear facts“. LiveScience.com.

Burch, Sarah. 2013. “The myological consequences of extreme limb reduction: new insights from the forelimb musculature of Abelisaurid theropods“. 73rd Meeting of the Society of Vertebrate Paleontology: Abstracts of papers.

Lamanna, Matthew C. et al. 2010. “A definitive abelisaurid theropod dinosaur from the early Late Cretaceous of Patagonia“. Journal of Vertebrate Paleontology 22(1):58-69.

Limarino, Carlos Oscar et al. 2017. “Diagenetic model of the Bajo Barreal Formation (Late Cretaceous) in the south-western flank of the Golfo de San Jorge Basin (Patagonia, Argentina)“. Marine and Petroleum Geology 88(September 2017).

Nature:Prehistoric Life. No date. “Abelisaurs“. BBC Nature.

Novas, Fernando E. 2009. “The Age of Dinosaurs in South America“. Indiana University Press.

Rodríguez, Jorge F. R. 2001. “The deposition of the green sandstones (Bajo Barreal Formation- Late Cretaceous) and their taphonomic implications“. Translated by Matthew C. Lamanna.

Sabrina. 2018.”I Know Dino Podcast show notes: Xenotarsosaurus (episode 167)“. I Know Dino, The Big Dinosaur Podcast.

Theropoda:Ceratosaurida:Abelisauridae“. No date. Palaeos.org.

Xenotarsosaurus bonapartei“. No date. Dinodata.

Xenotarsosaurus bonapartei Martínez et al. 1986 (ceratosaur)”. No date. Fossilworks.org.

 

Featured image credit: Sergey Krasovskiy/atrox1