It is said that they could break the legs of T. rex and co with their hammer-like tail clubs. But maybe the ankylosaurs used their famous weapons against each other, say paleontologists. They have discovered broken and healed shell elements on the flanks of a fossil that has been preserved in a particularly detailed manner. These battle scars are thus more likely to be from the sideways blows of another ankylosaur than from the attack of a predatory dinosaur. According to this, the clubs may have played a similar role to the weapons of some modern-day animals: they could have served primarily for intraspecific competition, the researchers say.
With their spectacular "equipment" the ankylosaurs advanced to celebrities of the Cretaceous animal world. It is an extremely successful group of four-legged walking dinosaurs. During the Cretaceous period, they produced different species that grew up to nine meters long and populated large parts of the world. An important recipe for the success of the heavy-weight herbivores was their ability to defend themselves: thick armor and some spike-like structures protect them from attacks by predatory dinosaurs such as tyrannosaurus and the like. In the case of the particularly famous representatives, there was a massive thickening at the end of the appendage. Apparently, this element provided particularly intense punching power.
A "hammer dinosaur" in sight
It was therefore obvious that the ankylosaurs swung their hammer clubs in the fight against predatory dinosaurs. This was initially assumed in the case of an approximately six meter long and 2.5 ton representative whose fossil was found in the north of the US state of Montana: Zuul crurivastator was named after the fictional monster "Zuul" from the film "Ghostbusters". and the nickname crurivastator means something like "destroyer of the shins". What sets the approximately 76-million-year-old Zuul fossil apart from other known ankylosaur remains is its amazingly detailed preservation.
Originally, the skull and tail were cleared of the surrounding rock, but other parts of the body remained trapped in the sandstone for a long time. In the meantime, however, a Canadian paleontological team has been able to uncover them. It became more and more apparent that large parts of the skin and the bone armor have been preserved over the entire back and flanks. "It's like a snapshot of what the animal looked like when it was alive. And the injuries it sustained throughout its life could give us clues as to how it might have interacted with other animals," says senior author David Evans of the Royal Ontario Museum in Toronto.
The team reports that Zuul's tail was about ten feet long and had spikes on the sides. The back half was stiff, and the tip was encased in huge lumps of bone that formed a massive, hammer-like weapon. It was also revealed that Zuul's entire body was covered with bony plates and spikes of various shapes and sizes. It is striking that the plates on the sides in particular appear particularly stable. A detailed look at the spine structures in this area then provided particularly interesting findings: the tips were missing on a number of these spines near the hips on both sides of the body. It became apparent that the bone and horny sheaths had healed into a blunt form. The researchers assume that these are traces of violent effects.
Reference to internal conflicts
However, as they explain, it seems implausible that these striking lateral marks of violence come from the attack of a two-legged predatory dinosaur. Instead, according to them, it is likely that they were caused by the blows of another ankylosaur's tail club. Thus, it is an indication of a use of the mace that lead author Victoria Arbor of the Royal BC Museum in British Columbia has previously considered as a possibility: intraspecific disputes. "I've been interested in how ankylosaurs use their tail clubs for years, and now we have a new piece of the puzzle," says Arbour. "We know that ankylosaurs could use their tail clubs to deliver very powerful blows to an opponent." But as it turns out, these opponents may not have been predatory dinosaurs: "Maybe ankylosaurs like Zuul fought each other," says the paleontologist.
The new research results do not refute the idea that the tail clubs were also used for self-defense against predators. However, its use as a weapon for intraspecific conflicts may have played an even more important role in the development of the concept. Because this is also the case with some of today's weapon carriers in the animal kingdom: For example, deer or antelope species use their horns primarily in intraspecific conflicts. This can also apply to both sexes. In contrast, the use of weapons against robbers is often of secondary importance.
It remains unclear exactly what role the clubs played for the ankylosaurs. However, based on the fossil evidence, the researchers suspect a significant intraspecific importance: These animals may have had complex social behavior, fought for dominance and territorial supremacy or even had a "rutting season" with pronounced rivalry, say the paleontologists.
Source: Royal Ontario Museum, Specialist article: Biology Letters, doi: 10.1098/rsbl.2022.0404