Pachycephalosauria: ramrods of the Cretaceous

Figure 8.1 (see p. 147). The flat-headed, thick-headed Homalocephale, best-known of all pachycephalosaurs.

Through a dense thicket come the deep sounds of thuds, slaps, and scuffling. Beyond these shrubs in a large clearing are a dozen or more pachycephalosaurs - Homalocephale - their broad thickened heads fringed with an array of small knobs and horns. Many are slowly foraging for succulent leaves and fruits in the undergrowth, while two of the largest individuals, some 2.5 m long, are kicking up a storm of dust in the center of the group. At regular intervals, they turn toward each other, then rapidly lunge forward, colliding head-to-head, head-to-sides, head-to-thighs (Figure 8.1). After more than an hour, one tires and is forced from the group by the victor.

Fact or fiction? How can we possibly tell? What sort of data would we look to in order to assess these ideas about pachy-cephalosaur behavior? What was the lifestyle of these unusual animals, many of which had large domes on their heads?

Pachycephalosaurs, bipedal ornithischians so far known from only the continents in the Northern Hemisphere and from Cretaceous rocks, all had thickened skulls, but not all were domed. Some like the Asian Homalocephale had flattened skulls. These flat-headed pachycephalosaurs were thought to represent a clade different from the round-headed forms, but, as we will see later in this chapter, they actually represent an evolutionary trend in pachycephalosaurs to thicken and then dome the top of the head (Figure 8.2).

Figure 8.1 (see p. 147). The flat-headed, thick-headed Homalocephale, best-known of all pachycephalosaurs.

Pachycephalosaur We begin with the global distribution of pachycephalosaurs (Figure lives and lifestyles 8.3). All are known from northern hemisphere continents, suggesting that their origin is related to the northern hemisphere supercontinent of Laurasia. In fact, all of the primitive members of Pachycephalosauria (e.g., Wannanosaurus, Goyocephale, and Homalocephale) are known from central or eastern Asia, which implies that they arose in this part of the world.

Figure 8.2. The left side of the skull of Stegoceras. (Photograph courtesy of H.-D. Sues.)
Figure 8.3. Global distribution of Pachycephalosauria.

Concentrating for a moment on the material from North America, most of what we know about pachycephalosaurs comes from isolated skull caps, many of which are highly water-worn. In fact, only single specimens of Stegoceras and Pachycephalosaurus are represented by more than just these skull caps, the former by a well-preserved skull and partial postcranial skeleton and the latter by a nearly complete skull. Still, these skull caps are very abundant in Upper Cretaceous rocks of North America, suggesting that some pachycephalosaurs - particularly Stegoceras - may have constituted up to 10% of the dinosaur fauna of the time. What is controlling the bias against the preservation of better specimens? C. M. Sternberg and, later, P. Dodson have suggested that North American pachycephalosaurs may have lived some distance away from those rivers and lakes whose sediments now preserve their remains. Alternatively, the habitats of the North American forms might have been far away from the kinds of environment that are preserved as sedimentary rocks; for example, if they lived in more upland or mountainous settings, only the most robust of their bones - their skull caps - would survive the long journey in the rivers that drained these regions to the flatlands where burial (and deposition) occurred.

In contrast, when we turn to Asia, the situation is quite different. Here, the combination of desert conditions and fluvial environments of Mongolia and China must have created favorable conditions for pachycephalosaur preservation. The nearly complete skulls and associated skeletons clearly lack traces of the kinds of long-distance transport seen in North American forms, suggesting that these animals must have lived very close to where they were eventually buried and preserved.

10 cm

10 cm
10 cm

10 cm

10 cm

10 cm

Figure 8.4. Left lateral view of (a) Homalocephale, (b) Prenocephale, (c) Stegoceras, (d) Pachycephalosaurus, and (e) Tylocephale.


5 mm

Figure 8.5. An upper cheektooth of Pachycephalosaurus.

5 mm

Figure 8.5. An upper cheektooth of Pachycephalosaurus.

Like many herbivorous dinosaurs, pachycephalosaurs must have spent a great deal of their time foraging on many of the low-growing plants of the time (because of their stature, pachycephalosaurs probably browsed no more than a meter or so above the ground). The herbivorous habits of these animals are evident not only from their teeth but also the impressive volume of their abdominal region. At the front of the jaws were simple, peg-like gripping teeth, the last of which were sometimes enlarged in a canine-like fashion (Figure 8.4). It is likely that these teeth were surrounded by a small, horny rhamphotheca as in all ornithischians. Further back, the cheek teeth of pachycephalosaurs were uniformly-shaped, with small, triangular crowns (Figures 8.4, 8.5). The front and back margins of these crowns bear coarse serrations, the better for cutting or puncturing plant leaves or fruits. This kind of chewing produced a variety of kinds of tooth wear, implying that different pachycephalosaurs fed on different kinds of vegetation.

At the other end of the feeding apparatus, pachycephalosaurs must have had a large internal vat for fermenting their food (Figure 8.6). The rib cage is very broad, a great girth that extends backward to the base of the tail. These anatomical modifications of the more primitive condition seen in ceratopsians (see Chapter 9), ornithopods (see Chapter 10), and other ornithischians suggest a backward migration and enlargement of the digestive tract to occupy a position between the legs and under the tail. Much like the case for thyreophorans (see introductory text to Part II: Ornithischia, and Chapter 9), simple styles of chewing may have combined with more extensive chemical digestion via the development of a huge gut to solve the problem of making a living as a plant-eating dinosaur.

Thoughts of a pachycephalosaur

Pachycephalosaur neuroanatomy suggests that, despite having only an average-sized brain for their body size, these animals may have had a quite acute sense of smell; the olfactory lobes of the brain were enlarged. The front half of the brain (the cerebrum) is highly flexed relative to the rest of the brain (to give it a horizontal orientation) as it was nestled inside the skull, while the back half (the pontine region) was less flexed than in other dinosaurs. With a smaller degree of pontine flexure, this region of the brain is downwardly inclined. As reported by E. B. Buchholz (formerly Giffin), the reduction in pontine flexure appears to reflect the rotation of the back of the skull (the occiput) to face not only backward, but also slightly downward. She has also shown that that the most extreme degree of occipital inclination is associated with more prominent doming of the frontal-parietal slcull cap. And prominent doming, of course, is the feature that has led paleontologists to suggest that pachycephalosaurs head-butted.

Social behavior

The idea that pachycephalosaurs used their thickened skull roofs as battering rams first came from suggestions made in 1955 by E. H. Colbert and independently by the Russian evolutionary biologist L. S. Davitashvili in 1961. In the years since, head-butting among pachycephalosaurs has been analyzed in considerable detail particularly by P. M. Galton and H.-D. Sues in the 1970s.1

50 cm

Figure 8.6. Dorsal view of the skeleton of Homalocephale.

I Very recently M. Goodwin and J. Horner have challenged whether the thick skulls were able to sustain heavy blows; using bone histology they suggested that the domes grew very fast and therefore were more likely used in display rather than combat. (See Goodwin, M. B. and Hornen J. R. 2004. Cranial histology of pachycephalosaurs (Ornithischia; Marginocephalia) reveals transitory structures inconsistent with head-butting behavior Paleobiology, 30,253-267.

Stegoceras Skeletal
Figure 8.7. Head-on pushing and butting in (a) Homalocephale and (b) Stegoceras.

Let's look at what Galton and Sues had to say about pachy-cephalosaurs as living battering rams (Figure 8.7). Each noted that the outside of the very thick dome is often smooth. Some specimens bear what look like scars on this external surface. Internally, the structure of the dome is very dense, consisting of fine bony columns that radiate so as to be approximately perpendicular to the external surface of the dome. Such an arrangement is ideal for resisting forces that come from strong and regular thumps to the top of the head and transmitting such forces around the brain, much like an American football player's helmet channels forces around the head. Adding strength to this functional interpretation of bone structure is Sues's simulation of how such forces would pass through the dome. Using special clear plastic cut to resemble a cross-section of the dome of Stegoceras (Figure 8.8), Sues stressed this plastic model in way that simulated head-butting and, voilà, the stress lines (which can be seen under ultraviolet light) had the same orientation as the columnar bone. The match between stress lines and bony columns strongly suggests that the latter have optimal orientation to resist the former.

Even though there is a match between stress-line and bone-column directions in Sues's functional model, the microscopic arrangement of the bone within the dome suggests that something else besides shock absorption was going on. In a paper on the histological structure of the dome of Stygimoloch, a pachycephalosaur from the Late Cretaceous of Wyoming, M. Goodwin and co-workers discovered that the enlarged frontal bone contains abundant microscopic openings for blood vessels.

Figure 8.8. (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 (a). (Photographs courtesy of H.-D. Sues.)

Because of this pattern of vascularity, these authors inferred that the skull of pachycephalosaurs were not well designed to sustain either front or side impacts and that domes functioned principally as display structures. We don't yet know how many other pachycephalosaurs had this land of bone histology, although it is unlike that seen in the skulls of other dinosaurs that have been investigated.2

If domes were used in some sort of intraspecific behavior, whether display or as a battering ram, we might expect that these structures should show some degree of sexual dimorphism. The first suggestion of such dimorphism in domes was made by Galton and W. P. Wall, but by far the best treatment is by R. E. Chapman and several of his colleagues. Their work assessed variation in the size and shape of the frontal-parietal dome using a large sample of a single pachycephalosaur species, Stegoceras validum. Many of these domes were from juveniles, while others - the largest specimens - obviously came from old individuals. On the whole, variation within the Stegoceras validum sample was fairly homogeneous - none of the specimens stood out from the main group, which is what one would like when assessing a single species known only from the fossil record.

2 Goodwin and Horner have recently done just that, which promises to give us a much more complex story about pachycephalosaur behavior

Figure 8.9. Two forms of the dome of Stegoceras.The shallower dome is thought to pertain to a female (a), while the deeper dome may pertain to a male (b).

Some heterogeneity in size and shape occurs in the domes of Stegoceras. In fact, the sample can be segregated into two groups on the basis of relative sizes of domes and braincases (Figure 8.9). One group, which shows slight acceleration in the rate of growth of the dome relative to the braincase, naturally formed a larger dome. At the same time, these larger-domed individuals had a slightly greater growth rate for dome thickness as well. Those individuals with larger, more convex domes were arbitrarily designated by Chapman and collaborators as males. The other group, distinguished by having thinner, flatter domes (as a consequence of lower growth rates than in the other, more prominently domed group), was presumed to be female.3 It is additionally interesting that these assignments to male or female come out to be a one-to-one ratio. Since these Stegoceras individuals are thought to be sympatric (living in the same place at the same time), such a sex ratio is to be expected. Finally, in terms of the subject at hand - the functional significance of differences between sexes - it is the males with their large and more convex domes that are better designed to initiate and receiving head-butting blows, while the females have domes less well developed and in fact very similar in the general profile to juvenile or young adult males, exactly those individuals not yet likely to engage in head-butting. If Chapman and his colleagues are correct, then we have a much more subtle perspective on the relationship of dome size and shape in males and females to the use of the domes in head-butting. But what else can be said about pachycephalosaur battering rams in terms of other regions of the body? We turn next to the orientation of the head on the neck.

As we have already noted, the back of the pachycephalosaur skull is progressively rotated forward beneath the skull roof. In this way, the line of action of any impact force to the top of the head passes close to the

3 Why is maleness assigned to the larger-domed forms, and femaleness assigned to the smaller-domed forms? Is there any biological validity to this, or are vertebrate paleontologists simply atavistic chauvinists? While we might not exonerate all of our colleagues from the charge, the fact is that, in birds, the closest living relatives of pachycephalosaurs, it is predominantly the males who are larger and who are disposed toward sexual display, mostly through coloration and behavior For this reason, those pachycephalosaurs with large domes, for whom head-butting in sexual display may be indicated, are reasonably - if arbitrarily - inferred to have been males.

I Ocm

Figure 8.10. Rear view of the skull of (a) Homalocephale and (b) Stegoceras.

I Ocm

Figure 8.10. Rear view of the skull of (a) Homalocephale and (b) Stegoceras.

occipital condyle (the bony joint between the skull and neck vertebrae). With the head in a downward position - the only position that makes sense for head-butting - rotation of the back of the skull minimizes the chance of violent rotation or even dislocation of the head on the neck. These sorts of injury represent the Cretaceous equivalent of something like automobile whiplash: immediately debilitating and often life-threatening. Therefore, we might expect some corrective measures to be seen in the neck region as well. Unfortunately, the neck itself is not known in any pachycephalosaur. It is nevertheless clear from the back of the skull of these animals (Figure 8.10) that the extremely well-developed and very strong neck musculature was used to position the head at the right attitude for head-butting and to resist some potentially injurious rotations of the skull.

What about further down the animal? Are there skeletal measures to ensure that the body did not torque around itself, injuring the spinal cord and the nerves that extended from it? The answer is a resounding "Yes." Unique to pachycephalosaurs, the tongue-and-groove articulations between vertebrae along the back are very much like the carpentry joints of the same name that give strength to, and make rigid, adjoining pieces of wood. In a similar fashion, the pachycephalosaur tongue-and-groove condition would have provided a great deal of rigidity to the back, preventing the kinds of violent lateral rotations of the body that would otherwise have been suffered at the time of impact.

The skull and the back vertebrae all speak strongly for head-butting in pachycephalosaurs. With the phylogeny of the group in mind (see below), we can now develop some of the history of this kind of behavior in the clade. Most primitively, pachycephalosaurs all bear thickened, yet flat heads. This feature alone strongly suggests that these animals appear to have been head-pushers. Such encounters were probably made with the vertebral column held horizontally, an obvious advantage when competing males approached one another, and in any case it was the primitive condition for these bipedal ornithischians. It is heartening to note that analogous kinds of pushing match are also found in modern marine iguanas, which use these types of confrontation to establish social hierarchies. Perhaps the social structuring described here for primitive pachycephalosaurs had an even wider phylogenetic distribution, as ceratopsians are also thought to have evolved a similar social order. If so, then perhaps social hierarchies and head-on confrontations constitute the ancestral behavioral pattern for at least Marginocephalia and retained in all members of Pachycephalosauria.

It was not until the evolution of fully domed forms that battering-ram behavior appears to have developed. Again, approaches of competing males were probably done with the vertebral column held horizontally. Perhaps these males faced each other and made dome-to-dome blows or else charged at each other and lowered their heads just before impact. But these head-to-head collisions did not come cost-free: very severe injuries to the head and neck were to be expected simply because these animals lacked self-correcting mechanisms of the kind seen in modern head-butters such as goats and big-horn sheep that would have kept the tremendous forces aligned with the rest of the skeleton. Glancing blows would have been the worst for these head-on battering rams. Debilitating, or even lethal, injuries to the brain or spinal cord were to be avoided at all cost. So without precision headbutting, these animals instead may have butted each other along their flanks. It was much safer that way and the results were presumably the same - winners and losers.

Whatever region of the body that pachycephalosaurs butted, a myriad of anatomical modifications took place to reduce the possibility of injury. As we have already outlined, the tremendous forces applied to the top of the dome would have been transmitted by the reoriented occiput through the long axis of the neck, thereby reducing the chances that the skull was dislocated by the jolt of head collision. Any additional tendency toward head dislocation would have been prevented by the large and powerful muscles of the neck. Finally, whiplash to the back would have been prevented by the rigid construction of the vertebral column. All in all, these animals were very well designed for the rough and tumble lifestyle of the ram-rods of the Late Cretaceous.

Integrated with head-butting is a suite of features related to visual display. First, there are the canine-like teeth. These were probably used in threat display or biting combat between rival individuals, much as pigs and primitive deer do today. If we only knew more about them, we might discover that these teeth may be sexually dimorphic, large in males and less prominent, more normal-sized in females. Alas, all this remains speculation. More informative are the knobby and spiny osteoderms that covered the snout, the side of the face, and most extensively on the back of the parietal-squamosal shelf. Assuredly these distinctive knobs and spines - especially in Stygimoloch - were used to show off, males alternately displaying to females and threatening rival males.

Sexual selection The "glue" holding all of these evolutionary modifications together is sex.

The establishment of dominance hierarchy gives some males - those that send the right signals about their qualifications for breeding -

preferred access to females. And it is the females who choose not only their mates but by their very actions what those signals are. Males that show off in ways that are acceptable to females are those that gain reproductive access. These same males must also fend off competitors for access to females. To do so requires some land of threat display and/or offensive weaponry that is used to establish dominance between greater and lesser males. In modern bighorn sheep it is the curl of horns, while in modern anole lizards it is the colorful dewlap that unfolds under the chin. In general, this practice of female choice and its effect on establishing dominance hierarchies constitute what is called sexual selection, selection not between all individuals within a species but between males alone. Sexual selection emphasizes features related to display and combat, principally of males to females and males to males. In pachycephalosaurs, this emphasis is on the prominence of domes, knobs, and spikes, structures that acted in ritual display and, should the need arise, in actual violent clashes. The winner, presumably the male with the best-fashioned head-thumper, either in terms of showiness or safest design, perpetuates its family line, but nevertheless must be ever vigilant for other males that want to knock his block off - or at least knock him off his block.

The evolution of Today we recognize over a dozen pachycephalosaur species, principally chyceDhalosauria fr°m the Northern Hemisphere and all but two (Yaverlandia and

Stenopelix) from the Late Cretaceous. With all of these different forms, their taxonomy and more especially their phylogenetic relationships -both with other ornithischians and among themselves - have been the focus of much attention.

The most obvious thing about pachycephalosaurs is their thickheadedness. With clear modifications of the skull, it is not surprising that Pachycephalosauria has long been considered a monophyletic group. Where the controversy lies is "Which other ornithischian group represents the next closest relatives of pachycephalosaurs?" As we have seen, the interpretation of derived versus primitive characters used to support a monophyletic group depends on the group's next closest relatives. You can't have one without the other, as they say L. M. Lambe originally thought pachycephalosaurs had affinities with stegosaurs (which in those days also included ankylosaurs), while more modern studies have placed these animals within, or as close relatives to, ornithopods, ankylosaurs, or ceratopsians. It is this latter relationship that now appears to be the best supported (see introductory text to Part II: Ornithischia, and discussion later in this chapter).

With this information in hand, it is possible to define and diagnose Pachycephalosauria (Figure 8.11). This clade consists of the common ancestor of Stenopelix and Pachycephalosaurus, and all of the descendants of this common ancestor. Ceratopsians appear to be most closely related to this pachycephalosaur clade and together Pachycephalosauria and Ceratopsia form the clade called Marginocephalia. More distant relationships within Ornithischia are with Ornithopoda, Thyreophora,

0 0

Post a comment