Marginocephalia

Marginocephalia is a group of special dinosaurs known for having bony shelves or edges on the back of their skulls. These edges were probably used to show off to others. This group was officially described in 2021 and includes two main types of dinosaurs: the thick-skulled pachycephalosaurs and the horned ceratopsians. All of these dinosaurs mostly ate plants, although some pachycephalosaurs might have also eaten other small animals.

Pachycephalosaurs, meaning "thick-headed reptiles," started out small and walked on two legs. Over time, they developed very thick skulls, sometimes with dome shapes and horn-like bumps. Some scientists think these domes were used like helmets when these dinosaurs bumped heads with each other, although there is debate about whether their necks were strong enough for this.

Ceratopsians, or "horned-faces," had a special bone on the front of their skulls that formed a beak. They also had jugal horns and a thin shelf of bone that stretched back and up into a frill. This frill might have helped hold jaw muscles and was also likely used for showing off. Their horns could have been used to show who was in charge or to defend their areas. One of the earliest members of this group was Psittacosaurus, which lived in Asia. Later, ceratopsians grew very large and developed elaborate horns, such as Triceratops, Styracosaurus, and Centrosaurus.

Feeding

Marginocephalians had simple, peg-like teeth with a tough outer layer, arranged in groups that made it easy to replace worn teeth. These teeth likely helped them cut plants for food. Different types of Marginocephalians developed special ways to digest their food. For example, some had large stomachs that may have helped break down tough plants with help from tiny organisms. Others developed strong chewing muscles and special tooth shapes to grind up their food better. Some also swallowed small stones, which could have helped break down food in their stomachs.

Margins and social behavior

Marginocephalian dinosaurs had special bony parts on their skulls that likely helped them interact with each other. These parts might have been used to show off, defend themselves, or communicate with others of their kind.

Pachycephalosaurs, with their round heads, are thought to have bumped heads together. Their brains and necks were built to handle these bumps. But some of them had skull shapes that wouldn't have worked well for head-butting, so their heads might have been used just for looking fancy or for gentle nudging.

Ceratopsians had frills that came in many shapes and sizes. These frills might have helped protect their necks, but they probably also showed off to others or helped them recognize each other. Some groups of ceratopsians stayed together in herds, and some even cared for their young. Their horns might have been used for protection, showing how old they were, or even for locking horns in playful or defensive behavior.
interspecific communication Stygimoloch agonistic behavior Tyrannosaurus isotope analysis

Sexual dimorphism

Studying differences between males and females in dinosaurs is very hard. In Marginocephalia, the bony shelves or fringes on the back of the skull show some signs that might be differences between males and females. For example, Stegoceras validum, a type of dinosaur with a thick head, can be split into two groups based on the size and shape of their skulls. This split is almost like half the population being one type and half another, which might mean these are differences between males and females. But some think these might actually be two different species.

Another dinosaur, Protoceratops, also shows possible differences between males and females. However, the frills on their heads don’t fully grow until they are older and might be related to when they become adults.

Locomotion

Early marginocephalians could walk on two legs or sometimes on four legs. As these dinosaurs evolved, most of them walked only on four legs. This is very clear in the group called Ceratopsia, where only the early dinosaur, Psittacosaurus, walked on two legs. All the later forms walked strongly on four legs, but scientists debate how they held their bodies. Some believe they stood tall with their front legs straight to move faster. Others think their legs were spread out more, which would have helped them eat plants close to the ground more easily. We know less about how the bipedal pachycephalosaurs moved, but they likely had a wider body to hold their large stomachs.

Classification

The diagram below shows how scientists group certain dinosaurs together, based on work done in 2009 by Zheng and others.

Cladogram after Butler et al., 2011.