Sauropedia

A tribute to the 1993-1998 'Dinosaurs!' collection by Orbis Publishing Ltd.

Stegoceras June 27, 2011

Filed under: Pachycephalosauria — muzillu @ 4:34 pm

An adult male Stegoceras would have fought fiercely with another for control of the herd. Although quite small, Stegoceras was a tough creature. A bipedal herbivore, Stegoceras was a member of an unusual group, the pachycephalosaurs (‘thick-headed reptiles’).

This is the best known of the pachycephalosaurids, with dozens of skull fragments known and also a partial skeleton. The structure of the bone in the head dome was such that the bone fibres aligned to absorb impact from the top. The vertebrae of the neck and back were very strong, lashed together with strong tendons that prevented twisting, and aligned to absorb shocks emanating from the head end. The hips were particularly wide and solid. All this is consistent with the idea that the dome was used as a weapon, like a battering ram.

 

 

Factbox

Name: Stegoceras, meaning ‘roof horn’

Size: 2.5-3m long and 1.5m high

Food: plants and ferns

Lived: 75 million years ago in the Late Cretaceous Period in North America

Pachycephalosaurs had one special feature in common – a thick, rounded skull. On Stegoceras’ head was a semicircle of small bony lumps. These bumps ran above its eyes and around the back of its neck. Experts are not certain what the lumps were for, but they could have been used in flank-butting, a widely evidenced theory. The skull was not very thick when Stegoceras was born, but it became thicker as the dinosaur got older.

Some experts believe that they have discovered examples of both male and female Stegoceras. They have found that some skulls are thicker than others and believe that these thick skulls may have belonged to the males. A male Stegoceras could have a skull up to 6cm thick – which is half as thick as a brick.

It was initially proposed that male Stegoceras (and individuals of other pachycephalosaur species) would ram each other headlong, not unlike contemporary bighorn sheep or musk oxen. It was later suggested that they engaged in flank-butting rather than ramming, a theory for which there is a great deal of evidence. Foremost, the rounded shape of the skull roof would lessen the contacted surface area during headbutting, resulting in glancing blows. Second, pachycephalosaurs would not have been able to align their head, neck and body in a perfect horizontal line (which would be needed to transmit stress) – it was more likely that they carried their neck in an S- or U-shaped curve (this to a lesser extent in Stegoceras, due to their thick neck muscles). Lastly, the relatively large width of most pachycephalosaurs would have served to protect vital organs from harm during flank-butting.

For most of the time, Stegoceras was a peaceful herbivore. It moved through the Late Cretaceous vegetation, pulling leaves and flowers from trees and low-lying plants with its beaked mouth. Its teeth were sharp and serrated like a saw. Stegoceras used them to shred leaves and plants, rather like modern goats.

The dome on the head of Stegoceras is high, but not as high as that of others in the group, and is surrounded by a frill of little horns and knobs. The teeth at the front of the jaw are very widely set and the muzzle is broad compared with other pachycephalosaurids. This may indicate a less selective feeding strategy. The very broad hips suggest that the pachycephalosaurids gave birth to live young (this is not widely accepted).

 

Stygimoloch May 14, 2011

Filed under: Pachycephalosauria — muzillu @ 5:31 pm

Stygimoloch’s tough, bony skull was thought to have protected its brain during its amazing headbutting fights. This hypothesis has been disputed in recent years.

Stygimoloch lived in herds and grazed in woodland areas. It had short front legs, but far longer back legs. It also had a lengthy tail which it held level with its body when running. On its head were prominent horns. These were originally thought to have functioned solely for show and not used as weapons.

The name of this pachycephalosaur derives from its frightful appearance. Moloch was a horned devil in Hebrew mythology, and in Greek legends the river Styx was the river that the dead had to cross to reach the underworld. The fossils were found in the Hell Creek formation in Montana, and this was a further inspiration for the name. The first Stygimoloch horn core was found in 1896 and regarded as part of a Triceratops skull. In the 1940s, when pachycephalosaurs were recognized, it was classed as a species of Pachycephalosaurus.

Factbox

Name: Stygimoloch, meaning ‘horned devil from the river of death’

Size: 3m long

Food: plants

Lived: about 70 million years ago during the Cretaceous Period in North America

The most obvious feature of Stygimoloch is the array of horns projecting from the rim of the dome. The head is quite long and the dome is high, narrow and thin. From the front, this presents a startling apparition of ornamentation, with long horns surrounded by clusters of more stubby spikes, that would evidently have been very effective as a threat or defence display, very much like those of some of the horned ceratopsians.

Unlike other pachycephalosaurs, the domed skull is relatively small, slightly flattened from side to side, and pear-shaped; even when isolated this unusual dome can easily be distinguished from the broader, larger domes of Pachycephalosaurus. While the dome is reduced in size, the ornamentation over the skull is more elaborate than in any other pachycephalosaur. Short, conical hornlets covered the nose, and the back corners of the skull bore an enormous pair of massive, backward-pointing spikes, up to 5cm in diameter and 15 cm long; these are surrounded by two or three smaller spikes. The function of this unusual ornamentation is unknown. Even if other pachycephalosaurs did butt heads (which is a subject of continuing debate), the small dome of Stygimoloch suggests that this behaviour was not as important. Instead, the skull ornament might have functioned for display, may have been used for self-defence, or perhaps were locked together and used in shoving matches, like the horns of deer. More likely, however, is that the squamosal horns were used to inflict pain during flank-butting.

Stygimoloch is known mostly from the skull. There have been five partial skulls found, but there have been other parts of the skeleton found in remains from North and South Dakota, USA.