Zuul: what a nearly complete ankylosaur can tell us

Zuul crurivastator is an ankylosaur from the Campanian Judith River Formation of Montana. Its holotype, ROM 75860, is unusually complete: the skull and tail club are preserved in the same individual, together with much of the skeleton, armour and patches of skin. The specimen gives researchers a rare reference for matching isolated ankylosaur bones and studying how armour was arranged.

Quick facts

A tail club uncovered in Montana

The fossil was found in 2014 in the Coal Ridge Member of the Judith River Formation near Havre, Montana. Quarry work first exposed the tail club, leading the field team to recognise that a large part of an ankylosaur was preserved in the rock. The specimen entered the collection of the Royal Ontario Museum as ROM 75860.

Victoria Arbour and David Evans named the animal in 2017. The genus name comes from Zuul, the horned creature in the film Ghostbusters; the skull reminded the authors of the character. The species name combines Latin roots for shin and destroyer, referring to the proposed striking role of an ankylosaur’s tail club. The name expresses a hypothesis about function, not a fossil record of a particular blow.

Why the skull and tail in one skeleton matter

Ankylosaur bones are often found separately. After death, the heavy armour can detach from the body and spread across a site. ROM 75860 preserves a relatively complete skull and the terminal tail club in one individual, along with an extended sequence of vertebrae and many armour elements. This lets researchers compare parts that are rarely associated in other specimens.

The skull is well preserved in three dimensions and carries small bony tiles called caputegulae. Osteoderms remain close to their life positions along parts of the body. Skin impressions and dark films are also present; some dark material was interpreted as probable keratin associated with the armour. These traces preserve local surface information, not a complete colour pattern or an untouched skin covering.

Armour, tail and the limits of function

The tail club consists of a stiffened handle formed by fused tail vertebrae and enlarged terminal osteoderms. Its size and structure make it capable of resisting substantial forces. Comparative and biomechanical work can test how the tail moved and what loads the bones might withstand, but a fossil cannot show how often Zuul swung it or what an individual used it for.

Defence against predators is one plausible function. Competition between ankylosaurs is another possibility, particularly where armour shows healed damage. These are not mutually exclusive roles. A club’s potential mechanics do not by themselves establish the behaviour that produced a particular injury.

Healed injuries and the combat hypothesis

Further preparation and study documented pathological osteoderms on ROM 75860. Some lesions healed while the animal was alive, meaning the injuries were not all inflicted at the moment of burial. Their position and form prompted researchers to consider interactions with other ankylosaurs, including blows from a tail club.

That interpretation is evidence-based but not certain for every damaged bone. A healed lesion does not identify the attacker, the exact weapon or the circumstances. The fossil does not preserve a fight in progress, and it cannot tell us who won. The strongest conclusion is that Zuul survived significant injuries to its armour; combat is a plausible explanation for some of them.

Geology and preservation

The Coal Ridge Member was deposited during the Campanian, roughly 76–75 million years ago. This interval is a stratigraphic estimate, not an exact date for the animal. The Judith River Formation preserves several fossil-rich environments along the margin of the Western Interior Seaway, but its many species accumulated over time and across a broad region.

The preservation of ROM 75860 involved rapid burial and unusual conditions that kept bones, osteoderms and impressions associated. The skeleton is not perfectly articulated: some elements shifted, and preparation revealed the fossil progressively. Its completeness should not be confused with a record of the animal’s final behaviour. Cause of death remains unknown.

A reference specimen for other ankylosaurs

Because ROM 75860 links the skull, tail club and armour in one animal, it helps palaeontologists interpret fragmentary ankylosaur fossils from other sites. The specimen also clarifies that armour elements can preserve shape and position that would otherwise be difficult to reconstruct. Its value lies in the combination of evidence, not only in its dramatic appearance.

Zuul is placed within Ankylosaurinae, a branch of Ankylosauridae. Phylogenetic analyses compare its skull, vertebrae, armour and tail with those of other ankylosaurs; the exact relationships can change as more specimens are described. For other armoured dinosaurs and their evidence, use the A–Z dinosaur catalogue.

What the fossil cannot show

ROM 75860 preserves a great deal, but it does not record the animal’s colour, calls, social group or exact diet. It does not preserve the cause of death, a fight or an opponent. The healed armour shows that Zuul survived earlier damage; anything beyond that must be presented as a testable interpretation rather than a witnessed scene.