A newly discovered species of ankylosaur protected itself from predators with a tail weapon that is unlike anything in the paleontological record. Indeed, in an effort to describe it, the best comparison dinosaur describers could find was the Mesoamerican war club, the macuahuitl. The advantages and disadvantages of this slashing blade compared to the spikes and maces developed by other armored dinosaurs remain a mystery, but the discovery proves Gondwanan Cretaceous species were in need of as much protection as their northern hemisphere counterparts.
Faced with the fearsome predators of the era, many herbivorous dinosaurs found armor insufficient, developing tails that could do serious damage to the shins, and possibly soft underbellies, of anything that tried to eat them. Some may have also found these useful in mating conflicts with their own species.
Having failed to develop a more scientific term, paleontologists adopted the name “the thagomizer” after a Gary Larson cartoon. A paper in Nature describes a newly identified late Cretaceous ankylosaur from Chile, whose defining feature is its thagomizer, which even the paper’s headline calls “bizarre”.
The discovery is different enough from anything known it needs a new genus, and the authors have called it Stegouros, which confusingly has nothing to do with the famous Stegosauruses. The name comes from the Greek words for “roof” and “tail”; Stegouros lived 80 million years later and on the other side of the world from its near namesake. The species name is elengassen.
Only one Stegouros fossil has been found – another major contrast to the common Stegosaurus – but that one is almost complete. At 180-200 centimeters (6-7 foot) long, tail included, it is believed to have been fully grown.
The tail was relatively short by the standards of armored dinosaurs and ended in seven pairs of flat bony deposits that form a shape somewhat like a fern frond, for those unfamiliar with ancient Aztec weaponry. It could probably slice deep into any perceived threats.
S. elengassen had a skull and teeth similar to other ankylosaurs, including the much more common representatives in the northern hemisphere. The rest of its body, however, looks like a throwback to earlier times, including some features that do indeed resemble stegosauruses.
S. elengassen was found in a layer dated between 74.9-71.7 million years old from Magellanes as the southern tip of Chile. The site was a delta at the time in which many dinosaurs and other animals were trapped. At the time South America was still connected to Antarctica and Australia as part of the supercontinent Gondwana. It’s nearest known relatives appear to be the larger Antarctopelta and Australia’s Kunbarrasaurus, but Stegouros had several noticeable differences from each. The authors propose a new clade called Parankylosauria ("next to the ankylosaurs") to combine these. “The Parankylosauria lack many features of the 'true' ankylosaurs that were already present in the middle Jurassic, about 165 million years ago. Therefore, the roots of Parankylosauria must be very old , before that date ” said Dr Alexander Vargas of the University of Chile said in a statement.
The authors still believe ankylosaurs were genuinely less common in the southern hemisphere than in the north, but our limited knowledge of the Gondwana species also partially reflects the amount of exploration done there. No dinosaurs from Chile were described before 2011, Stegouros is the fourth in ten years.
Although they may not have been common, the fact the southern hemisphere ankylosaurs represented a line that stretched back almost 100 million years suggests tanks of the animal kingdom might still be around today, were it not for a meddling asteroid.