Pachycephalosaurus wyomingensis

Coloration:
Adult and juvenile (both sexes) - Blue and brown mottling.

Diet:
Herbivore. Pachycephalosaurus is not particular about what it eats, as it does not have to bother with chewing, instead relying on gastroliths to help digest its food. Being restricted to highlands forces its food preference to consist of low-growing shrubbery and other flora evolved to flourish in cold, low oxygen areas. These include perennial grasses, sedges, forbs, cushion plants, mosses, and lichens.

Preferred Habitat:
Upland forests of the Mountain Ranges or Southern Highlands, but will make do with scattered brush elsewhere.

Social Structure:
Small groups to medium-sized herds consisting of several females, their young, and a few subordinate males led by a  single dominant male who guards his herd fiercely. Some males may be solitary, usually as a result of being evicted from their herd by a  stronger male. Younger, adolescent males chased from their natal herds form small bachelor herds, but with males being less tolerant of each other - especially as they grow into adulthood - their only incentive for remaining together is for protection against carnivores. When they reach breeding age, the males leave to seek lone females to claim; or challenge another male for his herd. Sometimes the more passive males of a bachelor herd will follow a more dominant male to help him take over a herd. The less dominant males help the dominant male keep order and protect the herd from predators and rival males.

Description:
Bipedal herbivore. Pachycephalosaurus is a tough animal, quite large and bulky, and easily recognized for its tall, domed head. Well known for its aggressive head-ramming, the dome of Pachycephalosaurus' skull is 30 cm thick. Surrounding the dome and decorating the snout are bony knobs and short bony spikes. These are more pronounced in males than in females. Infants of both sexes also have blunt knobs (males develop spikes as they age). Hatchlings lack the familiar dome of the adults; instead their skulls are flat and quite soft, rising and thickening into the distinctive dome as they grow. Higher layers of fat to endure the lower temperatures of the highlands also prevent Pachycephalosaurus from staying in the lowlands and hotter areas for long periods.

Behaviors:
Pachycephalosaurus is well-known for its aggressive, unpredictable nature, and both sexes will use their domed skulls as weapons. When faced with danger, Pachycephalosaurus is likely to try and defend itself by facing the predator with its head lowered, snorting and stamping its feet before charging. Although not so effective against large carnivores, such as Tyrannosaurus, charging at smaller predators such as Velociraptor and ramming into their sides can be fatal; having the potential to break bones and crush organs. Pachycephalosaurus herds will often bunch together in a circle when under threat, protecting the  more vulnerable young in the center.

Males are much more aggressive than the females, and usually one dominant male leads over a harem of females, their young, and a few subordinate males. During the breeding season, the aggression of the males increases as they enter the rut, and they will compete with each other for females and ultimate dominance over the herd. The spikes on the base of their domes will grow dramatically, inducing further pain and frustration in the rutting males, and they will use these spikes to  impress females and warn off less experienced males. Equally-matched males will face one another, posturing, roaring, and scraping the ground with their clawed feet; and unless one backs down they may fight, using their domed skulls to batter each other into submission. The winner will then gain control of the females while the loser is usually evicted from the herd.

Females lay their eggs high in the mountains, where few predators can reach. All the young in the herd belong to the dominant male - rarely ever are they sired by a subordinate male. The young are able to move about with the herd soon after hatching, are nimble of foot and have superb balancing skills. They are born with a strong awareness of heights and danger, so they instinctively know where to be cautious. When they reach adolescence, the young males start to become troublesome and rowdy, often sparring with one another as they prepare for adulthood. To prevent the problem from growing worse, the dominant male chases his sons out of the herd before they are old enough to challenge him. His daughters are allowed to stay, and may even become a part of  his harem - resulting in the problem of inbreeding.

Typically, females are subordinate to the males, and often greatly outnumber the males in a herd. However, in the absence of males, the strongest, most dominant female in an all-female herd of Pachycephalosaurus starts to undergo a change in which the blunt knobs at the back of her skull become somewhat spikier. Her body muscles develop; and she becomes more aggressive and vocal to compensate for the lack of males which normally protect the herd. Even in the event that a true male takes over the herd, the change of the affected female is  permanent. Being female, she is not a threat to the new lead male, and in fact she is of use to the herd in acting as a defending force.

The Pachycephalosaurus' vocalizations include bleats, growls, bellows and snorts, and some kind of shriek, similar to a pig's squeal, produced when the animal is scared. Sometimes, Pachycephalosaurus herds can also quite happily mingle with other herbivores while grazing, particularly with its smaller, more timid relative, Homalocephale. Brooding mother Pachycephalosaurus with young are often keen to care for the Homalocephale as well; due to the Homalocephale bearing a resemblance to  the juvenile form of Pachycephalosaurus.