Pachycephalosaurid dinosaurs (sometimes colloquially referred to as a pachys), of which the famous Pachycephalosaurus is a member, are typically shown headbutting each other much like you would see in rams, goats and oxen. They've even been shown to headbutt other things with a devastating amount of force without sustaining any major injury. This was popularized by the story "A Gun for a Dinosaur", of the Rivers Of Time series by L. Sprague de Camp.
Pachycephalosaurs in fiction will be far more aggressive at the very least. They will often be shown charging at top speed into everything in sight, be it other members of their own species, bigger dinosaurs, or time machines with people inside. They're also likely to be depicted as aggressive and violent, in a departure from the usual depiction of plant-eating dinosaurs in fiction (though pachycephalosaurs were most likely omnivorous). Also, especially in older works, pachycephalosaurs will sometimes be shown as far larger than any real-life species, being on the same scale as hadrosaurs or ceratopsids, even though Pachycephalosaurus (the largest of the bunch) was no bigger than a horse, and most of its kin, like Stegoceras and Prenocephale, were dog-sized at best.
The head-butting behavior is debated in paleontology circles: some argue that the necks were too weak to support head-on collisions, or else that smoothly rounded domes would have been too likely to slide off of each other, and point to the spikes and sides of the head to say pachycephalosaurs swung their heads more like one does a hammer. On the other hand, others say the injuries found on pachycephalosaur skulls match head-on collisions similar to sheep and goats. Others still argue that some mixture of behaviors may have occurred, so that fights between pachycephalosaurs may have consisted of mixtures of head-to-head collisions, shoving matches, and strikes to the legs and flanks.
Naturally this is a subtrope of Artistic License – Paleontology, due to it being about the exaggerated portrayal of a prehistoric animal, and of Use Your Head. It's also a subtrope of Animal Stereotypes, in a rare case where the stereotype belongs to a long-extinct species. Compare Raptor Attack, Gentle Giant Sauropod, and Social Ornithopod for stereotypical depictions of other dinosaur species, and Temper-Ceratops and Tough Armored Dinosaur in particular for another cases of herbivorous dinosaurs being portrayed as aggressive and dangerous. It also doubles as a form of Beastly Bloodsports when they're ramming each other for territory or for a mate. Don't confuse with pachyderms headbutting; those would be an example of Rhino Rampage, Cruel Elephant, or Angry, Angry Hippos.
- Daikyouryu no Jidai: A pair of of Pachycephalosaurus are shown to be headbutting each other so hard that it not only gets the attention of other dinosaurs, but scares them too. Not the Tyrannosaurus rex though, he came for dinner.
- Doraemon: Nobita's Dinosaur briefly showed a pair of male Pachycephalosaurus head-butting each other to win over a female. Later, one Pachycephalosaurus rams the ship of some time-travelling poachers as the other captive dinosaurs are breaking out.
- One episode of the 2005 anime made in the 2010s had Nobita getting chased by a Pachycephalosaurus which tries to smash him with its head. Amusingly, this is subverted when Doraemon comes to his rescue and accidentally head-butts the Pachycephalosaurus on its domed head, knocking it out.
- Jabberwocky: Tuzo, a bodyguard for Dorohov, is a humanoid Pachycephalosaurid who fights by using his thick bone skull to ram through his opponents
- One Piece has Ulti of the Animal Kingdom Pirates, whose Devil Fruit allows her to turn into a Pachycephalosaurus, complete with a headbutt-based moveset.
- Magic: The Gathering: Territorial Hammerskull, a pachycephalosaur-like dinosaur shown lifting a much larger predator off its feet with a headlong charge.
From the eyes up, it's solid bone and stubbornness.
- Weird N Wild Creatures: The fact card for the Pachycephalosaurus mentions that they use their heads as battering rams for head-on collisions. Interestingly earlier it mentions that they could have used their heads to slam into the side of their rival which would be a aversion of this trope since they'd be targeting a far softer area.
- Yu-Gi-Oh! has the similarly-named "Hyper Hammerhead", a Dinosaur-type monster resembling a Pachycephalosaurus with a head shaped like a literal hammer.
- Heavily downplayed with the pachycephalosaurs in Prehistoric Park Reimagined when first introduced, as none of the adult pachycephalosaurus rescued in t-rex centric episode Return of the King are featured even charging at or ramming their heads against anything, let alone doing so against each other, but some of the juveniles are shown to playfully headbutt and ram each other in the safety of their new paddock at the park after their rescue. The texacephale rescued in D-Bags likewise choose to flee instead of fight back against a pack of newly discovered tyrannosaurs that try to hunt them. Ultimately played straight later, however, in Big Storm Episode Hell's Labyrinth when the entire pachycephalosaurus engage in this behavior against an invading escaped pack of Morrison Formation allosaurus.
- Averted in Ice Age: Dawn of the Dinosaurs where Pachycephalosaurus are never shown head-butting anything. However, this is played straight in the tie-in video game where Buck must ride on a Pachycephalosaurus and get it to break through rock walls by head-ramming them.
- The Land Before Time: In the first film a group of Pachycephalosaurus are seen ramming their heads together and into the ground in an attempt to attack Cera, all without any injury.
- Speckles: The Tarbosaurus: Ironically shown with a bit of Truth in Television the losing Pachycephalosaurus ending up knocked out, which as stated above in the description could easily end up being a death sentence for an animal.
- Jurassic Park makes use of this alongside a wide variety of other stereotypical — and generally inaccurate — dinosaur traits. Justified In-Universe due to the dinosaurs having been created to have audience-pleasing traits such as increased strength and aggressiveness, and having been designed to align with what audiences wanted to see rather than with scientific accuracy.
- The Lost World: Jurassic Park: The scene with the Pachycephalosaurus getting captured shows it ramming into the side of a jeep. This not only causes the jeep's door to be torn in half, but also sends the man inside flying out the other side. This shouldn't be possible since (hopefully) like most of the other vehicles used by Ludlow's Team, it was intentionally modified to help them against the dinosaurs.
- Jurassic World: Not seen, but it is discussed that when Pachycephalosaurus butt heads they temporarily disable their implants, implying that their implants are in or around the pachys' heads, without harming themselves.
- Jurassic World: Fallen Kingdom: Downplayed, but still ultimately present with the Stygimoloch who manages to ram down a brick wall and a steel cage door. Although it did take the dinosaur a unspecified amount of time it is the only dinosaur to have broken out despite the fact that several dinosaur powerhouses (T. rex, Triceratops, ect.) were also kept in similar cages. The experience does leave its mark on the animal, as it seems fairly dizzy and disoriented for some time after this.
- One of the many dinosaurs who appears in Tyranno's Claw is a pachy, who initially appears docile until a Dark Tribe caveman tries provoking it. The Pachy then reacts violently and even headbutts one of the tribesmen to death.
- The magic Wannanosaurus in Dinosaur Cove has the tendency to do this.
- Dinotopia averts this, never depicting pachycephalosaurs as headbutting anything. Although some Pachycephalosaurus are shown using their heads to push a large, wooden device operating a lathe room in a Volcaneum .
- The Fantastic Dinosaur Adventure: When the protagonists visit the Cretaceous period, they observe two male Pachycephalosaurus engaging in head-butting combat over a harem of females.
- In The Lost World (1995) paleontologist Levine points out several pachys to the kids, noting their reinforced skulls presumably used to butt heads in dominance displays. Malcolm dryly notes "Yes, butting heads. Exactly as you see them doing right now," (the pachys are just milling about and grazing, utterly peaceful). Levine exasperatedly retorts that such thick skull reinforcement must have had some purpose. Later, Sarah Harding runs afoul of a herd of pachys, specifically comparing them modern animals like cows, American bison, and African buffalo, who are three distinctly different degrees of dangerous. The pachys turn out to have something of a Hair-Trigger Temper, and charge at anything they perceive as a threat. . . which, in this instance, includes Sarah Harding.
- The Magic School Bus: In the Time of the Dinosaurs mentions that pachycephalosaurids "may have butted heads like rams", which normally wouldn't be a problem since it's stated as a theory instead of fact, but then proceeds to illustrate two ramming each other head on in the background.
- Rivers Of Time: As stated above, the story "A Gun For A Dinosaur" popularized the idea of Pachycephalosaurus violently ramming things with their domes, making it the Trope Codifier.
- Dino Dan: Two Stygimoloch are shown head-butting each other like battering rams.
- Power Rangers Dino Charge: Justified due to being a Mecha, the pachy Zord naturally headbutts enemies.
- Super Sentai: Justified, as the Pachycephalosaurus are Mecha:
- Bakuryuu Sentai Abaranger has Bachycelo, a Bakuryuu whose thick skull can then be weaponized as a Power Fist for the giant mechas. His powerset is even compared to boxing and punches, and he even sports a natural pair of gloves.
- Zyuden Sentai Kyoryuger: Zigzagged, one of the Zyudenryu, Bunpachy, can headbutt enemies like a stereotypical pachycephalosaurus, but its main power is swinging its mace on its tail. Curiously, the mace is attached to its head before activation. Its Ranger partner has also used a headbutt technique on occasion.
- Prehistoric Planet: In the "Swamps" episode, male Pachycephalosaurs are depicted this way, crashing heads together to impress females. The end of the episode features a "Prehistoric Planet: Uncovered" segment, which breaks from the usual mockumentary format to ask Was Pachycephalosaurus Really a Headbutter?, laying out the evidence for (the many lesions found on the skulls themselves) and against (the argument that the animal's bones were not strong enough to absorb the impact). The showrunners commissioned an experiment using CT scanning technology of the same kind that is used to test the structural integrity of airplanes parts, and concluded that the bones were indeed strong enough.
David Attenborough: We now believe pachycephalosaurs used their head both as a formidable weapon and as a colourful display. Two ways to demonstrate who was boss in their prehistoric world.
- Primeval: New World: A Pachycephalosaurus goes on a rampage through the city of Vancouver, smashing storefronts with its head after mistaking its reflection for a rival.
- Pathfinder: Pachycephalosaurus is an otherwise relatively unremarkable herbivorous animal distinguished chiefly by its ability to make powerful headlong charges against its foes.
- ARK: Survival Evolved: For the most part the game seems to avert this trope, with any combat excused to this for easily being Justified due to the limited Game Mechanics, and then you discover that Pachycephalosaurus leniproelia can knock trees over by ramming them.
- Carnivores: The game's instruction manual mentions that Pachycephalosaurus use their thick skull rimmed with knobby spikes as a battering ram against other dinosaurs for defense. Ironically this isn't seen in the game and their neck is a target for hunters.
- E.V.O.: Search for Eden: Happen to appear occasionally in the third chapter "The Age of Dinosaurs", using the headbutt attacking method either against you or themselves. They player could attack them for some experience points or just leave them peacefully and fight each other instead.
- Fossil Fighters: Several different species of Pachycephalosauria are present and all have skills that have the word "Headbutt" and/or "Ram" in them, implying this trope.
- LEGO Jurassic World: In the story mode there is a show that has a Pachycephalosaurus ram three pillars of solid rock.
- Monster Hunter: World: The Kestodon and Gastodon, two species chiefly based off of pachycephalosaurs, attack exclusively by headbutting foes.
- Monster Sanctuary: Rampede is a bipedal, Mix-and-Match Critter dinosaur who can break crystal blocks and hidden walls in the overworld by charging into them.
- Pokémon Diamond and Pearl: Cranidos and its evolution Rampardos are both based on pachycephalosaurs, and both are routinely shown performing headbutting attacks such as Headbutt, Take Down, Zen Headbutt and Head Smash. Their Pokédex entries also focus almost entirely on their propensity for headbutting things, mentioning how their skulls were hard as iron and used for smashing inconveniently placed trees and taking down prey. Rampardos' entries claim it to have a foot-thick skull that shields it from any harm its headbutting may cause, although it also stifles the growth of its brain, and that a cloned specimen destroyed a skyscraper by headbutting it.
- Primal Carnage: The Pachycephalosaurus is the first non-carnivorous animal (although it still heals by eating meat in-game) to be made playable. Since it has no sharp claws or teeth, its only method of attacking humans is.... well, guess.
- Temtem: Vulcrane vaguely resembles Pachycephalosaurus, with its armoured head, bipedal stance, and long tail. It's also described as fierce and learns the attack Head Ram.
- Dinosaurs: Terrible Lizards features two Pachys (who look like purple hotdogs with legs) attempting to headbutt one another, completely missing each other, and continuing to charge blindly for several miles until one of them eventually falls off a cliff. This is one of the LESS weird things in the animation.
- Mighty Magiswords: One episode has Prohyas and Vambre going up against one of King Rexxtopher's minions which is a Pachycephalosaurus, who headbutts Vambre and later No Sells a pogo stick attack on the head from the duo.
- Pinkfong: The Dinosaur series of the Web Animation has a Pachycephalosaurus that sings about being the "king of head-butting" and using this against predators like Tyrannosaurus rex by charging, which would have in most cases have actually caused them more harm than good.
- Dumbing of Age: After Sal gets into Dina and Amber's room by naming Pachycephalosaurus as her favourite dinosaur her and Dina end up talking about the trope in an attempt to break the ice. A Patreon-exclusive flashback strip has Sal hitting Walky with a flying headbutt and the dino's name as a battle cry.
- Dink, the Little Dinosaur: "The Reluctant Head-Banger" had Shyler befriending a Pachycephalosaurus named Nobbie who lacks confidence challenging his herd members in head-butting contests. At one point, Dink mentions "dome-heads" cannot feel a thing when they ram heads since her skulls are "really solid". Eventually, Nobbie gets over his fear when he saves the kids from a rockslide by smashing the rocks with his head.
- Dino Ranch has Bopper, a baby Pachycephalosaurus who has a habit of using his species' famous thick skull to butt anything he comes across.
- Dino-Riders: The Valorians/Dino-Riders use Pachycephalosaurus to smash large rocks with their domed heads. They're also mentioned to be used for clearing rubble, presumably the same way. Both actions would have likely have caused the dinosaurs harm.
- Dinosaucers: Normally averted with Bonehead, who is not usually shown head-butting things and isn't even drawn with a visible skull dome, but played straight in an episode where he "Dinovolves" and rams his head into a building.
- Dinosaur Train: Defied in an episode where the kids meet a pair of Pachycephalosaurus siblings who headbutt each-other in the sides.
- Dinotrux: Cementasaurs are a species of Dinotrux who are half-Pachycephalosaurus and half-cement mixer, who use their domed heads to ram into walls and attack enemies. In the episode where the species is introduced Chunk and Crunk accidentally ram themselves while trying to hit Waldo.