Marginocephalia Pachycephalosauria In Domes We Trust

Pachycephalosaurs were bipedal ornithischians with thickened skull roofs (Figure 6.2). In the North American pachycephalosaurs, this took the form of high domes, but several Asian varieties had flattened, thickened skulls (Figure 6.3); some, however, are considered to be juvenile forms of fully adult dome-headed pachycephalosaurs. Figure 6.4 shows the distinctive Northern Hemisphere distribution of pachycephalosaur sites.

Pachycephalosaurs
Figure 6.2. The flat-headed, thickheaded Homalocephale, best-known of all pachycephalosaurs.

Pachycephalosaur lives and lifestyles

Where did a pachycephalosaur call home? In Asia, pachycephalosaurs apparently lived in a Sahara-like desert, punctuated by ephemeral streams in small drainage basins. Their remains are commonly found as nearly complete skulls and beautifully articulated skeletons. These fossils show little evidence of transport, and were apparently fossilized close to where the living animal died.

In North America, by contrast, the environments were very different. The rocks where marginocephalian remains are found represent a broad, Cretaceous coastal plain in a then-temperate climate - built from sediment eroded as the Rockies mountain range rose

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Tylocephale

Figure 6.3. Left lateral view of (a) Homalocephale, (b) Prenocephale, (c) Stegoceras, (d) Pachycephalosaurus, and (e) Tylocephale.

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Figure 6.3. Left lateral view of (a) Homalocephale, (b) Prenocephale, (c) Stegoceras, (d) Pachycephalosaurus, and (e) Tylocephale.

Figure 6.4. Global distribution of Pachycephalosauria.

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10 cm to the west (Figure 6.5). The most common finds are isolated, thickened skull caps, whose water-worn appearance suggests that they were transported long distances in rivers before burial and fossilization (Figure 6.6). The frequency of skull cap finds suggest that only the most robust of bones - in this case, the skull caps - survived long journeys. The absence of

OVERTHRUST BELT

Sea level

Volcanic belt

Oceanic Plate

■ Lance Formation

North American Plate

Medicine Formation

Niobrara Chalk Greenhorn Formation Dakota Sandstone

■ Hell Creek Formation Fox Hills Formation Bear Paw Shale Pierre Shale -Judith River Formation Mancos Shale

Figure 6.5. Late Cretaceous paleogeographic of North America. As the ancestral Rocky Mountains were uplifted and then drained and eroded by rivers, a thick sedimentary sequence was deposited into geological basins directly to the east of the rising Rockies. Fossil material was carried from the highlands by those rivers, and deposited onto the flat coastal plain to the east.

Figure 6.5. Late Cretaceous paleogeographic of North America. As the ancestral Rocky Mountains were uplifted and then drained and eroded by rivers, a thick sedimentary sequence was deposited into geological basins directly to the east of the rising Rockies. Fossil material was carried from the highlands by those rivers, and deposited onto the flat coastal plain to the east.

Museum Tray

Figure 6.6. A museum tray filled with the isolated skull caps of pachycephalo-saurs. Camera is 10 cm.

articulated specimens in coastal plain sediments implies that, in life, North American pachy cephalosaurs likely lived toward, or even in, mountains, where sediments were more likely to be eroded rather than deposited.

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Figure 6.7. An upper cheek tooth of Pachycephalosaurus.

Figure 6.8. Dorsal view of the skeleton of Homalocephale.

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Figure 6.7. An upper cheek tooth of Pachycephalosaurus.

Feeding. The herbivorous habits of pachycephalosaurs are evident not only from their teeth, but also from the impressive volume of their abdominal regions. At the front of the mouth, the jaws contained simple, peg-like gripping teeth, the last of which were sometimes enlarged in a canine-like fashion (see Figure 6.3). These teeth were surrounded by a small rhamphotheca. Further back, the cheek teeth of pachycephalosaurs were uniformly shaped with small, triangular crowns (Figure 6.7). The front and back margins of these crowns bear coarse serrations, for cutting or puncturing plant leaves or fruits.

Turning to the opposite end of the gastrointestinal system, the rib cage of pachycephalosaurs was very broad, extending backward to the base of the tail. Its size suggests that it accommodated a large stomach (or stomachs) which broke down tough vegetation via bacterial fermentation (Figure 6.8).

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Pachycephalosaur brains. Pachycephalosaurs had typical ornithischian brains, and doubtless aspired to typical ornithischian thoughts. Atypical, though, were the enlarged olfactory lobes of the brain, suggesting a better-than-average sense of smell. What they smelled, however, is a secret that died with them.

For all its unremarkable-ness, the pachycephalosaur brain was uniquely oriented in its skull. The back half of the brain is angled downward, which is reflected in the rotation of the back of the skull (the occiput) to face not only backward, but also slightly downward. It's been shown that the higher the dome, the more downward the orientation of the occiput. And that might be related to pachycephalosaurs using their heads for something other than profound thought.

Using your head . . . for a battering ram? The morphology of the domes has suggested to many scientists that, incredibly, pachycephalosaurs used their thickened skull roofs as battering rams (Figure 6.9). Internally, the structure of the dome is very dense, with the bone fibers oriented in columns approximately perpendicular to the external surface of the dome. Such an arrangement may be ideal for resisting forces that come from strong and regular thumps to the top of the head and for transmitting such forces around the brain, much as the helmet worn by sparring boxers is supposed to channel forces around the head.

Using special clear plastic cut to resemble a cross-section of the high-domed pachycepha-losaur Stegoceras (Figure 6.10), paleontologist H.-D. Sues stressed the model in a way that simulated head-butting. The stress lines, seen under ultraviolet light, mimicked the orientation of the columnar bone, reinforcing the suggestion that the fibrous columns evolved to resist stresses induced by head-butting.

Building a better head-butter. If head-butting was in fact the preferred means of pachycephalosaur expression, the body had to be set up to allow for it. Recall that the back of the pachycephalo-saur skull is rotated forward beneath the skull roof. With the head in a downward position - the only position that makes sense for headbutting - rotation of the back of the skull minimizes the chance of violent rotation or even dislocation of the head on the neck.

We might hope to see some protective measures in the neck as well; unfortunately, we know virtually nothing about the neck of pachycephalosaurs. Still it is clear from the occiput (Figure 6.11) that the neck musculature was unusually well developed and very strong. We surmise that it was used to position and hold the head correctly for head-butting.

Further down the spinal column, and utterly unique to pachycephalosaurs, the vertebrae are reported to have distinctive tongue-and-groove articulations, which must have provided rigidity to the back. These articulations would have prevented the kinds of violent lateral rotations of the body that would otherwise have been suffered at the time of impact.

Conscientious objectors? But not so fast - or at least not so hard. The thickened skull cap of one North American pachycephalosaur - Stygimoloch - has been shown to contain abundant microscopic openings for blood vessels. With so much vascularization, the skulls of pachy cephalosaurs may not have done well with either front or side impacts; this leaves the domes in this genus, and probably others, principally as display structures rather than WMDs.

Stegoceras
Figure 6.9. Head-on pushing and butting in (a) Homalocephale and (b) Stegoceras.

Socializing pachycephalosaur style

Either way - display or as a battering ram - social behavior is strongly implied, and some degree of sexual dimorphism might be expected. And so this idea was tested using a large sample of a

Figure 6.10. (a) Vertical section through the dome of Stegoceras. Note the radiating organization of internal bone. (b) Plastic model of the dome of Stegoceras in which forces were applied to several points along its outer edge and seen through polarized light. Note the close correspondence of the stress patterns produced in this model and the organization of bone indicated in (c) The left side of the skull of Stegoceras.

Figure 6.10. (a) Vertical section through the dome of Stegoceras. Note the radiating organization of internal bone. (b) Plastic model of the dome of Stegoceras in which forces were applied to several points along its outer edge and seen through polarized light. Note the close correspondence of the stress patterns produced in this model and the organization of bone indicated in (c) The left side of the skull of Stegoceras.

single pachycephalosaur species Stegoceras validum. It turns out that the domes of Stegoceras can be segregated into two groups on the basis of relative size and dome shape (Figure 6.12). One group had larger, thicker domes than the other. Strikingly, the ratio of larger-domed to less-large-domed individuals was one-to-one; exactly what you might expect if one population was male and the other, female. Which was which, however, is anybody's guess.

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Figure 6.12. Two forms of the dome of Stegoceras. The shallower dome (a) is thought to pertain to a female, while the other dome (b) may pertain to a male.

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Sexual selection. Pachycephalosaurs all had, along with, and perhaps including, the dome, a suite of features related to visual display. Firstly, there are the canine-like teeth. These could have been used in threat display or biting combat between rival individuals, much like pigs and some deer do today. Equally suggestive are the knobby and spiny osteoderms that covered the snout, the side of the face, and most extensively on the back of the marginocephalian shelf (see Figure 6.1, node 2; see also Figure 6.3). These distinctive features were likely all about showing off and establishing dominance.

The establishment of dominance gives one gender - males, if living reptiles are any guide - preferred reproductive access to the other gender, who then select a mate. These same males must also fend off competitors and establish dominance. In general, this practice of establishing dominance hierarchies constitutes sexual selection, selection within one gender (generally, males), rather than among members of a single species.

In pachycephalosaurs, domes, knobs, and spikes all acted in ritual display and, potentially, violent clashes. The winner, likely the male with the best-fashioned cranial hardware, got to perpetuate the family line. But he always had to be vigilant for other males that wanted to literally knock his block off - or at least knock him off the block.

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